;co

:CO

VK 597 U49 1921

>rial No. 142

DEPARTMENT OF COMMERCE

U. S. COAST AND GEODETIC SURVEY

E. LESTER JONES, Director

:

GENERAL INSTRUCTIONS

FOR THE

FIELD WORK OF THE U. S. COAST AND GEODETIC SURVEY

SPECIAL PUBLICA SECOND ED

WASHINGTON

GOVERNMENT PRINTING OFFICE 1921

Serial No. 142

DEPARTMENT OF COMMERCE

U. S. COAST AND GEODETIC SURVEY

E. LESTER JONES, Director

GENERAL INSTRUCTIONS

FOR THE

FIELD WORK OF THE U. S. COAST AND GEODETIC SURVEY

SPECIAL PUBLICATION No. 26 SECOND EDITION

WASHINGTON

GOVERNMENT PRINTING OFFICE 1921

CONTENTS.

Latitude aud longitude determinations 5

Triangulation 6

Secondary triangulation 7

Tertiary triangulation 12

Elevations by vertical angles 58

Gravity 71

Traverse 72

Precise leveling 72

Reconnoissance for triangulation 72

Azimuth 73

Topography 74

Hydrography 85

Coast pilot 128

Tides 136

Currents 188

Magnetic observations 195

Descriptive reports 201

Progress sketches 204

Geographic names 205

Additional instructions -. 207

Index 211

3

6 GENERAL INSTRUCTIONS FOR FIELD WORK.

TRIANOULATION.

3. Classification of triangulafion. — Triangiilation in the United States Coast and Geodetic Survey is divided into three classes :

4. Precise triangulation. — Triangiilation which has an accuracy represented by an average closing error of a triangle of about one second with all stations occupied. It is to be used in extending arcs over long distances. The network of arcs of precise triangu- lation constitutes- the bases from which secondary and tertiary tri- angulatiou are extended. The general instructions for precise triangulation, reconnoissance, and base measurement are given in Appendix 4, Report for 1911, and in Special Publication No. 19.

5. Secondary triangulation. — Triangulation which has an accu- racy represented by an average closing error of a triangle of be- tween two and three seconds. Some of the stations need not be occupied. This class of triangulation is usually employed when carrying the control from precise triangulation to the locality where topographic, hydrographic, boundary, and other surveys are to be made. Such distances are comparatively short, usually than 100 miles. In the Philippine Islands the main scheme of tri- angulation is secondary in character. The general instructions for secondary triangulation are given below.

6. Tertiary triangulation. — Triangulation of an accuracy repre- sented by an average closing error between three and five seconds. The coast triangulation of the Survey is of this character and is used for the immediate control of topographic and hydrographic surveys. At intervals- along the coast the tertiary scheme is controlled by secondary and even precise triangulation. All triangulation done by United States Coast and Geodetic Survey parties or vessels along the harbors and rivers of the United States. Alaska, Philippine Islands, or other coasts under the jurisdiction of the United States will be tertiary in character unless special instructions designate another class.

7. If it is found impracticable on account of the physical condi- tions or lack of time to obtain the accuracy in the triangulation called for by these general instructions or by special instructions, the chief of party will make a special report to the office calling attention to the discrepancies and the conditions causing them. In the Philippines such report will be made to the Director of Coast Surveys.

8. The instructions which apply only to secondary triaugulation are given in paragraphs 10 to 21. Complete instructions are given

SECONDARY TRIANGULATION. 7

for tertiary triaugulation. Some of these apply also to secondary triangulation, and a reference to them is given in paragraph 22.

9. The instructions for observation of vertical angles are given in paragraphs 112 to 138 and are to be followed in both secondary and tertiary triangulatlon.

SECONDARY TRIANGULATION.

10. Character of figures. — The chain of triangulation between base nets shall be made up of figures of from four to seven points each, in which certain stations may be left unoccupied, as indi- cated in paragraph 25. It may be allowed to degenerate to single triangles in exceptional cases where otherwise the cost and time would be excessive. There must be no overlapping of figures, ex- cept that in a four-sided central-point figure one of the diagonals of the figure may be observed, and no excess of observed lines beyond those necessary to secure a double determination of every length. If it is necessary to occupy other stations than those in the main scheme, in order to fix certain positions which are required by instructions to be fixed, connect these additional occupied stations (which will be called supplementary stations) with the main scheme by the simplest figures possible in which there is a check and preferably by single triangles with all angles measured.

11. Strength of figures. — This is the same as for tertiary trian- gulation. (See pars. 25 to 42.)

12. Length of lines.— The lower limit of length of line is fixed by two considerations. On very short lines it is difficult to get obser- vations of the degree of accuracy necessary to close the triangles within the required limit. Very short lines are apt to be accompa- nied, though not necessarily so, by poor geometric conditions as ex- pressed by large values of R. (See par. 25.) Past experience in precise triangulation indicates that observations over a line 6 kilometers long are of practically the same accuracy as over the longest lines, and that there is no advantage, in so far as accuracy in the measurement of the angles is concerned, in making the linn much longer than this. The lower limit for secondary trian- gulation is probably considerably below this. Therefore endeavor, in laying out the main scheme, to use the economic length of line ; that is, endeavor to use in each region lines of such lengths as to make the total cost of reconnoissance, building, triangulation, and base measurement a minimum per mile of progress, subject to the limitations stated in these instructions. If the economic length of the line is very great, then supplementary or intersection sta-

8 GENERAL INSTRUCTIONS FOR FIELD WORK.

tioas must lx> introduced to meet the needs of those \vho may wish to start other triangulation from the scheme.

13. Frequency of bases. — The requirements are identical with those for tertiary triangulation (see par. 44) except that the ZR, must be reckoned between measured bases or one measured base and a line of precise triangulation. Also the discrepancy between bases must not exceed 1 part in 10000 instead of 1 part in 5000. which is the requirement for tertiary triangulation.

14. Base sites and base nets. — Follow the direction given under tertiary triangulation, paragraph 46.

15. Base measurements. — In the base measurements such ap- paratus and methods should be used as to insure that the constant error does not exceed 1 part in 75000 and that the accidental er- rors are not greater than that represented by a probable error of 1 part In 200000 in the length of the base. No difficulty will be encountered iu keeping both classes of errors far within these limits, even when the base is over very rough ground, if it is measured twice with invar base tapes properly standardized. The required accuracy may be obtained by making the measurements with steel base tapes if the work is done at night or on cloudy days and provided the tapes have been well standardized. All of the base tapes of this Survey are standardized at the Bureau of Standards.

16. The method of measurements, form of record, and of compu- tations are given in paragraphs 47 to 55. Base measurements with tapes are described in some detail in the following publications of this Survey : Appendixes 8, Report of 1893, 3 of 1901. 4 of 1007, 4 of 1910, and Special Publication No. 19.

17. Horizontal angle measurement — Standard of accuracy. — In selecting the instrument to be used, the methods of observation, the signals to be used, and the conditions under which to observe, proceed upon the assumption that what is desired is the maximum speed and minimum cost consistent with the requirement that the closing error of a single triangle in the main scheme shall seldom exceed 6 seconds and that the average closing error shall be be- tween 2 and 3 seconds. The observations connecting supplemen- tary stations with the main scheme should be of this same degree of accuracy. This standard of accuracy used in connection with other portions of these instructions defining the necessary strength of figures, frequency of bases, and accuracy of base measure- ments, will, in general, insure that the probable error of any base line (or line of precise triangulation used as a base) as computed from an adjacent base for line of precise triangulation used as

SECONDARY TKIASGULATION. 9

a base) is about 1 part in 35000 and that the actual discrepancy between such bases is always less than 1 part in 10000.

18. Selection of instruments. — Either a direction or a repeating instrument may be used in triangulution of this class. In select- ing the size of instrument to be used two opposing factors must be taken into account. If small, light instruments are used and ii" sun and wind shields are not used, then the weight of the outfit which it is necessary to take to a station will be light, and the cost in time and money to transport the observing party and its outfit will not be large. On the other hand, the larger and better the instrument, up to the limit of the best theodolite available, the more fully it is protected from the sun and wind and the moiv stable the support provided for it the smaller will be the number of observations necessary to secure the required degree of accuracy and the shorter will be the observing period at the station.

19. Observations in the main scheme with the direction instru- ment.— An 8-inch direction instrument (No. 140, for example), used on its own tripod and protected from "sun and wind simply by an umbrella, will give the required accuracy with from 5 to 10 measures, a direct and reverse reading being considered 1 meas- urement. This is the type of instrument recommended for sec- ondary triangulation. Five positions of the circle should be used, corresponding approximately to the following readings on the initial signal : No. 1, 0" 1' ; No. 2, 72° 3' ; No. 3, 144° 5' ; No. 4, 216° 7' ; No. 5, 288° 9'.

The minimum number of measurements shall be 5, 1 in each position, and the maximum number 10, 2 in each position, un- less it shall be found that under particular conditions encountered a larger number is necessary to secure the required degree of accuracy. The backward (additive) reading of the micrometer only should be taken in each position of each microscope. At least once a month a few special readings both backward and forward should be taken on various graduations of the circle to determine the run of each micrometer and placed in the record as a test for run. If the average value of the run for either micrometer is found to be greater than two divisions (== 4 seconds), the micrometer should be adjusted for run. Under these conditions and with the specified positions of the circle the run will be eliminated from the results with sufficient accuracy by the process of taking means. For any other direction instru- ment the system of positions to he used must be selected with reference to the number cf measurements found to be necessary.

10

GENERAL INSTRUCTIONS FOR FIELD WORK.

With any direction instrument when a broken series is observed the missing signals are to be observed later in connection with the chosen initial, or with some other one, and only one, of the signals already observed in that series. With this system of observing no local adjustment is necessary. Little time should be spent in waiting for the doubtful signal to show. If it is not showing within, say, 1 minute when wanted, pass to the next. A saving of time results from observing many or all of the signals in each series, provided there are no long waits for signals to show, but not otherwise. When the elevations of the stations differ greatly, it is necessary to keep the horizontal axis of the instrument level in order to avoid large and troublesome errors. The magnitude of these errors for various conditions is shown in the following table. Of course, releveling should only be done between positions. (For form of record, see par. 62.)

Corrections to directions for inclination- of the horizontal axis of tl>c direction instrument or theodolite.

Inclina- tion of the horizontal axis.

Vertical angle of the line of collimation.

Correction

to hori- zontal direction.

(0

(A)

(i tan it)

10 20 30

20 20 20

0.06

a 12

0.17

10 20 30

40 40 40

0.12 0.23

0. 35

10 20 30

60 60 60

0.17

0.35 0. 52

20. Observations In the main scheme with a repeating instru- ment.— A 10-inch Gambey repeating theodolite, used on its own tripod and protected from sun and wind simply by an umbrella, will give the required accuracy with from one to two sets of observations. This is the type of repeating theodolite recom- mended for secondary triangulation. A set of observations should consist of six repetitions of the angle with the telescope in the direct position and six repetitions of the explement of the angle with the telescope in the reversed position. The method used in making a set of observations is as follows : Set the circle approximately at zero and record the initial reading. Point on

SECONDARY TRIANGULATION. 11

the left-baud object by ineaus of the lower motion and then unclamp the upper motion and point on the right-hand object. Record the approximate reading of the circle. This completes the first repetition of the angle. Next loosen the lower motion and point again on the left-hand object, then unclamp the upper motion and point again on the right-hand object, and so on. A careful reading of the circle must be made and recorded after three repetitions of the angle and again after six repetitions. Next reverse the telescope about- the horizontal axis and by means of the lower motion repoint on the right-hand object. Then loosen the upper motion and point on the left-hand object. This constitutes one repetition of the explement of the angle. After six repetitious of the explement the instrument should be back very nearly to the initial setting. The circle should then be carefully read and recorded as before. (See sample record in par. 65.) Slightly ch:mge the setting of the circle at the completion of each set of observations and make an entirely independent reading for the initial of the next set. When two or more sets of observations are made on the same angle the initial setting for each set should differ by an amount approxi- mately equal to 180° divided by the number of sets. For example, if an angle is to be measured with two sets of observations, the initial settings should be about 90° apart; if with three sets, 60° apart, etc. If some of the stations observed upon are much higher or lower than the station occupied, it is necessary to keep the horizontal axis of the instrument level, in order to avoid large and troublesome errors. (See table above.) With any repeating theodolite measure only the single angles between adja- cent lines of the main scheme and the angle necessary to close the horizon. In the comparatively rare case in which the failure of julja^ont signals to show at the same time prevents carrying out this program, make as near an approach to it as possible and then take the remaining signals in another series together with some one, and only one, of the signals observed in the first series, and •.ire in the new series only the single angles between adjacent signals and the angle necessary to close the horizon. With this scheme of observing, no local adjustment is necessary, except to distribute each horizon closure uniformly among the angles meas- ured in that series. If an attempt is made to use 7-inch repeating theodolites on triangulation of this class, it may be found nec- essary to make three sets of observations as defined above. (For a form of record of observations with a repeating theodolite, see par. 65.)

12 GENERAL INSTRUCTIONS FOR FIELD \VORK.

21. Observations on intersection stations. — An intersection .sta- tion is one which is not occupied and whose position is determined by observations upon it from stations of the main scheme, or from supplementary stations. If a direction theodolite is used, one such measurement as is outlined in paragraph 19 must be made on each line to each intersection station. A second such measurement should be made if this can be done without ma- terially delaying the progress of the work. Each series of obser- vations on intersection stations with a direction instrument is to contain some one, and only one, line of the main scheme (or a line used in fixing the position of a, supplementary station). If a repeating theodolite is used, the direction to each intersection station is to be fixed by measuring the angle between it and a line of the main scheme (or a line used in fixing the position of a supplementary station) by one set of observations consisting of three repetitions upon the angle with the telescope in the direct position and three repetitions upon its explement with the telescope in the reverse position. No measures introducing station conditions are to be made on intersection stations. It is important with either form of theodolite to have lines to each intersection station from at least three occupied stations in order to secure a check, but a possible intersection station .should not be neglected simply because only two lines to it can be secured.

22. Paragraphs 56 to 5S and 69 to 111 also apply to secondary triang-ulation, and all persons engaged on- that work will comply with the directions given.

TEKTIABY TBIANGTTLATION.

23. Character of figures. — The main scheme of the triangulation shall be made up of figures of from four to seven points each, in which certain stations may be left unoccupied as indicated under paragraph 25, "Strength of figures. " It may be allowed in exceptional cases to degenerate to single triangles with all angles observed where other- wise the cost and time would be excessive. On the other hand, tLvve must be no overlapping of figures, except that in a four-sided, cen- tral-point figure one of the diagonals of the figure may be observed . and no excess of observed lines beyond those necessary to secure a double determination of every length. Observations over lines which will make the main scheme any more complicated than that defined above would practically be wasted. The main scheme should be extended to within sight of all portions of the area to be controlled

TERTIARY TKIANGUI^ATION. 13

by the triangulation. Ii it is necessary to occupy other stations than those in the main scheme in order to reach by intersection certain stationa which must be fixed to control hydrographic or topographic operations, connect these additional occupied stations (which will be called supplementary stations) with the main scheme by the simplest figures possible in which there is a check. Single triangles with all the angles measured will, in general, be sufficient for the purpose. It frequently happens that tertiary triangulation stops at a place from which it is probable that it will be extended at some future date — as, for example, at the head of a bay or part way up a river. In such a case it is desirable to stop on a line rather than a point, and the last figure should be quadrilateral with one point left unoccupied rather than a single triangle.

24. In the coast triangulation iu the Philippine Islands the stations are usually located near the coast and on off-lying islands or on the first, foothills back from the coast.

35. Strength of figures. — In the. main scheme of triaugulation. the

value of the quantity R=( 7— )2 [52A+5A«B+«3B] for any one

figure must not in the selected best chain of triangles (call it /»', } exceed 50, nor in the second best (call it R2) exceed 150 in units in the sixth place of logarithms. These are extreme limits never to be exceeded. except when it is extremely difficult under existing conditions to keep within them. Keep the quantities Rl and /?u down to the limits 25 and 80 for the beet and second beat chains, respectively, whenever the estimated total cost does not exceed that for the chain barely within the extreme limit* by more than 25 per cent. The values of R may be readily obtained by use of the "Table for determining relative strength of figures. " (See paragraphs 26 and 27 for this table and explanation of formula for R.) One station in each figure may be left unoccupied whenever to do so docs not increase the values of R beyond the specified limits. In a figure in which all stations are occupied, if any inter- ruption (as, for example, the failure of a signal to show) makes it prob- able that such a procedure would save considerable time, certain lines not exceeding three may be observed over in one direction only. In such a case Ri and R.z shall be computed as if one outside station of the figure had been left unoccupied, and the value so computed must not exceed the specified limits. For no triangle used in connecting a supplementary station with the main scheme should the value of R be greater than 50.

14

GENERAL INSTRUCTIONS FOR FIELD WORK.

26. Table for determining relative strength of figures in tri- angulation. —

ID"

1-'°

\4t

w

18°

Up*

^2°

24 "

26"

»'

',0°

:;5°

40°

J5C

50°

55"

60 =

65°

7ti°

!->'

<0°

85"

90°

10

128

3,09

12

359

2<).r

253

14

315

253

214

187

16

28-1

225

187

162

143

18

262

204

108

143 126

113

20

245

18!)

153

130

113

100

91

22

232

177

142

119

LOB

91

81

74

24

221

167

134

111

M

83

74

67

61

26

213

160

120

104

8)

77

08

til

66

51

28

206

153

12u

99

83

72

83

57

51

47

43

30

199

148

115

94

79

68

59

53

48

43

40

33

35

88

137

1( if

85

71

80

52

46

41

37

83

27

22

40

179

129

99

79

(U

54

47

41

36

32

29

23

19

16

45

172

124

93

74

GO

50

43

37

32

28

25

20

16

13

11

50

167

119

89

70

57

47

39

34

29

26

23

18

14

11

9

8

55

162

115

86

67

54

44

37

32

27

"A

21

16

12

10

8

7

5

60

159

112

83

64

51

42

35

30

25

22

10

14

11

9

7

5

4

4

65

165

109

80

62

49

40

33

28

24

21

18

13

10

7

6

5

4

3

2

70

152

106

78

60

48

38

32

27

23

It1

17

12

9

7

5

4

3

2

2

1

75

50

104

76

58

4fi

37

30

25

21

18

16

11

8

0

4

3

2

2

1

1

1

80

147

102

74

57

45

36

89

24

20

17

15

10

7

5

4

3

2

1

1

1

0

0

85

45

100

73

55

43

34

28

23

ID

1C

14

10

7

5

3

2

2

1

1

0

0

0

0

90

143

98

71

54

42

33

27

22

19

16

13

9

6

4

3

2

1

1

1

0

0

0

0

95

140

96

70

53

41

32

26

22

18

15

13

9

6

4

3

2

1

1

0

0

0

0

100

38

C5

68

51

40

31

25

21

17

14

12

8

6

4

3

2

1

1

0

0

0

105

36

<j3

67

50

39

30

25

20

17

14

12

8

5

4

2

2

1

1

0

0

110

134

91

C5

49

38

30

24

19

16

13

11

7

5

3

•2

2

1

1

1

115

32

89

04

48

37

29

23

19

15

13

n

7

5

3

2

2

1

1

120

129

88

62

40

36

28

22 18

15

12

10

7

5

3

2

2

1

125

127

M

61

45

35

27

22

18

14

12

10

7

1

4

3

2

130

125

84

59

44

34

26

21

17

14

12

10

7

5

<.A

3

135

122

82

58

43

33

26

21

17

14

12

10

7

5

4

140

119

80

6(5

42

32

25

20

17

14

12

10

8 6

145

116

77

55

41

32

25

211 17| 15

13

11

9

160

112

75

54

40

32

26

21

IS

16

15

13

152

111

75

53

40

32

26

22

19

17

16

154

110

74

53

41

33

27

23

21

19

156

108

74

54

42

34

28

25

22

158

107

71

54

43

35

30

27

160

107

7-1

56

45

38

33

162

107

76

59

48

42

164

109

79

C3

54

166

113

86

71

168

122

98

170

143

1

27. In the table above the values tabulated are 6A2+*A*B+ V« The unit is one in the sixth place of logarithms. The two argu- ments of the table are the distance angles In degrees, the smaller

TERTIARY TRIANGULATION. 15

distance angle being given at the top ol' the table. The distance angles are the angles in each triangle opposite the known side and the side required. 5A and SB are the logarithmic sine differences corresponding to one second for the distance angles A and B of a triangle.

28. The square of the probable error of the logarithm of a side

4 n ri

of a triangle is g- (d?) — ^— S [52A+SA5B+32B] in which d is the

probable error of an observed direction, D is the number of direc- tions observed in a figure, and C is the number of conditions to be satisfied in the figure. (See Wright and Hayford's Adjust- ments of Observations, 2d ed., pp. 168 and 169.) The summation indicated by 2 is to be taken for the triangles used in computing the value of the side in question from the side supposed to be absolutely known.

J) £1

29. In the above formula the two terms ~ and S [S2A-f5AoB+62B]

depend entirely upon the figures chosen and are independent of (he accuracy with which the angles are measured. The product of these two terms is therefore a measure of the strength of the figure with respect to length, in so far as the strength depends upon the selections of stations and of lines to be observed over. The strength table is therefore to be used, in connection with the

D—C

values of — ~— given hereafter, to decide during the progress of

the reconnoissance which of the two or more possible figures is the strongest, and to determine whether a sufficiently strong scheme has been obtained to make it inadvisable to spend more time in reconnoissance.

30. To compare two alternative figures, either quadrilaterals or central point figures for example, with each other in so far as the strength with which the length is- carried is concerned, proceed as follows :

(a) For each figure take out the distance angles, to the nearest degree if possible, for the best and second best chains of triangles through the figure. These chains are to be selected at first by estimation, and the estimate is to be checked later by the results of comparison.

(b) For each triangle in each chain enter the table with the distance angles as the two arguments and take out the tabular value.

(c) For each chain, the best and second best, through each figure, take the sum of the tabular values.

K) GENERAL INSTRUCTIONS FOB FIELD WORK.

(d) Multiply each sum by the factor — -~ — for that figure. The

J) _ Q

quantity so obtained, namely, — — 2 [^A+SA^B-T-^B]* wiH ior con-

venience, be called U, and R~ for the best and second best chains. respectively.

(c). The strength of the figure is dependent mainly upon the strength of the best chain through it, hence the smaller /?,, the greater the strength of the figure. The second best chain con- tributes somewhat to the total strength, and the other weaker and progressively less independent chains contribute still smaller amounts. In deciding between figures they should be ch- according to their best chains, unless said best chains are very nearly of equal strength and their second best chains differ greatly.

D-C 81. Some values of the quantity -~ — • —

The starting line is supposed to be completely fixed .

4—1 For a single triangle, — j- =0.75.

10—4 Fora completed quadrilateral, -~^r— =0.60.

For a quadrilateral with one station on the- fixed line unoccupied,

For a quadrilateral with, one station not on the fixed line pied, ^=0.71.

10—4 For a three-sided, central point figure, ,„ =0.60.

For a three-sided, central point figure with one Nation on the fixed

g _ o line unoccupied, —?- =0.75.

o

For a three-sided, central point figuv -vith one station not on the

7—2 fixed line unoccupied, —=—=0.71.

j4 _ 5 For a four-sided, central point figure, — jj-=»0.64.

For a four-sided, central point figure v.-ith one corner station on the

12—3 fixed line unoccupied, ~^2-=0.7.">.

For a four-sided, central point figure v.-ith one corner station not on

jj _ g the fixed line unoccupied, - =0.73.

TERTIARY TRIA>7GUL,ATION. 17

For a four-sided, central point figure with, the central station not on

10—2 the fixed line unoccupied, — ;^-=0.80.

For a four-sided, central point figure with the central station not on

12—4 the fixed line unoccupied and one diagonal observed, =0.67.

jg _ y For a five-sided, central point figure, - -,— =0.67.

lo

For a five-sided, central point figure with a station on a fixed outside

16-4 line unoccupied, — ,-£—=0.75.

For a five-sided, central point figure with au outside station not on

15—4

the fixed line unoccupied, •'•--.— =0.73. xo

For a five-sided, central point figure with the central station not on

13—2 the fixed line unoccupied, —IT,— =»0.85.

22 — 7 For a six-sided, central point figure. — 7^— =0.68.

For a six-sided, central point figure with one outside station on the

i 2°-r> , fixed line unoccupied, ~k =0.7').

For a six-sided, central point figure with one outside station not on

jf) _ g the fixed line unoccupied, -W^=0.74.

For a six-sided, central point figure with the central station not on

16—2 the iixed line unoccupied, ..„ =0.88.

i 'or a four-sided, central point figure with one diagonal also observed,

For a four-sided, central point figure with one diagonal also observed,

1?_4 with the central station not on the fixed line unoccupied, — ^o" —0.67.

32. Examples of various triangulation figures. — The following fourteen figures are given to illustrate some of the principles in- volved in the selection of the strong figures and to illustrate the use of the Strength Table.

33. In every figure the line which is supposed to be fixed in length, and the line of which the length is required, are repre- sented by heavy lines. Either of these two heavy lines may be

130278— 21 - 2

18 GENERAL INSTRUCTIONS FOR FIELD WORK.

considered to be the fixed line and the other the required line. Opposite each figure Ri and Rz, as given by the Strength Table, are shown. The smaller the value of Ri the greater the strength of the figure, /fe need not be considered in comparing two figures unless the two values of JBi are equal, or nearly so.

34. Compare figs. 1, 2, and 3. Fig. 1 is a square quadrilateral ; fig. 2 is a rectangular quadrilateral, which is one-half as long in the direction of progress as it is wide; fig. 3 is a rectangular quadrilateral twice as long in the direction of progress as it is wide. The comparison of the values of Ri in figs. 1 and 2 shows that shortening a rectangular quadrilateral in the direction of progress increases its strength. A comparison of figs. 1 and 3 shows that extending a rectangular quadrilateral in the direction <>f progress weakens it.

35. Fig. 4, like fig. 2, is short in the direction of progress. Such short quadrilaterals are in general very strong, even though badly distorted from the rectangular shape, but they are not economical as progress with them is slow.

36. Fig. 5 is badly distorted from a rectangular shape, but is still a moderately strong figure. The best pair of triangles for carrying the length through this figure are D 8 R and R »S' P. As a rule, one diagonal of the quadrilateral is common to the two triangles forming the best pair, and the other diagonal is common to the second best pair. In the unusual case illustrated in fig. 5 a side line of the quadrilateral is common to the second best pair of triangles.

37. Fig. 6 is an example of a quadrilateral so much elongated. and therefore so weak, that it is not allowable in any class of triangulation.

38. Fig. 7 is the regular three-sided, central-point figure. It is extremely strong.

39. Fig. 8 is the regular four-sided, central-point figure. It is very much weaker than fig. 1, the corresponding quadrilateral.

40. Fig. 9 is the regular five-sided, central-point figure. Note that it is much weaker than any of the quadrilaterals shown in figs. 1, 2, or 4.

41. Fig. 10 is a good example of a strong, quick expansion from a base. The expansion is in the ratio of 1 to 2.

42. Figs. 11 and 12 are given as a suggestion of the manner in which, in secondary and tertiary triangulation, a point (A), dif- ficult or impossible to occupy, may be used as a concluded point common to several figures.

TERTIARY TR1ANOULATION.

19

FIG. 1. — AH stations occupied. Rx= 5

Same, any one station Rt*=6 not occupied.

k63 63,

Fio. 2. — All stations occupied.

tf

Same, any one station not occupied. Ri=2

FIG. 3. — All stations occupied. Ri=22

R2=22

Same, one station on Ri=27 fixed line not occupied Rs=27

20 GENERAL INSTRUCTIONS JTOR FIELD WORK.

Km. 4. — All stations occupied. R1=l

FIG. 5.—- All stations occupied.

TERTT Al.'V TRIASTGtnLATION.

21

6. — All stations occupied.

IJ1=1G4 (approx.) R2=176 (approx.)

FIG. 7.— All stations oc- Hx= 2 cupied. R2=12

One outside sta- tion, on fixed line, not occupied.

l-"rc. 8. — All stations occu- 1^=13

pied. Ra=13

Same, one corner Ri=16

xtatlon not occupied. R2==16

Same, central sta- Ri=17

1 on not occupied. Rj=17

22

GENERAL ' NM KUCTIOXS FOR FIELD WORK.

FIG. 9. — All station* ._Ri=10

occupied. H»=li>

Same, any one Rt=l1

outside station R...= l»» not occupied.

Same, central Ii-i=13

station not <><•- J!.j=10 cupied.

FIG. 10. — All stations H,=5 occupied. R2=5

FIG. 11. — Unoccupied station Rj= not on fixed line. Rj=

TERTIARY TRIANGULATION.

23

. 12.— Unoccupied station R,= 4 at intersection of fixed line and Jino to be de- termined.

YIG. 13. — All stations occupied.

(A strong and quick expansion figure.) •

24

GENERAL INSTRUCTIONS FOR MELD WORK.

PIG. 14. — Central .station not Ri=18 occupied. R»=24

^02

43. length of lines. — The lower limit of length of line is fixed by two considerations. On very short lines it is difficult to get observations of the degree of accuracy necessary to close the triangles within the required limit. They require extreme caution in centering and plumbing signals so that all eccentricity due to these causes may be avoided. Very short lines are apt to be accompanied, though not necessarily so, by poor geometric con- ditions as expressed fry large values of R. The extreme lower limit fixed by these two considerations should be avoided. There is no advantage in so far as accuracy is concerned in using very long1 lines. Long lines are apt to introduce delays, due to signals not being visible. With long lines supplementary stations to reach required points in all portions of the area covered are much more apt to be needed than with short lines. Therefore endeavor in laying out the main scheme to use the economic length of line — that is, endeavor to use in each region lines of such lengths as to make the total cost of reconnoissance, signal building, triangula- tion, and base measurement a minimum for the area to be covered, s-ubject to the limitations stated in these instructions.

44. Frequency of bases. — If the character of the country is such that a base site can be found near any desired location, 2.Ri between base lines, or between a base line and a line of precise or secondary triangulation used as a base, should be made about 130. This will be found to correspond to a chain of from 10 to 35

TERTIARY TRIANGULATION. 25

triangles, according to the strength of the figures secured. With strong figures but few base lines will be needed, and a correspond- ing saving will be made on this part of the work. If topographic conditions make it difficult to secure a base site at the desired location, 2Ri may be allowed to approach but not exceed 200. There will be danger when this is done that an intervening base will be necessary, for the reason stated in the next sentence. If in any case the discrepancy between adjacent bases (either measured bases or lines of precise or secondary triangnlation used ds bases) is found to exceed one part in 5000, an intervening base must be measured or the intervening triangulation strength- ened.

45. Base sites and base nets. — A base may be measured over rough ground and steep slopes with steel or invar tapes with the degree of accuracy specified in the following paragraph. Smooth, level ground is a convenience, but not a necessity, for base meas- urement of this grade of accuracy. There should be no hesitancy in placing the base on rough ground if by so doing the geometric conditions in the base net are improved — that is, values of R made smaller. The length of a base is to be determined primarily by the desirability of securing small values of R in the base net The longer the base the easier it will be found to secure small values of R, and the smaller the values of R the longer the chain of triangles through which the lengths may be carried before an- other base becomes necessary. The base net shall consist of a figure or figures of the same character and subject to the same conditions as to strength as the main scheme previously described. If the net is made up of two or more figures they may overlap in space, but there should be no overlapping of figures in the sense of the existence of observed lines which tie together the separate figures. Broken bases are permissible when found advantageous.

46. Base measurements. — In base measurements select apparatus and methods which insure that the constant error does not exceed one part in 30000, and that the accidental errors are not greater than that represented by a probable error of one part in 100000, in the length of the base. No difficulty will be encountered in keeping both classes of errors within these limits, even if the measurement is over very rough ground and steep slopes, pro- vided that the vertical measurements on steep slopes are made with sufficient accuracy, that two measurements are made of each section of the base with 50-meter steel or invar tapes, and that the tapes have been properly standardized. The tape should be used on the field under the same conditions as to tension and

2|6 GENERAL INSTRUCTIONS FOR FIELD WORK.

number of supports tluit obtained during the standardization. Alter the measurement of a base or a series of bases the tape should, if practicable, be returned to the office for restandardiza- tlon.

47. Measurements made with steel tapes in daylight, particu- larly in sunlight, are subject to constant errors in the determina- tion (of the temperature by mercurial thermometers. These errors may exceed 3° G. and produce a constant error in length as great !i.-< one part in 30000: therefore preference should be given to overcast days when practicable, or to measures at night.

48. Bases will in general be measured by steel or invar tam-s suspended from stakes. The tape lengths may be marked on the tops of the stakes by marks on copper strips or zinc plates: a 50-meter tape should be supported at 25-meter or 12.5-meter inter- vals, the support (conveniently a nail in the side of a stake t being in line vertically and horizontally between the end supports. The smaller interval should be used whenever the wind is troublesome. The base tapes are usually standardized while being supported at each of these intervals. When the topography is such as to re- quire high end supports, the intermediate support, may bo placed above (never below) the grade of the end supports and each half of the tape corrected for grade. Two thermometers should be used, fastened to the tape toward either end. The tape should be carried clear of the ground. Two measurements of a base should usually suffice. Set backs and set forwards may be made with a quarter-meter scale and dividers, and where the lengths run off the stakes, with a pocket tape having proper subdivisions. A ten- sion of 15 kilograms should be applied with a spring balance attached to the forward end of the tape. It is preferable to use a complete tape-stretching outfit, as described and shown in illus- trations in Appendix 4, Report for 1910, but a simple staff may be used at each end of the tape. Strips of signal muslin are satis- factory in fastening the tape to the staffs. The use of hard twine or rope for that purpose tends to twist the tape.

49. The spring balance used in the base measurement should be verified before and after the measurement by comparison with a spare or standard spring balance, or with a standard testing weight furnished for that purpose.

50. The base measurements may be recorded in " Horizontal angle " record books by changing the headings or in the " Traverse measurements" record book (Form 590). A duplicate of the base measurement record should be made on computing paper.

TERT1 A R V TKI A N GUL A 'I ION.

27

mid attached to the List of Directions. Each record of base meas- uroinont should be preceded by a description of the measurement showing what tapes were used, their lengths, coefficients of ex- pansion, method of support, number of supports, the tension ap- plied, how the thermometers were used, and all information neces- sary to a clear understanding of the measurement.

51. Form for record of base measurement. —

From stake X<>.

To stake No.

Tliennomcters.

! Set

Set forwards.

2183.

backs. 21S4.

NE.base 176 175

176 175 174

° C. 5.0 4.7 4.9

' r. ^fcler.f. 4.9 i

5.0 ;

5.3

.Iftter*. 0.0121

REMARKS..

Time of beginning: 9h. 5m. p. m.

Began at NE. base station mark.

Backward measure.

Fifteen kilograms tension applied with spring balance No. 170. M*

Tape No. 403 on three supports.

Balance tested and found correct.

Wind, moderate, NW., at right angles to base.

Under the column of remarks give also the names of the persons recording and making the forward and rear contacts.

52. Example of computation of length of base. — Standardization formula for tape No. 403, supported at ends and in middle, with a tension of 15 kilograms:

0 to 50m. =50 m. +8.32 mm. +0,568 nun. (/-14°.56 C.) ± .039 mm. ± .003 mm.

Twenty tape lengths; mean corrected temperature 15°. 58 C.

Meters.

20 (50 m. +8.32 mm.)=20 (50.00832)= 1000. 1664

Temperature correction: 20 (15.58-14.56) (0.568 mm.)=. . -f . 0116

Set forwards, sum + . 0133

Set backs, sum - . 0060

Inclination corrections (see table, par. 55) sum — . 2283

Length of base 999. 9570

To be entered on right-hand page.

28 GENERAL INSTRUCTIONS FOlt FIELD WORK.

63. In case the standard length is given for a tape supported through- out its length, the catenary correction C is to be applied to each span of tape, as follows:

Where TF=weight in grams per meter of tape (21.6 grams for tape in use, 25 to 27 grams for invar tapes).

i=horizontal tension in grams= 15000 grams in general.

J=length of span in meters.

54. The following publications of this Survey deal with tape meas- urements: Appendix 7, Report for 1893; Appendix 3, Report for 1901; Appendix 4, Report for 1907; Appendix 4, Report for 1910; Special Publications Nos. 19 and 58.

55. Inclination correction tables for 25-meter tape lengths. — Correc- tion for inclination =— (I— -^P — h2) where I is inclined distance and h is difference of height. For Z=25 meters and h in feet, the correc- tion for inclination=-. 00186 h2 -.000000069 h*— The sec- ond term may be neglected for differences of height of 5 feet or less.

For 50-meter spans take one-half the correction given in the table. For instance, for a span of 50 meters and a difference in elevation of the two ends of the span of 2 feet (0.6096 meter), the correction is 0.0037 meter.

TERTIARY TRJANGULATIOX.

29

Difference in etovation.

Correc- tion.

Difference in elevation.

Correc- tion.

Difference in

elevation.

Correc- tion.

Foot.

Meter.

Meter.

Foot.

Meter.

Meter.

Feet.

Meier.

Meter.

0.00

0.0000

-0.0000

0-50

0.1524

-0.0005

1.00

0.3048

-0.0019

.01

.0030

0

.51

.1554

5

.01

.3078

19

.02

.0061

0

.52

.1585

5

.02

.3109

19

.03

.0091

0

.53

.1615

5

.03

.3139

20

.04

.0122

0

.54

.1646

5

.04

.3170

20

.05

.0152

0

.55

.1676

6

.05

.3200

20

-06

.0183

0

.56

.1707

6

.06

.3231

21

.07

.0213

0

-5V

.1737

6

.07

.3201

21

.08

.0214

0

.58

.1768

6

.08

.3292

22

.09

.0274

0

.59

.1798

6

.09

.3322

22

0.10

0.0305

0

0.60

0.1829

7

1.10

0.3353

22

.11

.0335

0

.61

.1859

7

.11

.3383

23

.12

.0366

0

.62

.1890

7

12

.3414

23

.13

.0396

0

.63

.1920

i

.13

.3444

24

.14

.0427

0

.64

.1951

8

.14

.3475

24

.15

.0457

0

.65

.1981

8

.15

.3505

25

.16

.0488

0

.66

.2012

8

.16

.3536

25

.17

.0518

1

.67

.2042

8

.17

.3560

25

.18

.0549

1

.68

.2073

9

.18

.3597

26

.19

.0579

1

.69

.2103

9

.19

.3627

26

0.20

0.0610

1

0.70

0.2134

9

1.20

0.3658

27

.21

.0640

1

.71

.2164

9

.21

.3688

27

.22

.0671

1

-72

.2195

10

.22

.3719

28

.23

.0701

1

.73

.2225

10

.23

.3743

28

.24

.0732

1

.74

.2256

10

.24

.3780

29

.25

.0762

1

.75

.2286

10

.25

.3810

29

.26

.0792

1

.76

.2316

11

.26

.3840

29

.27

.0823

1

.77

.2347

11

.27

.3871

30

.28

.0853

1

.78

.2377

11

.28

.3901

31

.29

.0884

2

.79

.2408

12

.29

.3932

31

(1.30

0.0914

2

0.80

0.2438

12

1.30

0.3962

31

.31

.0945

2

.81

.2469

12

.31

.3993

32

.32

.0975

2

.82

.2499

12

.32

.4023

32

.33

.1006

2

.83

.2530

13

.33

. 405 S

33

.34

.1036

2

.84

.2560

13

.34

.4084

33

.35

.1067

2

.85

.2591

13

.35

.4115

34

.36

.1097

2

.86

.2621

14

.36

.4145

34

.37

.1128

3

.87

.2652

14

.37

.4176

35

.38

.1158

3

.88

.2682

14

.38

.4206

35

.39

.1189

3

.89

.2713

15

.39

.4237

36

0.40

0. 1219

3

0.90

0.2743

15

1.40

0.4267

36

.41

.1250

3

.91

.2774

15

.41

.42C8

37

.42

.1280

3

.92

.2804

16

.42

.4328

37

.43

.1311

3

.93

.2835

16

.43

.4359

38

.44

.1341

4

.94

.2865

16

.44

.4389

38

.45

.1372

4

.95

.2896

17

.45

.4420

39

.46

-1402

4

.96

.2926

17

.40

.4450

40

-47

.1433

4

.97

.2957

18

.47

.4481

40

.18

.1463

4

.98

.2987

18

.48

.4511

41

.49

.1494

4

.99

.3018

18

.49

.4542

41

30

GENERAL INSTRUCTIONS FOR FIELD WORK.

Difference in elevation.

Correc- tion.

Difference in elevation .

Correc- tion.

Difference in elevation.

Correc- tion.

Feet.

Meter.

Meter.

Feet.

Meter.

Meter.

Feet.

Mettt.

Meter.

1.60

0.4572

-0.0042

2.00

0.6096

-0.0074

2.60

0.7620

-0.011C

.51

.4602

43

.01

.6126

75

.51 I .7650

117

.52

.4633

43

.02

.6157

76

.52

.7681

us

.53

.4663

44

.03

.6187

76

.53

.7711

119

.54

.4694

44

.04

.6218

77

.54

.7742

120

.55

.4724

45

.05

.6248

78

.55

.7772

121

.56

.4755

46

.06

.6279

79

.56

.7803

122

.57

.4785

46

.07

.6309

80

.57

.7833

123

.58

.4816

47

.08

.6340

80

.58

.7864

124

.59

.4846

47

.09

.6370

81

.59

.7894

125

1.60

0.4877

48

2.10

0.6401

82

2.60

0.7925

126

.61

.4907

48

.11

.6431

S3

.61

.7955

127

.62

.4938

49

.12

.6462

84

.62

.7986

128

.63

.4968

50

.13

.6492

84

.63

.8016

129

.64

.4999

50

.14

.6523

85

.64

.8047

130

.65

.5029

51

.15

.6553

86

.65

.8077

131

.66

.5060

52

.16

.6584

87

.66

.8108

131

.67

.5090

52

.17

.6614

88

.67

.8138

132

.68

.5121

53

.18

.6645

88

.68

.8169

133

.69

.5151

53

.19

.6675

89

.69

.8199

134

1.70

0.5182

54

2.20

0.6706

90

2.70

0.8230

135

.71

.5212

55

.21

.6736

91

.71

.8260

136

.72

.5243

55

.22

.6767

92

.72

.8291

137

.73

.5273

56

.23

.6797

92

.73

.8321

138

.74

.5304

56

.24

.6828

93

.74

.8352

139

.75

.6334

57

.25

.6858

94

.75

.8382

141

.76

.5364

58

.26

.6888

95

.76

.8412

142

.77

.5395

58

.27

.6919

%

.77

.8443

143

.78

.5425

59

.28

.6949

96

.78

.8473

144

.79

.5456

59

.29

.6980

97

.79

.8504

145

1.80

0.5486

00

2.30

0.7010

98

2.80

0.8534

146

.81

.5517

61

.31

.7041

99

.81

.8565

147

.82

.5547

61

.32

.7071

100

.82

.8595

148

.83

.5578

62

.33

.7102

101

.83

.8626

149

.84

.5608

63

.34

.7132

102

.84

.8656

150

.85

.5639

64

.35

.7163

103

.85

.8687

151

.86

.5669

64

.36

.7193

103

.86

.8717

152

.87

.5700

65

.37

.7224

104

.87

.8748

153

.88

.5730

66

.38

.7254

105

.88

.8778

164

.89

.5761

66

.39

.7285

106

.89

.8809

166

1.90

0.5791

67

2.40

0.7315

107

2.90

0. 8839

166

.91

.5822

68

.41

.7346

108

.91

.8870

157

.92

.5852

68

.42

.7376

109

.92

.8900

158

.93

.5883

69

.43

.7407

110

.93

.8931

159

.94

.5913

70

.44

.7437

111

.94

.8961

160

.95

.5944

71

.45

.7468

112

.95

.8992

162

.96

.5974

71

.46

.7498

112

.96

.9022

163

.97

.6005

72

.47

.7529

113

.97

.9063

164

.98

.6035

73

.48

.7559

114

.98

.9083

165

.99

.6066

73

.49

.7590

115 |l .99

.9114

166

TERTIARY TRIANGULATION.

Difference in

elevation.

Correc- tion.

Difference in

elevation.

Correc- tion.

Difference in elevation .

Correc- tion.

Feet.

Meters.

Meter.

Feet.

Meters.

Meter.

Feet.

Meters.

Meter.

3.00

0. 9144

-0. 0167

3.50

1.0668

-0. 0228

4.00

1.2192

-0.0297

.01

.9174

168

.51

.0699

229

.01

.2223

299

.02

.9205

169

.52

.0729

231

.02

.2253

300

.03

.9235

171

.53

.0759

232

.03

.2283

302

.04

.9266

172

.54

.0790

233

.04

.2314

303

.05

.9296

173

.55

.0820

235

.05

.2344

305

.06

.9327

174

.56

.0851

236

.06

.2375

306

.07

.9357

175

.57

.0881

237

.07

.2405

308

.08

.9388

177

.58

.0912

238

.08

.2436

309

.09

.9418

178

.59

.0942

240

.09

.2466

311

3.10

0.9449

179

3.60

1.0973

241

4.10

1.2497

312

.11

.9479

180

.61

.1003

242

.11

.2527

314

.12

.9510

181

.62

. 1034

244

.12

.2558

315

.13

.9540

182

.63

.1064

245

.13

.2588

317

.14

.9571

183

.64

.1095

246

.14

. 2619

318

.15

.9601

185

.65

.1125

• 248

.15

.2649

320

.16

.9632

186

.66

.1156

249

.16

.2680

322

.17

.9662

187

.67

.1186

250

.17

.2710

323

.18

.9693

188

.68

.1217

251

.18

.2741

325

.19

.9723

189

.69

.1247

253

.19

.2771

326

3.20

0.9764

190

3.70

1. 1278

254

4.20

1.2802

328

.21

.9784

191

.71

.1308

255

.21

.2832

330

.22

.9815

192

.72

.1339

257

.22

.2863

331

.23

.9845

194

.73

.1369

258

.23

.2893

333

.24

.9876

195

.74

.1400

260

.24

.2924

334

.25

.9906

196

.75

.1430

261

.25

.2954

336

.26

.9936

197

.76

.1461

262

.26

.2985

338

.27

.9967

198

.77

.1491

264

.27

.3015

339

.28

.9997

200

.78

.1521

265

.28

.3045

341

.29

1.0028

201

.79

.1552

267

.29

.3076

342

3.30

1.0058

202

3.80

1.1582

268

4.30

1.3106

344

.31

.0089

203

.81

.1613

270

.31

.3137

346

.32

.0119

205

.82

.1643

271

.32

.3167

347

.33

.0150

206

.83

.1674

273

.33

.3198

349

.34

.0180

207

.84

.1704

274

.34

.3228

350

.35

.0211

209

.85

.1735

276

.35

.3259

352

.36

.0241

210

.86

.1765

277

.36

.3289

354

.37

.0272

211

.87

.1796

279

.37

.3320

355

.38

.0302

212

.88

.1826

280

. .38

.3350

357

.39

.0333

214

.89

.1857

282

.39

.3381

358

3.40

1.0363

215

3.90

1.1887

283

4.40

1.3411

360

.41

.0394

216

.91

.1918

284

.41

.3442

362

.42

.0424

218

.92

.1948

286

.42

.3472

363

.43

.0455

219

.93

.1979

287

.43

.3503

395

.44

.0485

220

.94

.2001.

289

.44

.3533

367

.45

.0516

222

.95

.2040

290

.45

.3564

369

.46

.0546

223

.96

.2070

291

.46

.3594

370

.47

.0577

224

.97

.2101

293

.47

.3625

372

.48

.0607

225

.98

.2131

294

.48

.3655

374

.49

.0638

227

.99

.2162

296

.49

.3686

375

GENERAL INSTRUCTIONS POH FIELD WORK.

Difference in elevation .

Correc- tion.

Difference in elevation.

I Correc- tion.

Difference in elevation.

Correc- tion.

feet.

^fctCTX.

Meter.

Feet.

Meters.

Meter.

Feet.

3/£££Tf.

Meter.

4.5»

1.3716

-a 0377

5.00

1.5240

-a 0465

5.50

1.6764

-0.0563

.51

.3747

379

.01

.5271

467

.51

.6795

565

.52

.3777

380

.02

.5301

469

.52

.6825

567

.53

.3807

382

.03

.5331

471

.53

.6855

569

.54

.3838

384

.04

.5362

473

.54

.6886

571

.55

.3868

386

.05

.5392

475

.55

.ft) lf>

573

.56

.3899

387

.06

.5423

476

.56

.6947

575

.57

.3929

389

.07

.5453

478

.57

.6977

577

.58

.3660

391

.08

.5484

480

.58

.7008

579

.59

.3690

392

.09

.5514

482

.59

.7038

581

4.60

1.4021

394

5.10

1.5545

484

5.60

1. 7069

583

.61

.4051

396

.11

.5575

486

.61

.7099

585

.62

.4082

397

.12

.5606

488

.62

.7130

587

.63

.4112

399

.13

.5636

490

.63

.7160

689

.64

.4143

401

.14

.5667

492

.64

.7191

591

.65

.4173

403

.re

.5607

494

.65

.7221

594

.66

.4204

404

.16

.5728

495

.66

.7252

596

.67

.4234

406

.17

.5758

497

.67

.7282

598

.68

.4265

408

.18

.5789

499

.68

.7313

600

.69

.4295

409

.19

.5819

501

.69

.7343

602

4.70

1.4326

411

5.20

1.5850

503

5.70

1.7374

604

.71

.4356

413

.21

.5880

505

.71

.7404

606

.72

.4387

415

.22

.5911

507

.72

.7435

608

.73

.4417

416

.23

.5941

509

.73

.7465

611

.74

.4448

418

.24

.5672

511

.74

.7496

613

.75

.4478

420

.25

.6002

513

.75

.7526

615

.76

.4509

422

.26

.6033

515

.76

.7557

617

.77

.4539

424

.27

.6063

517

.77

.7587

619

.78

.4569

425

.28

.6093

519

.78

.7617

622

.79

.4600

427

.29

.6124

521

.79

.7648

624

4.80

1.4630

429

5.30

1.6154

523

5.80

1. 7678

626

.81

.4661

431

.31

.6185

525

.81

.7709

628

.82

.4691

433

.32

.6215

527

.82

.7739

630

.83

.4722

434

.33

.6246

529

.83

.7770

633

.84

.4752

436

.34

.6276

531

.84

.7800

635

.85

.4783

438

.35

.6307

533

.85

.7831

637

.86

.4813

440

.36

.6337

535

.86

.7861

639

.87

.4844

442

.37

.6368

537

.87

.7892

641

.88

.4874

443

.38

.6398

539

.88

.7922

644

.89

.4905

445

.39

.6429

541

.89

.7953

648

4.90

1.4935

447

5.40

1.6459

543

5.90

1.7983

648

.91

.4966

449

.41

.6490

545

.91

.8014

650

.92

.4996

451

.42

.6520

547

.92

.8044

C52

.93

.5027

452

.43

.6551

549

.93

.8075

655

.94

.5057

454

.44

.6581

551

.94

.8105

657

.95

.5088

456

.45

.6612

553

.95

.8136

659

.96

.5118

458

.46

.6642

555

.96

.8166

661

.97

.5149

460

.47

.6673

557

.97

.8197

663

.98

.5179

461

.48

.6703

559

.98

.8227

666

.99

.5210

463

.49

.6734

561

.99

.8258

668

TERTIARY TRIAXGULATION.

33

Difference in elevation.

Correc- tion.

Difference in elevation .

Correc- tion.

Difference in elevation.

Correc- tion

Feet. Meters. Meter.

Feet.

Meters.

Meter.

Feet. Meters.

Meter.

6. 00 1. 8288

-0. 0670

6.50

1.9812

-0. 0786

7.00

2.1336

-0. 0912

.01 .8319

672

.51

.9843

789

.01

.1367

915

. 02 . 8349 674

. 52 . 9873

791

. 02 . 1397

917

.03

. 8379 677

. 53 . 9903

794

. 03 . 1427

920

. 0 1 . 8410

679

.54

.9934

796

.04 .1458

922

. O.r. . 8440

681

. 55

.9964

799

.05 .1488

925

. Oti . 8471

683

.56

.9995

801

.06

.1519

928

.07 .8501

685

.57

2.0025

804

.07 .1549

930

. 08 . 8532

688

. 58 . 005»i

806

. 08 . 1580

933

.09 .8562

690

.59

.00X6

809

.09

.1610

935

(i. 10 1.8563

692

6.60

2.0117

811

7.10

2.1641

93S

.11 .8623

694 . 61

.0147

814

.11

.1671

941

.12

.8654

.697 ! .62

.0178

816

.12

. .1702

943

.13

.8684

699 i .63

.0208

819

.13

.1732

946

.14

.8715

701

.64

.0239

821

.14

.1763

949

.15

.8745

704 . 65 . 0269

824

. 15 . 1793

952

.16

.8776

706 .66 ! .0300

826

. 16 . 1824

954

.17

.8806

708 . 67 . 0330

829

.17

.1854

957

.18

.8837

710

. 68 . 0361

831

.18

.1885

960

.19

.8867

713

. 69 . 0391

834

.19

. 1915

962

6.20

1.8898

71.3

6.70

2.0422

836

7.20

2.1946

965

.21

.8928

717

.71

.0452

839

.21

.1976

968

.22

.8959

720 .72 .0483

841

.22

.2007

970

.23

.8989

722 .73 .0513

844

.23

.2037

973

.24

.9020

725 . 74

.0544

846

.24 .2068

976

.25

.9050

727 .75

.0574

849

.25

.2098

979

.26

.9081

729 .76 .0605

851

.26

.2129

981

.27

.9111

732 .77 .0635

854

.27

.2159

984

.28

.9141

734 . 78 ! . 0665

856

.28

.2189

987

.29

.9172

737 . 79 | . 0696

859

.29

.2220

989

6.30

1.9202

739

6.80 2.0726

861

7.30

2.2250

992

.31

.9233

741

.81 .0757

864

.31

.2281

995

.32

.9263

744

. 82 . 0787

866

.32

.2311

998

.33

.929-1

746

.83

.0818

869

.33

.2342

.1000

.34

.9324

748

.84

.0848

871

.34

.2372

.1003

.35

.9355

751

.85

.0879

874

.35

.2403

.1006

.36

.9385

753

.86

.0909

876

.36

.2433

.1009

.37

.9416

755

.87

.0940

879

.37

.2464

.1012

.38

.9446

757

.88

.0970

881

.38

.2494

.1014

.39

.9477

700

.89

.1001

884

.39

.2525

.1017

6.40

1.9507

762

6.90

2.1031

886

7.40 ! 2.2555

.1020

.41

.9538

764

.91

.1062

889

.41

.2586

.1023

.42

.9568

767

.92

.1092

m

.42

.2616 ;

.1026

.43

.9599

769

.93

.1123

894

.43

.2647 !

.1028

.44

.9629

772

.94

.11.53

896

.44

.2677

.1031

.45

.9660

774

.95

.1184

899

.45

.2708

.1034

.46

.9690

776

.96

.1214

902

.46

.2738

.1037

.47

.9721

779

.97

. 1245

904

.47

.2769

.1040

.48

.9751

781

.98

.1275

907

.48

.2799

.1042

.49

.9782

784

.99

.1306

909

.49

.2830

. 1045

13027°— 21-

34

GENERAL INSTRUCTIONS FOR FIELD WORK.

Difference in elevation.

Correc- tion.

Difference in elevation.

Correc- tion.

Difference in elevation.

Correc- tion.

Feet.

deters.

Meter.

Feet.

Meters.

Meter.

Feet.

Meters.

Meter.

7.50

2.2860

-0. 1048

7.80

2.3774

-0.1133

8.10

2.4689

-0.1222

.51

.2891

.1051

.81

.3805

.1136

.11

.4719

.1225

.52

.2921

.1054

.82

.3835

.1139

.12

.4750

.122s

.53

.2951

.1056

.83

.3866

.1142

.13

.4780

.1231

.54

.2982

.1059

.84

.3866

.1145

.14

.4811

.1234

.55

.3012

.1062

..85

.3927

.1148

.15

.4841

-1237

• 56

.3043

.1065

.86

. 3957

.1150

.16

.4872

.1240

.57

.3073

.1068

.87

.3988

.1153

.17

.4902

.1243

.58

.3104

.1070

.88

.4018

.1156

.18

.4933

. 124H

.59

.3134

.1073

.89

.4049

.1159 '

.19

.4963

.1249

7.60

2.3165

.1076

7.90

2. 4079

.1162

8.20

2.4994

.1252

.61

.3195

.1078

.91

.4110

.1165 1

.21

.5024

.1255

.62

.3226

. . 1082

.92

.4140

.1168

.22

.5055

.1258

.63

.3256

.1084

.93

.4171

.1171

.23

.5085

.1261

.64

.3287

.1087

.94

.4201

.1174

.24

.5116

.1264

.65

.3317

.1090

.95

.4232

.1177

.25

.5146

.1268

.66

.3348

.1093

.96

.4262

.1180

.26

.5177

. 1271

.67

.3378

.1096

.97

.4293

.1183

.27

.5207

.1274

.68

.3409

.1098

.98

. 4323

.1186

.28

.5237

.1277

.69

.3439

- . 1101

.99

.4354

.1189

.29

.5268

.1280

7.70

2.3470

.1104

8.00

2.4384

.1192

8.30

2.5398

.1283

.71

.3500

.1107

.01

.4415

.1195

.31

.5329

.1286

.72

.3531

.1110

.02

.4445

.1198

.32

.5359

.1289

.73

.3561

.1113

.03

.4475

.1201

.33

.5390

.1292

.74

.3592

.1116

.04

.4506

.1204

.34

.5420

.1295

.75

.3622

.1119

.05

.4536

.1207

-35

.5451

.1299

.76

.3653

.1121

.06

.4567

.1210

.36

.5481

.1302

.77

.3683

.1124

.07

.4597

.1213

.37

.5512

.1305

.78

.3713

.1127

.08

.4628

.1216

.38

.5542

.1308

.79

.3744

.1130

.09

.4658

.1219

.39

.5573

.1311

8.40

2.5603

.1314

56. Signals.— Various types of signals are used. A very satis- factory one is a single pole held in a vertical position by wire guys, with the foot of the pole resting on a low bench. The bench may be made of two stakes driven into the ground on either .side of the station mark, with a piece of scantling placed across and nailed to them. A hole is bored into the crosspiece directly over the station mark. The foot of the pole should have a spike placed in its center, projecting about an inch, and this spike should be placed in tlie hole in the crosspiece of the bench when the pole is erected. There should be four wires to each set of guys, the num- ber of sets depending upon the length of the pole. The pole is easily lowered when the station is occupied by loosening the guy or guys on only one side and letting the pole fall over. The guys on the other three sides are not disturbed from their anchors. To re- place the pole it is only necessary to stand it up on the bench and fasten the loosened guy to its anchor. The centering of the pole.

TEKTIAKY TRIANGULATION. 35

or that part upon which observations are made, should be tested. It will usually be found that the centering has uot been disturbed, by replacing the pole. Single pules with wire guys are not satis- factory when placed in a pasture where there are cattle, as their rubbing against the pole and wires will throw the former out of plumb. The part of the center pole of a signal observed upon should be accurately centered over the station mark, or if eccen- tric, the eccentric distance and angle should be measured and re- corded. Uncorrected eccentricity of signal i* the most frequent source of error in triangulation. If it becomes necessary to ele- vate the instrument more than a few feet, a double structure, like that described in Appendix 4, Report for 1903, should be used. That description gives bills of lumber, plans, etc.

57. Signal lamps.— The use of acetylene signal lamps, for long lines or even for lines of only moderate length, is recommended, if fhe atmospheric conditions are not favorable for observations on poles or targets. Illustrations in Special Publication No. 11 show the large and small lamps which are issued to field parties by the office. The large lamp is shown also in Special Publications Nos. 14 and 19. Large electric signal lamps may be used where conditions 41 re especially unfavorable.

58. Instructions to lightkeepers are given in detail in Special Publication No. 65. There are also given the code signals used between the observer and lightkeeper in precise triangulation and the continental Morse alphabet.

59. Horizontal angle observations — Standard of accuracy. — In se- lecting the instrument to be used, the methods of observation, the number of observations, the signals to be used, and the condi- tions under which to observe, proceed upon the assumption that what is desired is the maximum speed and minimum cost con- sistent with the requirement that the closing error of a single triangle in the main scheme shall seldom exceed 10 seconds, and that the average clpsing error shall be between 3 and 5 seconds. The observations connecting supplementary stations with the main scheme should be of this same degree of accuracy. This standard of accuracy, used in connection with other portions of these instructions defining the necessary strength of figures, fre- quency of bases, and accuracy of base measurements, will In general insure that the probable error of any base line (or line of precise or seoond.-u-y triangulation used as a base), as com- puted from an adjacent base (or triangulation line used as a base), is about 1 part in 20000, and that the actual discrepancy

•n si,,-}, ha«es is alway* Irs* than 1 part in 5000.

36 GENERAL, INSTRUCTIONS FOR FIELD WORK.

60. Selection of instrument. — Either a direction or a repeating instrument may be used in triangulation of this class. In select- ing the size of an instrument to be used, two opposing factors must be taken into account. If small, light instruments are used, and if sun and wind shields are not used, then the weight of the outfit which it is necessary to take to a station will be light, and the cost in time and money to transport the observing party and its outfit will not be large. On the other hand, the larger and better the instrument, the more fully it is protected from the sun and wind, and the more stable the support provided for it, the smaller will be the number of observations necessary to secure the required degree of accuracy and the shorter will be the observing period at the station.

61. Observations in the main scheme with a direction instru- ment.— An 8-inch direction instrument (No. 140, for example) used on its tripod and protected from sun and wind simply by an umbrella will usually give the required accuracy with two measure- ments, a direct and reverse reading being considered one measure-, ment. Any two positions of the circle may be used with this instrument for which the settings on the initial signal differ by approximately 90° 05'. The backward (additive) reading of the micrometer only should be taken in each position of each micro- scope. At least once a month, as a test for run, a few special readings both backward and forward should be takeu on various graduations of the circle to determine the run of each micrometer and placed in the record. If the average value of the run for either micrometer is found to be greater than two divisions (four seconds), the micrometer should be adjusted for run. Under these conditions and with the specified positions of the circle the run will be eliminated from the results with sufficient accuracy by the process of taking means. For any other direction instrument the system of positions to be used may be selected with reference to the number of measurements found to be necessary. With any direction instrument when a broken series is observed the missing signals are to be observed later in connection with the chosen initial, or with some other one, and only one, of the signals already observed in that series. With this system of observing no local adjustment is necessary. Little time should be spent in waiting for a doubtful signal to show. If it is not showing within, say, one minute when wanted, pass to the next. A saving of time results from observing many or all of the signals in each sei'ies, provided there are no long waits for signals to show, but not otherwise. When the elevations of the stations differ greatly

TERTIARY TRIANGULATION.

37

it is necessary to keep the horizontal axis of the instrument level in order to avoid large and troublesome errors. The magnitude of these errors for various conditions is shown in the table in paragraph 19. Any releveling should, of course, be done between jHisitions.

Example of record is given below.

62. Horizontal directions. —

Station: Gunton. Observer: .

Date: April 17, 1902.

Instrument: 8-inch theodolite No. 140.

•s

7*

•d

§

1

Objects observed.

gj

5

ft

jj

ft

^

E

I

1

.2

"§>

1

1

1

.1

1

e

e

a

2

«

a

a

ft

h. m.

0 ,

Div.

„

„

„

I

Benvenue

2 56

i)

A

0 00

07.0

B

04.5

'11.5

Benvenue

3 06

R

A

180 00

05. 0

B

11.0

16.0

13.8

00.0

White Stone

D

A

45 40

12.0

Point

B

07.0

19.0

White Stone

R

A

225 40

08.5

Point

B

16. 5

25.0

22.0

08.2

Stevenson

D

A

76 35

26.5

B

23.0

49.5

Stevenson

R

A

256 35

24.0

B

29.5

53.5

51.5

37.7

Gut

1)

A

87 05

25.5

B

24.5

50.0

Gut

R

A

267 05

23.5

B

26.0

49.5

49.8

36.0

11

Benvenue

3 10

R

A

270 05

03.0

B

09.5

12. 5

Benvenue

3 15

D

A

90 05

06.0

B

05.5

11.5

12.0

00.0

White Stone

R

A

315 45

05.0

Point

B

06.0

11.0

White Stone

r>

A

135 45

06.0

Point

B

07.0

13.0

12.0

00.0

Stevenson

K

A

346 40

24.0

B

22.0

46.0

Stevenson

D

A

166 40

20.5

B

23.0

43.5

44.8

32.8

Gut

R

A

357 10

22.0

B

21.5

43.5

Gut

D

A

177 10

18.0

B

22.0

40.0

41.8

29.8

1 Each division of the micrometer corresponds to 2" of arc, and therefore the "mean" for this instrument is the sum of the two readings.

38 GENERAL INSTRUCTIONS FOR FIELD WORK.

63. Observations in the main scheme with a repeating instru- ment.— A 7-inch Berger repeating theodolite used on its own tripod and protected from sun and wind by an umbrella will

the required accuracy with from one to two sets of observations on each angle, each set of observations consisting of six repeti- tions on the angle, with the telescope in the direct position, and six repetitions on the explemeut of the angle, with the telescope in the reversed position. This is the type of repeating theodo- lite recommended for tertiary triangulation in any region in which the convenience of transportation of the instrument is an im- portant consideration. When the elevations of the stations differ greatly it is necessary to keep the horizontal axis of the instru- ment level in order to avoid large and troublesome errors. (See table in paragraph 19.) Releveling may be done between sets or between the separate angle measures of a set; that is, when the lower clamp is loose. With any repeating theodolite, meas- ure only the single angles between adjacent lines of the ma in scheme and the angle necessary to close the horizon. In the comparatively rare case in which the failure of adjacent signals to show at the same time prevents carrying out this program, make as near an approach to it as possible and then take the remaining signals in another series together with some one, and only one, of the signals observed in the first series, and measure in the new series only the single angles between adjacent signals and the angle necessary to close the horizon. With this scheme of observing no local adjustment is necessary, except to distribute each horizon closure uniformly among the angles measured in that series. If the region is one in which there is no great inconvenience in transporting a heavier instrument, and a 10-inch Gambey repeat- ing theodolite or an equivalent instrument is used on triangula- tion of this class, it will probably be found that one set of ob- servations consisting of three repetitions on the angle and three on its explement, will be sufficient to secure the required accuracy.

64. An example of a record is given below. From this the re- sulting directions should be written in the "List of directions" (Form 24A) without any other abstract. It will be noticed in the sample below that, in addition to the usual practice of reading one repetition on the first measurement of each angle, there is a reading for three repetitions in each case. The latter gives a value of the angle correct to within 10 seconds, which will check the reading of the minutes for the six repetitions. The reading of one repetition on one vernier does not give a sufficiently accu- rate check. Tse the reading on three repetitions as ;i check only.

TERTIARY TRIANGULATION.

39

65. Horizontal angles. —

Station : Dab. Inland : Luzon. Observer :

Date : February 7, 1906.

Instrument : B. &. B. 7-inch theodolite No. 134.

Objects observed.

£

I

p

« g fi "3

EH

Repetitious, j

_« "Si a

«i

«

$

fl2J

|B

1

^S 2o

§P5

jj

Sn

«J

Pet-Dog

a. m. 8.00

0

1

0 00 88 59

00 50

00

00

3

266 55

20

20

D

6

173 58

40

40

40

40

88 59 46. 7

(Dog-Pet)

R

6

0 00

10

20

15

25

44.2

45.5-0.7-44.8

Dog-Bat

0

1

0 00 42 30

15

15

25

20

3

127 30

35

45

R

6

25501

15

25

20

00

42 30 10.0

D

6

0 00

25

25

25

55

09.2

09. 6-0. 7-08. 9

Bat-Kow

0

0 00

10

10

10

27 34

10

3

82 43

10

20

D

6

165 26

20

30

25

15

27 34 22. 5

R

C

0 00

55

00

55

30

25.0

23.7-0.8-22.9

Kow-Bol

0

000

00

10

05

1

37 40

40

3

113 02

10

20

R

6

226 04

20

30

25

20

37 40 43. 3

D

6

0 00

10

20

15

10

41.7

42.5-0.8-4LJ

Bol-Pet

0

000

20

30

25

1

163 15

10

3

129 15

30

30

D

6

259 30

40

40

40

15

163 15 02. 5

R

6

0 00

20

30

25

15

02.5

02.5-0.8=01.7

360 00

03. 8 00. 0

66. Observations on intersection stations. — An intersection sta- tion is one which is not occupied and of which the position is de- termined by observations upon it from stations of the main scheme, or. from supplementary stations. The direction method of obser- vation should be used in observations upon intersection stations oven if the theodolite is a repeater. Each series of observations on intersection stations should contain some one line, and only <>;ie, of the main scheme (or a line used in fixing the position of a supplementary station). Such a series of observations should commence with the selected line of the main scheme, with the tele- scope in the direct position, and with the circle reading approxi- mately zero. The intersection stations should then be observed in order of azimuth and the first half of the series closed with a

40

GENERAL INSTRUCTIONS FOR FIELD WORK.

reading on the line of the main scheme. The telescope should then be reversed and the same process repeated in the reverse order, beginning and ending as before with the selected line of the main scheme. A second set of observations should be made on each intersection station ( with the circle shifted in position, say, 100° ) if this can be done without materially delaying the party. It is important to observe at least three lines to each intersection sta- tion in order to secure a check, but a i>ossible intersection station should be observed upon even if only two lines to it can be secured.

67. Example of record is given below. From this the resulting directions should be written in the "List of directions" (Form 24A) without other abstract.

68. Horizontal angles —

Station: Rat. Island: Negros. Observer:

Date: January 10, 1902.

Instrument: 7- inch theodolite No. r-7.

Objects observed.

Time.

Tel. D.or R.

Angle.

A. B.

Mean of ver- niers.

Cor- rec- tion.

Direction.

Re- marks.

Tree

a. m. 9.50

D

0 00

00 50

+ 5

R

180 00

10 00

00

0 00 00

• Bell tower, Olon

D

21 18

30 20

R

201 18

40 30

30

21 18 35

Oil

D

176 13

10 30

R

20

+10

176 13 30

Fro?

D

209 59

50 40

R

29 59

60 50

50

+ 5

209 59 55

I,, tang. Parian? Id

D

232 18

R

52 18

232 18

Peak 17

D

241 11

00 40

R

61 10

50 40

48

+ 5

241 10 53

Tree

D

359 59

50 40

10.32

R

179 59

60 50

50

0 00 00

69. Eccentric stations and signals. — Whenever a station is not occupied centrally the distance and direction between the eccentric station and the center of the station must be carefully measured. A sketch, showing the relation between the two stations and also to one line of the main scheme of the triangulation, must be en- tered in the record book and also on the list of directions form for the station. If the station is a lighthouse, the eccentric dis- tance can be obtained by measuring the circumference of the tower

TERTIARY TRIANGULATION. 41

from which tlie radius can be computed. The length of the radius added to the distance between the eccentric station and the nearest point of the tower gives the eccentric distance sought. The direction to the center may be obtained by observing upon each side of the tower and entering the results in the record as left and right tangents. The mean of these two directions is the direction to the center. The eccentric angle should, if practicable, be measured from the station used as the initial in the regular observations.

70. If the part of a signal upon which observations have been made is eccentric, the eccentric distance and direction must be measured and entered in the record and on the list of directions. The eccentric distance for a signal is usually small, and it is

sufficient to state that the pole is off center meters on line

and in the direction to station, a station of the main

scheme or an intersection station. If the eccentric station is not in a line between the center and some other station, the eccentric

distance can be given, and then it should be added that it is

meters to the north of the line to station. In any event

the eccentric distance and direction must be measured with the greatest care. Making the measurements twice will nearly always insure against mistakes.

71. Observations on large objects. — When sighting on a gas tank, standpipe, large chimney, or other object which has a large diam- eter, it is often best to observe on the two sides, right and left, and enter the observations in the record book as left and right tangents to the object. The mean of the two observations will give the direction to the center of the structure and should be used in the computations of the triangles.

72. Observations on indefinite and temporary objects. — < )bserva- tions may sometimes be made ujxm indefinite objects, as, for example, mountain or hill tops which are comparatively flat or wooded, or upon points which are temporary in nature and not marked upon the ground, such as flags in trees, etc., which are to be used as hydrographic and topographic signals. The direc- tion method of observation shall be used in all such cases, even if the instrument is a repeater. One series of observations, such as is described in paragraph 66, is all that is necessary in such cases. The two pointings, one direct and the other reversed, are needed to check the degrees and minutes of the direction. The indefinite objects may be observed in the same series with other intersection stations. Each j)ointing upon an indefinite object should be clearly marked "indefinite," and all topographic and

42 GENERAL INSTRUCTIONS FOR FIELD WORK.

hydrographic points not permanently marked upon the ground should be marked in the record with a circle and a dot. For observations of these two classes a graphic treatment will fre- quently be all that is desired, and hence the necessity of dis- tinguishing them from other points for which a complete com- putation must be made. In selecting indefinite points to be ob- served it should be borne in mind that an approximate determi- nation of the position of a prominent mountain or hill too far inland to be included in the fringe of topography along the coast, or an island far out from the coast, is frequently of con- siderable geographic value.

73. Indefinite or other unoccupied stations which have been ob- served upon from two or more stations must have their identity established in the record either by means of sketches, estimated distances, or descriptive notes, so that they may be plotted or computed without probability of confusion. An object appearing in more than one list of directions must bear the same name or designation in each. Any characteristic features of hills, moun- tains, church spires, or other objects which would render them good landmarks must be clearly noted, as such notes are valu- able in chart construction and in hydrographie work. The loca- tion of prominent objects and the determination of elevations of mountains are to be considered important parts of the work essential to the completeness of the survey. This work is to be done even when the objects and mountains are beyond the limits of the area to be surveyed. In the case of imperfectly known regions tangents should be taken to points and islands outside of the region to be surveyed.

74. Value of intersection stations.— In selecting intersection sta- tions it should be kept in mind that the geographic value of a triangulation depends upon the number of points determined, the size of the area over which they are distributed, and the per- manence with which they are marked. The geographic value of a triangulation is lost for a given area, when stations can not be recovered within that area. The chance of permanency is made greater by increasing the number of stations as well as by thorough marking. For the reasons stated there should be deter- aiined as intersection stations many artificial objects of a per- manent character such as lighthouses, church spires, cupolas, towers, chimneys, and standpipes. Make the description definite whenever practicable. Instead of describing the object as " church spire" with the name of the town, make its identity certain by giving street location or denomination of church. There should

TEKTIARY TKJANGULA'flOX. 43

also be determined well-defined natural objects, such as sharp mountain peaks, waterfalls which show from the sea, and pin- nacle rocks along the coast. Occasionally intermediate stations should be established, permanently marked, and determined by intersections for the special use of topographic and hydrographic parties.

75. Report on aids to navigation. — At the end of each season the chief of party will make a list, of the objects whose positions have been determined by triangulation and which may serve as aids to navigation. For purposes of identification each object will be described us to its size, shape, color, and its relative prominence as seen from the water. This list and a description of each object must be sent to the office with a transmitting let- ter for the files of the chart construction division. (See also paragraphs 73 and 196.)

76. Marking stations. — Every station, whether it is iu the main scheme or is a supplementary or intersection station, which is not in itself a permanent mark, as are lighthouses, church spires, cupolas, towers, large chimneys, sharp peaks, etc., shall be marked in a permanent manner, except where the station is on ji shifting sand dune.

77. At every station a standard metal triangulation mark should, if practicable, be set in rock or concrete. Where digging is feasi- ble, there should be an underground mark which is separate from the surface mark. The use of a standard station mark hi the underground mark is desirable. The station mark, if on bed- rook, should be surrounded by a triangle or circle cut in the rock, which will make it possible to recover the station approximately if the metal disk should be removed through cupidity. Wooden -stakes will be acceptable as marks only where the station is on shifting sand dunes, where a concrete or stone mark could nor remain undisturbed.

78. When the triaugulation is on a narrow river or other body of water where it is difficult to identify topographic features, it is advisable to stamp numbers, by means of dies, on the metal disk station marks. The numbering of the stations in any one region need have no relation to the numbering of stations in oilier places. The number placed on the mark should be entered in the description of the station, but in no case should a station be designated by number only. Eacli station should be given si name to facilitate the filing and indexing of its position and de- scription in 'the office records. It is desirable to stamp the year in which the station is established on all metal disks.

44 GENERAL INSTRUCTIONS FOR FIELD WORK.

79. See paragraph 88 under the heading " Standard notes i\>r use in descriptions of stations."

80. Reference marks. — Two permanent reference marks and sev- eral witness marks should be established at each marked station. These marks should be referred to the station by theodolite angles and tape distances. The object of the reference marks is to serve in place of the station mark, if the latter should be de- stroyed, and also to serve as an aid in ihe recovery of the station mark. The reference marks should, therefore, be placed in secure positions, if practicable, and the directions and horizontal (not inclined) distances from the station to them should be measured with groat care. If the station is on a .shore which is liable to erosion, the reference marks should be placed some distancv. more than 20 meters, inland. If the station is in a field, the reference marks should be placed in a fence line, on the edj.v »>i' a road, or in some other place Avhere they are not likely ; disturbed. In each reference block of concrete or rock should be placed a standard metal reference mark which is similar to the station mark, except that it has an arrow in its center instead of a triangle, and contains the words " reference mark " instead of " triangulation station."

81. Where the shore and the area back of it are marshy, the station and reference marks may be made as follows : Drive a piece of scantling (preferably one 4 inches square) into the marsh and let its top project about 12 inches. Set over this projecting end a drain tile, the bottom of which is made to extend about 0 inches below the surface of the marsh. Surround the bottom of the tile and fill it with concrete, and set one of the standard marks in the top of the tile. As the wood is pr« >- tected from the air it is believed that it will last many years-. The fact that the mark projects from 1 to 2 feet will -aid in its recovery.

82. Witness marks. — These are used primarily to recover fhe general locality of a station and also to find the station mark. whether surface or underground. They may, therefore, be indefi- nite, such as the mouth of n creek, a fence corner, corner of a barn or other building, triangles cut into the bark of trees, mounds of earth, etc. In general, the distances from the station, us measured over the surface of the ground, are sufficient in the descriptions rather than the horizontal distance. The distances and directions need be only moderately accurate, but they should be free from gross errors. The objects used as witness marks

TERTIARY TRIANGULATION. 45

should be as widely separated as practicable to insure greater permanency of at least part of them. When close together the same cause may destroy all of them. On prairie or other uncul- tivated land where there are no objects available for witness marks one or more such marks should be constructed. A very permanent and satisfactory mark is a mound formed by digging ji circular trench. s;iy 10 feet in diameter, and throwing the dirt from the trench to the center of the circle formed by it. The trench and mound will probably soon become sodded over and will be easily found until the land is plowed (possibly even after that time). Blazed trees are frequently used as witness marks. Tlie.se are to be considered as only temporary marks. A good witness mark is a standard metal reference mark set firmly into a hole bored into a tree at the center of a triangular blaze. When the station is- in woods it is advisable to have a witness mark on the road which runs nearest to the station, and preferably the mark should be at the point on the road from which the path or trail leads to the station. It is also desirable to have a witness mark established in a conspicuous place on the seashore in certain cases opposite triangulation stations which would otherwise be difficult to recover. The witness mark should, if practicable, be a concrete mark with the standard reference mark set into its top. The arrow should point in the general direction of the station. The description should give the approximate bearing and distance between the witness mark and the station. See paragraph 88 under the heading " Standard notes for use in de- scriptions of stations."

83. If a recovered station is marked badly or in any other man- ner than with a standard metal mark, the re-marking should provide one of these marks which can be set into a block of con- crete or solid rock. Where it seems inadvisable to disturb th<- old center mark a standard reference mark should be established near the old mark. The distance between the two marks' need be only a few feet. With a standard reference mark near by, the station can be more easily recovered and it is less likely to be wantonly destroyed.

84. Re-marking stations of the Engineer Corps, IT. S. Army. — Wherever a station, only temporarily marked, of the Engineer Corps is connected with, it should be re-marked in a permanent manner. An especially inscribed disk mark for engineers' sta- tions, which is furnished by the offk-o, should he set into the concrete or stone used to mark the station, and reference and witness marks should be established. (See pars. 81 to 82.)

46 GENERAL INSTRUCTIONS FOR FIELD WORK.

85. Descriptions of old stations. — If the existing description of an old station is not exact as to the present topography around the station or as to the marking, a new description should he prepared and made complete in itself. The new descriptions of the old sta- tions should be made on Form 526, called recovery note, triaugula- tion station (see sample on p. 51), and should be written with a typewriter if possible. One of these recovery notes should be filled out for every station visited whether the station was re- covered or not. Do not report a station as lost unless a very thorough search has been made. In case the station mark found differs from the mark given in the description furnished by this office, full details should be stated in the recovery note.

86. Description of stations. — Descriptions shall be furnished of all marked stations. For each station which is in itself a murk, as are lighthouses, church spires, cupolas, towers, large chimneys, objects valuable for future hydrographic signals, sharp peaks, etc., either a description must be furnished or the records, list of direc- tions, and lists of positions must be made to show clearly in con- nection with each point by special words or phrases, if necessary, the exact point of the structure or object to which the horizontal and vertical measures refer. Every land section corner connected with the triangulation must be fully described. The purpose of the description is to enable one who is unfamiliar with the locality to find the exact point determined as the station and to know posi- tively that he has found it. Nothing should be put into the de- scription that does not serve this purpose. A sketch is not neces- sary, for the description can express in words tlie essential , facts which would be shown on the sketch. There is no objection to a sketch being made, but the written description must be complete without it.

87. Only one copy of the descriptions of stations need be sent to the office but these descriptions must be on form 525 (see sample description on p. 50) and should be written on the typewriter if possible. The essential information which should be contained in a description is as follows :

Locality (general and particular).

How marked.

Distances (by tape) and directions (by theodolite) from center of station to reference and witness marks; and, if necessary for the recovery of the station, directions or magnetic bearings to prominent objects in the vicinity, tangents to points, islands, etc. In any set of directions to prominent objects and to reference and

TERTIARY TRIAXGULATION. 47

witness marks, a main scheme or intersection station .should be used as the initial.

Note* describing the marks which are given in paragraphs 92 to 1)5 may be referred to by number, thus lessening the amount of work necessary in writing the descriptions. Wherever an excep- tional mode of marking is used, the marks must be described in full. The notes in paragraphs 92 to 95 cover the various methods of marking now in general use by parties <>f this Survey.

The height above the station mark of the top of the signal pole, and of any other part of the signal likely to be used in observing vertical angles, should be measured in meters and centimeters and so stated in the " Description of stations.''

88. Standard notes for use in descriptions of stations. — The fol- lowing notes on the marking of stations are made as general a> possible in order that it may not be necessary in the field to de- scribe small and unimportant variations. For instance, no dimen- sions are given for the different concrete blocks and bowlders, but it is understood that they will have a volume of at least 1 or 2 cubic feet as a general rule. Although it is not stated in the notes, the surface and reference marks should project a little above the ground, say from 1 to 6 inches. In the notes regarding the under- ground marks the distance of the mark below the surface of tin- ground is stated as 3 feet. This probably represents closely the average value. Unless the variation from this value exceeds G inches it need not be stated in the description. Whenever a type of marking is used which is not covered by these notes, the marks used should be described in the record.

89. The standard triangubiiion disk station mark referred to in the following notes consists of a disk and shank made of brass and cast in one piece. The disk is DO millimeters in diameter, with a small hole at the center surrounded by a 20-millimeter equilateral triangle, and has the following inscription : " U. S. Coast and Geodetic Survey triangulation station. For informa- tion write to Superintendent, Washington, D. C. $250 fine or im- prisonment for disturbing this mark." On marks established after the year 1920 the word Director Aviil supersede the word Super- intendent. The shank is 25 millimeters in diameter and 80 milli- meters long, with a slit at the lower end into which a wedge is in- serted, so that when it is driven into a drill hole in the rock it will bulge at the bottom and hold the mark securely in place. (See par. 91.)

90. The authority for the warning concerning punishment for disturbing the mark is contained In an act of Congress, approved

48 GENERAL, INSTRUCTIONS FOR FIELD WORK.

March 4, 1909, entitled "An act to codify, revise, and amend the penal laws of the United States," and reads as follows: "Who- ever * * * shall willfully deface, change, or remove any monument <>r bench mark of any Government survey shall he fined not more than $250, or imprisoned not more than six months, or hotli." 35 Statute 1088, section 57. Many States have also en- acted additional laws, among them being California. Connecticut, Georgia, Illinois, Indiana, Maine, Maryland, Massachusetts. Min- nesota, Michigan, Missouri, Mississippi, New Hampshire, New Jer- sey, Ohio, Oregon, South Carolina, Tennessee, Vermont, Virginia. and West Virginia.

91. The standard disk reference mark referred to in the follow- ing notes is similar to the standard disk triangulation-station mark described above, except that the center of the disk is in- scribed with an arrow instead of with the triangle and that the words "reference mark" replace the words " triangulation station" in the inscription. A short perpendicular groove across the shank of the arrow indicates the point to which the measurements are made. The mark is set so that the arrow points toward the sta- tion. Botli station and reference marks should have stamped upon the top, by means of steel dies, the name of the station and the year of its original location.

92. Surf ace- station marks. —

.Vote 1. — A standard disk station uiark set in the top of (a) a square block or post of concrete, (1>) a concrete cylinder. (<•) an irregular mass of concrete.

Note 2. — A standard disk station mark wedged in a drill hole in outcropping bedrock (a) and surrounded by a triangle chiseled in the rock, (b) and surrounded by a circle chiseled in the rock, (c) at the intersection of two lines chiseled in the rock.

Note 8. — A standard disk station mark set in concrete in a de- pression in outcropping bedrock.

Xote 4. — A standard disk station mark wedged in a drill hole in a bowlder.

Note 5. — A .standard disk station mark set in concrete in a de- pression in a bowlder.

Note 6. — A standard disk station mark set in concrete at the center of the top of a tile (a) which is embedded in the ground, (&) which is surrounded by a mass of concrete. (r-> which is fastened by means of concrete to the upper end of a long wooden pile driven into the marsh, (d) which is set in a block of concrete and projects from 12 to 20 inches above the block.

Special Publication No. 26. (2d ed.)

FIG. 15.— Standard station and reference marks used in marking triangulation

stations.

TERTIARY TRIANGULATION. 49

93. Underground-station marks. —

Note 7. — A block of concrete 3 feet below the ground containing at the center of its upper surface (a) a standard disk station mark, (&) a copper bolt projecting slightly above the concrete, (c) an iron nail with the point projecting above the concrete, (d) a glass bottle with the neck projecting a little above the concrete, (e) an earthenware jug with the mouth projecting a little above the concrete.

Note 8. — In bedrock, (a) a standard disk station and mark wedged in a drill hole, (&) a standard disk station mark set in concrete in a depression, (c) a copper bolt set in cement in a drill hole or depression, (d) an iron spike set point up in cement in a drill hole or depression.

Note 9. — In a bowlder 3 feet below the ground, (a) a standard disk station mark wedged in a drill hole, (6) a standard disk station mark set in concrete in a depression, (c) a copper bolt set with cement in a drill hole or depression, (d) an iron spike set with cement in a drill hole or depression.

Note 10. — Embedded in earth 3 feet below the surface of the ground, (a) a bottle in an upright position, (&) an earthenware jug in an upright position, (c) a brick in a horizontal position with a drill hole in its upper surface.

94. Reference marks. —

Note 11. — A standard disk reference mark with the arrow point- ing toward the station set at the center of the top of, (a) a square block or post of concrete, (6) a concrete cylinder, (c) »n irregular mass of concrete.

Note 12. — A standard disk reference mark with the arrow point- ing toward the station, (a) wedged in a drill hole in outcropping bedrock, (&) set in concrete in a depression in outcropping bed- rock, (c) wedged in a drill hole in a bowlder, (d) set in concrete in a depression in a bowlder.

Note 13. — A standard disk reference mark with the arrow point- ing toward the station set in concrete at the center of the top of a tile, (a) which is embedded in the ground, (&) which is sur- rounded by a mass of concrete, (c) which is fastened by means 01' concrete to the upper end of a long wooden pile driven into the marsh, (d) which is set in a block of concrete and projects from 12 to 20 inches above the block.

95. Witness marks.-

Note l'i. — A conical mound of earth surrounded by a circular trench.

13027°— U1 — -4

50

GENERAL INSTRUCTIONS FOR FIELD AVOBK.

Note 15, — A tree marked with, (a) -<\ triangular blaze with a nail at the center and each apex of the triangle, (6) a sqmm- blaze with a nail at the center and each corner of the square, (c) a blaze with a standard disk reference mark set at its center into the tree.

96. Sample descriptions. —

DESCRIPTION OF TRIANGULATION STATION.

DEPARTMENT OF COMMERCE, rj. a. COAS* AXD GEODETIC SURVEY. Form 525.

Name of station: Lopena. Chief of party: E. II. Pagenhart.

State: Tutu. Year: 191S.

County: JFtBocy. Locality: Laguna MaAre.

Surface-station mark, Note, Ib. Underground- station

mark, Note, 7c.

Reference mark, Note, lib.

Reference mark, Note,

Witness mark, Xote,

Witness mark, Note,

Height of signal above station

mark, / meter. Height of telescope above station

mark, / 1/3 meters.

Distances and directions to reference marks and prominent objects.

Object.

Distance.

Direction.

Azi- muth.

• i •'

A. coca

0 00

138 S6

247 85 361 41

Windmill (north- ern one of two). Windmill Reference mark

ImUe

318 mile 81. 63 meters .

Detailed description:

Near the southwestern end of a high sandy ridge partly covered with grots, on a large island known as Lopena. Island on the western side of Laguna Madrt. The ridge is near the western side of the island and about midway of tin length north and south. The station in in range with the left tangent of the. southernmost one of a group of four islands about 4 miles distant in a south-southwest direction and the left tangent of an island vest of the southern end of Lopena Island.

Described by E. H. Pagenhart. Marked by O. D. Cowi*.

NOTE.— The initial direction must be to a main scheme station.

TERTIARY TR1ANGULATION.

51

DESCRIPTION OF TRIANGULATION STATION. DEPARTMENT OF COMMERCE, 17. S. COAST AKD GEODETIC SURVEY.

Form 52.5.

Name of station: Swan Point 3. State: Maryland. Chief of party: C. C. Yalen. Year: 1909.

County: Queen Annes. Locality: Chesapeake Bay.

Surface-station mark, Note, la. Underground-station

mark, Note, 7d.

Reference mark, Note, lla. Reference mark, Note, Witness mark, Note,

Witness mark, Note,

Height of signal above station

mark, 2 meters. Height of telescope above station

mark, gt, meters.

Distances and directions to reference marks and prominent objects.

Object.

Distance.

Direction.

Azi- muth.

Love Point Light.

0 00

m 54 Z64 or

287 02 867 OH

m 12

SOS 49

Chimney of cabin . .

Gable of Rockhall wltarfhov.se. Reference mark, a copper bolt in a block of concrete. Reference mark (noti: I1(i). Chimney of house to riyht of Wind- mill Point. Gable of barn

S3 meters ±, Imile

../'.{ • meters..

ISM meter*.. A miles

Detailed description:

On Swan Point, a sand ami marsh point on the eastern shore of Chesapeake Bay, about 5\ miles south-southwest of Tolchexter Bench Wharf and 7 miles north of Love Point. The station is abou t %%!> meters from tht extremity of the point, about /-J meters back from the shore line, and about 65 meters southwest of a fisherman's cabin.

Described by C. C. Yates. Marked by J. J. Phelan.

NOTK.— The initial direction must be to a main scheme station.

RECOVERY NOTE, TRIANGQLATION STATION.

R

DEPARTMENT OF COMMERCE, U. S. COAST AND GEODETIC SURVEY. Form 52C.

Name of station: \firiinoille BtMe. State: California. County: Butter. Established by: W. Kimbeck. Year: 1870. Locality: Sacramento Valley. Recovered by: A. F. Rodgen. Year: 1904. Detailed statement as to the fitness of the original description:

About 16 miles went of MaryHville, on the soutlteastern summit of the south butte of the Warysville Buttes, about 6 meters northeast of the highest part of the summit, and near the steep cliff withe northern side. The station was marked originally by a copper bolt set in a drill hole in, a depression in outcropping bedrock. When recovered in ISOftkU mark teas found in good condition. As an additional m.ark the depression was filled with concrete in which o, standard disk shition mark was placed directly above the copper bott.

Distances and directions at stations. '

Distance.

Direction.

^tount Helena

0 00

Reference mark No. 1 ('note ISa)

meters..

2.916

5$ 04

Reference mark No. % (copper bolt set in solid rock) Reference mark No.S(sa,rrteas No. 2)

do.... do....

S-470 2.635

196 4S 315 OS

NOTE. — One of these forms must be used for every station recovered.

52 GENERAL INSTRUCTIONS FOR FIELt) WORK.

97. Land section corners and other survey marks. — Whenever it is feasible to do so without incurring undue expense, the section corners established by the Land Survey, and survey marks of any kind found upon the ground, including township, county. State, and international boundary monuments, shall be con- nected with the trangulation, either by direct measurement of a distance and direction from a triangulation station or by treat- ing them ns intersection stations.

98. It will insure the permanence of a -station if it is related by direct measures or otherwise to neighboring cadastral features, and a station located close to a line fence; is less liable to dis- turbance than one situated out in an open field.

99. Poor seeing. — Observations either in the main scheme or on intersection stations in triangulation of this class may be taken under any atmospheric conditions when the object to be pointed upon is visible and there should be no delay to secure good seeing before observing. If the seeing is very poor, it may be necessary to increase the number of observations on angles in the main scheme in order to secure the required accuracy. The decision in regard to the necessity of each increase should be based upon the triangle closures which are secured with such poor seeing rather than upon the appearance of the signals or even upon the range of the observations.

100. Field computations. — The field computations for the main scheme and supplementary stations are to be carried to even seconds in the angles and azimuths, to hundredths of seconds in the latitudes and longitudes, and to five places in the logarithms. The field computations for intersection stations, and for indefi- nite objects should be carried out to a sufficient number of deci- mal places to give two uncertain figures in each result. In gen- eral it will be necessary to carry the angles to even seconds and the logarithms to five places. The computation of the horizon- tal measurements up to and including the lists of directions for all stations and objects and the computation of the triangle sides of the main scheme should be kept up as closely as possible as the work progresses, to enable the observer to know that the ob- servations are of the required degree of accuracy and complete- ness. No least square adjustments are to be made in the field. All of the computation (taking of means, etc.) which is made in the record books and in the lists of directions, should be s<» thoroughly checked by some person, other than the one by whom it was originally done, as to render an examination in the office unnecessary. If there is no out: in the party besides the observer

TEUTIAKY TKIAXGULATIOX.

53

who is competent to check the computations, then it will be ac- ceptable for the observer to do the checking, but it should not be done immediately alter the computing. The initials of the per- sons making and checking the computations in the record books and the lists of directions should be signed to the record as the computation and checking progresses. Pointings upon indefinite objects should be caivfully examined, graphically or otherwise, the objects identified, and the identification clearly indicated in the records and computations. It is important to indicate clearly what lines are to objects on which no pointing was secured from a second station, us well as to indicate by common names or symbols what lines are to the same object. This must be done as the field work pro.mvs.ses.

101. In laying out the triangle side computation, the names of iho stations should be written in the triangle in a clockwise direc- tion, and the order of triangles should be such as to give two or more results for the side to be used as a base for going ahead.

102. For each quadrilateral figure the length of the base from which it is computed should be that resulting from the computa- tion of the two strongest triangles (those used in computing Ri) in the preceding quadrilateral.

The length computed through the two weakest triangles should be used only as a check. With well- shaped figures the two values for any one line will, in general, agree within 1 part in 5000, un- l«-ss a mistake has been made.

103. In the position computa- tion on Form 27 the position of a station should be computed from the two sides, radiating from the point, of one of the stronger triangles used for carrying for- ward the lengths, and from the angles of that triangle. In any triangle C B A, figure 16, C being the new point whose position is

desired, the line from B to C is computed on the left page of the form and from A to C on the right page. With the triangle side and position computation written as above (from left to right) the angles at B and A are always, respectively, + and — , and no sketch is necessary to write up the position computation. The

FIG. 16.

GENERAL INSTRUCTIONS FOR FIELD WORK.

factors for the position computation in the latitudes from 0° to 72° are given in Special Publication No. 8. There are also given in the preface to that publication detailed directions, with sample forms, for making the position computation.

104. Where connection is made with a base the measured length is to be used in going ahead.

105. Reduction to center of observations at eccentric station. — Use Form 382. The instructions for computing the reduction to center are given on the back of the form, a copy of which is given below. A sketch showing the relative position of the center and eccentric station with directions to one or inure stations must- be entered in the record and on the list of directions.

The required reduction to center is, in seconds. <•— — — > in

which d is the distance from the eccentric station to the true sta- tion, and * is the length in meters of the line between the true stations involved, and, therefore, log s is taken directly from the computation of triangle sides, a is the direction of the distant station involved, reckoned in a clockwise direction as usual, but referred to the direction from the eccentric to the true station, or center, taken as zero. This definition of a is true for the rase in which the object pointed upon is eccentric, as well us for the case in which the instrument is eccentric.

Carry « to minutes only and all logarithms to five decimal places only. Do not in any case carry the derived reductions to more than two decimal places. There is no advantage in carrying them to more decimal places than the directions to which they are to be applied are carried on Form 24A.

The preceding paragraph fixed the maximum number of decimal places to be used. In some cases a smaller number may he used as indicated in the following table:

And A is less than value stated below in meters —

If logarithm of shortest, line con- cerned is more than —

Use logarithms to four decimal places and « to minutes.

Use logarithms to three decimal places and a to degrees.

Primary trian- gulation.

Secondary or ter- tiary tnangu- lation.

I'rtmary trian- gulation.

Secondary or ter- tiary triangn- latioii.

2.5 3.0 3.5 4.0

4.5 5.0

0.6 2 6 20

0.02 0.06 0.2 0.6 2 6

0.6 2 6 20

0.02 0.06 0.2 0.6

TERTIARY TRIANGULATIOX. 55

REDUCTIONS FO15 AN ECCENTRIC INSTRUMENT.

If the instrument is eccentric, the first column of this form should contain the names of the stations observed from that eccentric position of the instrument.

The values in the fifth column are derived by subtracting those in the fourth column from those in the third. The values in the fourth column may need to be dSrived by successive approxima- tions from the triangle side computations if the eccentric reduc- tions are large. The values in the sixth column are obtained

from those In the fifth by adding log — — — derived as indicated

plO J

in the heading of the form, if <? is expressed in meters. If <l is expressed in feet, to the other two logarithms add also 9.48402 10 .-onvert to meters. To obtain a direction as shown on Form L'J A. subtract the reduction c for the station which is the initial on Form 24 A from the reduction c for the required direction and apply the difference to the observed direction. Similarly, the correction to any angle is the difference of the reductions on this form to the two directions involved in that angle.

REDUCTIONS yOR AN ECCENTRIC OBJECT OBSERVED.

'

If the object observed is eccentric, the heading " Eccentric

Station " should be changed to " Eccentric Observed Object

at Station ," the first column should contain the names of

the stations from which this eccentric object was observed, and in each case a is the direction from the eccentric object to the distant station involved, reckoned in a clockwise direction as usual, but referred to the direction from the eccentric object to the true station, or center, taken as zero. (No distinction need be made between the direction from the eccentric object to the distant station and the direction from the true station to the distant station except when the eccentric reduction is more than one minute.) The remainder of the computation on this form is made in the manner indicated above with reference to an eccen- tric Instrument. The reductions to directions are, however, to be applied to observed directions, at the stations named in the first column, to the eccentric object at the station named in the heading. The directions to which these reductions are to be applied are therefore found in various of .the lists of directions on Form 24 A. not all in one list as is the case when the instru- ment is eccentric.

56 GENERAL, INSTRUCTIONS FOR FIELD WORK.

Compare the following example with that given on Form 24. \.

BEDUCTION TO CENTER.

Eccentric Station: Chase. Log d = 1. 04088 Cologsinl" = 5.31443

d= 10. 987 meters. Sum 6.35531

- • —

Stations.

«.

Log sin a.

Log*.

Logfi^f.

Loga- rithms of reduc- tion in seconds.

Reduc- tion

=c.

Center Central

a

0 00 224 27 242 47 249 02 179 IS

-

9.84528 9.94904 9.97025 8.08696

4.40254 4. 51928 4.30616 4.49198

5. 44274 5. 42976 5.66409 3. 59498

1.79805 1.78507 2.01940 9.95029

- 62.81 - 60.96 -104.57

+ 0. 89

Little River..

Lyons, salt works .

Bossing

106. Spherical excess. — The spherical excess which is propor- tional to the area of the triangle becomes appreciable only when the sides are from 4 to 5 miles in length. One-third of the com- puted excess is deducted from each angle of the triangle, and the difference between the sum of the resulting angles of the triangle and 180° is the error of closure to be distributed. The formula for the spherical excess E is

E= mab sin C.

in which a, b are the triangle sides and C the included angle. The values of m are tabulated for every 30' of latitude, and are printed in Special Publication No. 8, page 16. A condensed table of log m to four decimal places for every 5° of latitude is given below. A rough approximation of the spherical excess of a triangle in sec- onds is obtained by multiplying its area in square miles by 1J and pointing off two decimal places.

107. Condensed table of log m. —

Lati- tude.

Log m.

Lati- tude.

Log m.

Lati- tude.

Logm.

0

1.4070

25

1.4059

50

1.4035

5

69

30

55

55

30

10

68

35

50

60

25

15

66

40

45

65

21

20

63

45

40

70

17

TERTIARY TRIAXGULATION.

57

CO

I

O

CM

o

108. Mathematical solution of the three-point problem. — If three points, forming a triangle of which the sides and angles are known or can be computed, be visible from a fourth point P, it is required to determine the position of P.

Set up the theodolite at P and measure the two angles subtended by any two of the given sides.

This problem is of use in cases where the regular tri- >angulation having been com- pleted, additional points are required for the topographic survey or are needed for special use. The angles should he carefully measured and in the computations the loga- rithms should be carried to the same number of places of decimals as in the regular tri- angle side computation.

Three cases of its applica- tion are given, depending upon the location of the point P with reference to the sides of the triangle. If P falls upon the prolongation of a side of the triangle the case resolves itself into the solution of a triangle with a side and all the angles given, while if P is situated on the circumference of the circle passing through the vertices of the triangle the problem is in- determinate.

Given the sides, a, 6, c, and the angle A.

Angles observed, APC—P' APB=P".

To find, ABP=x and ACP=y.

In cases I and II, let S=180°— i < A+P'+P") =i

In case III, S=J (A— P'— P")=i (a>+v).

c sin P'

O

Let tan Z=

b sin P"

58 GENERAL INSTRUCTIONS FOR FIELD \STORK.

then.

6 = 4

tan e=cot O+45° ) tan 8.

If tan g be positive, x=S+s., y=S — g-

If tan £ be negative, x=S — g, y=S+s..

Since all the angles and a side in each triangle are now known, the other sides, or the distances from J' to the three given points. can be readily computed.

The results are verified when both triangles -rive the same value for the line 7M.

109. Triangiilation records.— Do not duplicate volumes of hori- zontal angles. Do not make an abstract of angles. Make a com- plete list of directions on Form 24A, in accordance with the in- structions on the back of that form. The local adjustment cor- rections (to close horizon only) are to be written in the "Hori- zontal angle record," and the " List of directions " is to be made from that record directly.

110. Base, azimuth, and vertical angle observations may be recorded in " Horizontal angle record." Base and azimuth ob- sc rv.itions are to be duplicated -m computing paper and attached u> " List of directions."

111. All records of observations should contain an alphabetic index of stations occupied. When stations are occupied more than once, each record should have a cross reference to page and volume. Records should also contain a preface giving briefly number, make, and size of instrument, with direction and manner of graduation and method of observation, and any information necessary to a complete understanding of the record.

ELEVATIONS BY VERTICAL ANGLES.

[The instructions under this head apply to both secondary and tertiary triangulation.]

112. Scheme of observations. — In connection with secondary and tertiary triangulation a complete scheme of vertical angle ob- servations should be carried out, except in the cases stated below. This complete scheme should consist of a continuous series of ver- tical angle measures through the main scheme of the triangulation, observing each line over which horizontal angles are observed (the observations over each line to be made in both directions if both ends of the line are occupied), and should also include observa- tions of vertical angles upon all supplementary and intersection stations corresponding to the horizontal angles measured upon

ELEVATIONS BY VERTICAL ANGLES. 59

such stations. Connections should be made with elevations ac- curately determined by precise leveling, wye leveling, or tidal observations as frequently as possible. When a triangulation is carried along a coast or tidal stream, at each station near the shore where it is convenient to do so. a connection should be made with mean sea level and the connection recorded. Tide gauge marks should be included where available, but otherwise an ob- served vertical angle to the water's edge and an approximate dis- tance i<> the ]>oint sighted upon, with a note as to height of tide or the rime, may serve the purpose. If the plane-table topog- niphy tixe* the elevation in the region covered by the triaugu- lalion, that part of the observations of -vertical angles upon sup- plementary and intersection stations which would merely furnish reiVierminatious of elevations fixed by the topographic survey may be omitted, but the observations of vertical angles in the main scheme and upon supplementary and intersection points beyond the limits of the plane-table surveys should be made. If the scheme of triangulatiou is along » coast or river, with 110 high ground visible from the stations, and if at each station it is not difficult to make a sea-level connection, then a series of vertical nugles need not be carried through the main scheme. In this case only such vertical angles should be observed as are necessary to determine the elevations of the highest points of aids to naviga- tion, such as lighthouses,, standpipes, stacks, etc. If the eleva- tions of such aids to navigation throughout the scheme are known to be fixed previously in elevation, no observations whatever need be made. This condition will probably seldom occur, and then only on the Atlantic and Gulf coasts of the United States.

113. Plane of reference. — All heights will be referred to mean sea level.

114. Method of observation. — In the main triangulation scheme, two measures, each consisting of one pointing with the telescope in the direct position and one pointing with it in the reversed position, on each day of occupation is sufficient. For observations on intersection stations and indefinite objects two measures, each consisting of one pointing in each of the two positions of the tele- scojje, on only one day are all that are necessary. Such observa- tions should be taken on each intersection station or indefinite object from all the stations from which horizontal angles are measured to that station or object.

115. As far as practicable, the observations for vertical angles should be made as near the middle of the day as possible, and in any event not before 10 o'clock in the morning nor later than one

60 <;K. \ERAL -INSTRUCTIONS FOR FIELD WOHK.

hour before sundown. Early morning ami lute al'ternoou observa- tions are of lower accuracy, owing to rapidly changing vertical refraction.

116. An essential for the accurate measurement of vertical angles, whether in triangulation or for astronomic positions, is that the vertical axis be truly vertical or that the effect of errors of vertically be eliminated by the method of observation. Tun instrument should of course be leveled and placed in adjustment before beginning observations, but thereafter error in vertically of axis will be eliminated by the following system of observing for each elevation to be determined :

117. Directions for observations. — (a.) Point en object, bring horizontal thread to position by telescope-clamp slow-motion screw; (6) bring to the center of the vial the bubble attached to the verniers of the vertical circle by means of the vernier slow- motion screw; (c) read both verniers; (d) turn the instrument 180° in azimuth and transit the telescope. Repeat (a), (&), and (c) in same order. Do not change the relation between the axis of the bubble and the line joining the zeros of the verniers between the two pointings of a set. For all important objects determina- tion of elevation should be obtained from at least three station??.

118. If the instrument used is u theodolite of 7-inch circle or smaller, it is usually best to bring the bubble to the center of the scale for each pointing in order to avoid having level corrections. With the larger instruments, especially designed for trigonometric leveling, the bubble is very sensitive, and it will be found bpsr to make level readings for any position the bubble may be on The scale rather than attempt to center the bubble.. If the vertical circle is fastened rigidly to the telescope of the theodolite, the bubble may be brought to the center by means of the foot screws (before making the pointing) or by the slow-motion screw of the frame supporting the bubble and verniers after making the second pointing. If the instrument is one by which the vertical angles are measured by the method of repetitions, then the bubble must be brought to the center or on the scale, for the second pointing, by the foot screws only.

119. Record. — Observations should be recorded in the u-siut! Double Zenith Distance records except in work where very few stations are occupied for vertical angles, in which case they may be recorded in the Horizontal Angle record book and listed in the table of contents. The actual circle readings are always to be recorded.

ELEVATIONS BY VERTICAL ANGLES. 61

120. The D. Z. D. record is arranged for use with repeating ver- tical circle ; for observations as here proposed the columns headed •' Rep's of DZD," " Level," " C," and " D " may be left blank.

121. In every case the record must show clearly the height of the instrument in meters and centimeters above the surface mark at the station occupied and the exact point observed on at each distant signal with its height above the surface mark; whenever the entire signal is visible the ground should be observed and so noted. In observing objects other than signals care should b<> taken to note in the record the exact point sighted upon in each case, as, for example, for mountain peaks, " ground " or " tops of trees"; for a church, "top of dome" or "top of tower," etc. When tops of trees are observed the estimated height above ground should be noted in the record and the ground should be observed if practicable.

122. In the case of mountains and hilltops a small sketch show- ing the relation of the point determined to the outline of the sur- rounding elevations will be useful in representing the object on the chart, as well as to the -observer in identifying the object from another station.

123. Computation of elevations from observations of zenith dis- tances made in connection with tertiary triangulation. — Abstract all zenith distances on Form 29, bringing together all observations upon the same object from a given station, and taking the mean. If the observations are taken on more than one day, give the mean result for each day the same weight, regardless of whether many or few observations were made on that day.

124. In the record book and on Form 20 carry all angles to seconds only.

125. The value in the column headed " Object above station " is zero if the object pointed upon is the final mark for elevation, as, for example, the top of a chimney, top of a spire, etc.

126. Use the column headed "Reduction to line joining sta- tions" only when the observations are reciprocal — that is, arc made in both directions over the line in question. The quantity

in this column is an angle of which the value in seconds is = — p-/'

in which * is the horizontal distance between stations, t the ele- vation of the telescope above the station mark at the observer's station, and o the elevation of the object sighted on above the station mark at the distant station. This formula represents, therefore, a vertical eccentric reduction which is to be applied -as

f»2 GENERAL INSTRUCTIONS FOR riELD WORK.

a correction to the observed zenith distanre io obtain the cor- rected zenith distance. Four places in the logarithms are all ihar are necessary in computing these values.

127. If the observations are made in one direction only over a line, the above vertical eccentric reduction is not needed. Instead, the difference t — o, expressed in meters, is to be applied as a correc- tion to the computed difference of elevation, as indicated on Form 29B.

128. For reciprocal observations use Form 29A in computing differences of elevation. The lower part of the form, involving the weight p and the coefficient of refraction HI, is not used in field computations. The formula for the difference of elevation between stations 1 and 2 is :

ht-lti=* tan \ (i'=-fi) |.l H ('}.

In this formula hi is the elevation above mean sea level of station 1. which should be the station whose elevation is the more pre- cisely known : /;- is the elevation of station 2 ; x is the horizontal distance between the stations, reduced to sea level, log s being taken from the best available computation of triangle sides; f. is the mean corrected zenith distance of station 1, as observed from station 2 ; similarly, <Jj is the zenith distance of station 2 from station 1. The values of fs and j'i are to be taken from computing Form 29. A, B, and C are correction factors whose values are nearly unity and whose logarithms may be found in Tables «r. b, and c, on pages — and — . .1 is the correction factor for the ele- vation of station 1 ; its formula is

in which p is the radius of curvature of the arc between stations 1 and 2. B is the correction factor to the approximate difference of elevation, * tan J (&— fi). Its expression is :

Jt=-\+jj- tan Hfo-i-j). *p

(' is the correction factor for the distance between stations, its ex- pression being

s2

Further explanations in regard to A, B. and C will be found in connection with their respective tables.

ELEVATIONS BY VERTICAL ANGLES. 63

129. Compute through the form by horizontal lines. In the form a brace groups those quantities which, added together, give the quantity on rhc line immediately below the brace. In field computations carry the angles to seconds, the logarithms to five places of decimals, and the differences of elevation to hundredth*? of meters. In field computations the lines marked " J (ft— ft) in sees." and " log ditto" may be omitted and log tan i (ft — ft) may be taken directly from Vega's or Shortrede's tables and entered iu the line marked " T." Having found log [« tan i (ft — ft)], use it to take out log B from Table b. Add algebraically the log- arithms of A, B, and C to log [« tan * (ft— ft)] ; the sum will be log (h« — fti), /»* — hi being expressed in the same unit as s, in this case the meter, which is the unit throughout the computation. To convert meters to feet, which should be used in topographic work, multiply the number of meters by 3.28083 (log 3.28083= 0.51598).

130. For nonreciprocal observations use Form 29B in computing differences of elevation. The computation of weights provided for at the bottom of the form may lie omitted in a field computa- tion. The same rules as to the number of figures, etc., will apply here as to the computation of reciprocal observations and the braces have the same meaning as in Form 29A. The formula for difference of elevation is similar to that for reciprocal elevations, but since only one zenith distance (ft) is observed, the quantity \ (ft — ft) must be replaced by 90° 4- k— ft, the value in seconds of

k being given by the equation k= — '-^-r- r— In this equation

p SHl 1

m is the coefficient of refraction, which varies with varying at- mospheric conditions. In office computations the best available value of m will be used, but for field computations put

log (0.5— MI-) =9.63246-10

which corresponds to «i.=0.071. Log p comes from the t,able in paragraph 138. the arguments of which are the mean azimuth and mean latitude (a and 0) of the line. These quantities need not be known closer than the nearest tenth of a degree. Having found k (to the nearest second only for field computations) the formula for the difference of elevation is given by

Ji*—Jii=* tan (90° + Av- ft) [A B C].

The quantity ft is the mean observed zenith distance and comes from Form 29, as does also the quantity t — o which is to be

64

GENERAL INSTRUCTIONS FOR FIELD WORK.

applied as a correction to li-—lh as indicated in paragraph 12 1. No vertical eccentric angular reduction is to be applied to fi. This is contrary to the practice on reciprocal zenith distances.

131. In the field computations the lines marked "90°+fc — ft in sees." and "log ditto" may be omitted and log tan (90°-ffc — ft) taken directly from Vega's or Shortrede's tables and entered in the line marked "T." Log [s tan (90°+fc— ft)] is used as the argument for log B. The arguments of log .4 and log (' are hi and log s, respectively, as in the case of reciprocal observations. The logarithms of A, B, and C' added algebraically to lot,' [s tan (90°+/v— ft)] give log (tf-M.

132. Table a gives the values of log A, the correction factor for the elevation of the known station, by showing the limiting values of the elevation lh, between, which log A may be taken as 0. 1, 2. 3, etc., units of the fifth place of decimals. Log A is positive, except in the very rare case where hi corresponds to a point below mean sea level.

133. Table a.

«1

Log A, units of fifth place of decimals.

fti

IMS, A, units of fifth place of decimals.

fti

LOR A, units of fifth place of decimals.

ft,

Log A

units of fifth place of decimals.

• i

Meters.

Meters.

Meters.

Meters.

0 :

1541

3156

4770

0

11

22

33

73

Ittt

3303

4917

1

12

23

34

-20

1899

3449

5064

2

13

24

35

3fi"

1982

3596

5211

3

14

25

36

514 i

2128

3743

5357

4

15

26

37

661

2275

3S90

5504

5

16

27

38

807

2422

403*5

5651

'

6

17

2s

30

954

2569

4183

5798

7

IS

29

40

1101

I 2715

4330

6945

8

i

19

30

41

124N

2862

4477

6091

9

20

31

1394

3009

4624

10

21

32

1541

31-36

4770

. 134. Table 6 gives the values of log B, the correction factor for approximate difference of elevation, by showing the limiting values of log [5 tan ^ (&— fi)] or log [s tan (90°+fc— fi)] between which

ELEVATIONS BY VERTICAL ANGLES.

65

log li may be taken as 0, 1, 2, 3, etc., units of the fifth place of decimals. Log B has the same sign as the angle % (fr—fr) or 90°-f-fc— ft; for example, if log \s tan \ (ft— ft)] lies between 3.565 and 3.598 and \ (fr— ?,) is positive, log #= +0.00013. but if i (ft— i'i) is negative then log B= —0.00013, i. e., 9.99987—10, the former way of writing being usually more convenient in practice. 135. Table b.

Lo,; [s tan

[-11: i • tun

Log [« tan

'or log (s tan (90*+

Log B,

units of fifth place ofdecimals.

— i"i)J or lo£ [* tan (90°+ fc-ri)]. (s

units of fifth place ofitocitnals.

i (h-n)J or lee [« tan (90°+ k-fi). (*

LogJ?, units of fifth place ofdecimals.

in meters.)

in meters.)

in meters).

— oo

0

2.167

3.397

3.685

1

9

17

2.644

3.445

3.711

2

10

18

2.886

3.489

3.735

3

11

19

3.011

8. 528

3.7*

4

12

20

3.121

3.565

8.779

5

•

13

21

3.20B

3.598

3.800

6

14

22

3.281

3.629

3.820

7

15

23

:i :ti:>

3.658

8.839

8

M

24

;{. 3(17

::. eK

3.867

136. Table c gives the value of log C, the correction factor for tJistaiwv between stations, by showing the limiting values of log * between which log C may be taken as 0, 1, 2, 3, etc., units of the fifth place of decimals. Log C is always positive.

137. Table c.

Log C,

LogC,

Log « (s in meters).

units of fifth place ofderamals.

Log s (* in meters).

units of fifth place ofdecimals.

0.000

6. aw

0

4

4.875

5.352

1

5

5.113

5.395

2

6

5.224

5.432

3

7

5.297

5.463

13027°— 21-

GENERAL, INSTRUCTIONS FOR FIELD WORK.

138. Table of logarithms of radii of curvature of the earth's surface in meters for various latitudes and azimuths, based upon Clarke's ellipsoid of rotation (1866).

i

Azimuth.

0° lat.

1° lat.

2° lat.

3° lat.

4° lat.

5° lat.

6° lat.

Meridian.

6. 80175

6. 80175

6. 80175

6. 80176

6. 80177

6. 80178

6.80180

5

177

177

178

178

179

180

182

10

184

184

184

185

186

187

188

15

195

195

195

196

197

198

199

20

209

209

210

210

211

212

214

25

227

228

228

228

229

230

232

30

248

249

249

250

250

251

252

35

272

272

272

273

273

274

276

40

296

297

297

297

298

299

300

45

322

322

322

323

324

324

325

50

348

348

348

348

349

350

351

55

373

373

373

373

374

374

375

60

396

396

396

396

397

398

398

65

417

417

417

418

418

418

419

70

435

435

436

436

436

437

437

75

450

450

450

450

451

451

452

80

461

461

461

461

462

462

463

85

468

468

468

468

468

469

469

90

470

470

470

470

471

471

472

Azimuth.

6° lat.

7° lat.

8° lat.

9° lat.

10° lat.

11° lat.

12° lat.

Meridian.

6.80180

6. 80181

6.80183

6.80186

6.80188

6. 80191

6.80194

5

182

184

186

188

190

193

196

10

188

190

192

194

197

200

202

15

199

201

203

205

207

210

213

20

214

215

217

219

222

224

227

25

232

233

235

237

239

242

244

30

252

254

256

257

260

262

264

35

276

277

278

280

282

284

287

40

300

301

303

304

306

308

310

45

325

326

328

329

331

333

335

50

351

352

353

354

356

358

359

55

375

376

377

379

380

382

383

60

398

399

400

401

403

404

406

65

419

420

421

422

423

424

426

70

437

438

439

440

441

442

443

75

452

452

453

454

455

456

457

SO

463

463

464

465

466

467

468

85

469

470

470

471

472

473

474

90

472

472

473

474

474

475

476

ELEVATIONS BY VERTICAL ANGLES.

67

Azimuth.

12° lat.

13° lat.

14° lat.

15° lat.

16° lat.

17° lat.

18° lat.

Meridian.

6. 80194

6. 80197

6.80201

6.80204

6.80208

6.80213

6.80217

5

196

199

203

206

210

215

219

10

202

206

209

213

217

221

225

15

213

216

219

223

227

231

235

20

227

230

233

236

240

244

248

25

244

247

250

254

257

261

265

30

264

267

270

273

276

280

284

35

287

289

292

295

298

301

305

40

310

313

315

318

321

324

327

45

335

337

339

342

344

347

350

50

359

361

364

366

368

371

373

55

383

385

387

389

391

394

396

60

406

407

409

411

413

415

417

65

426

427

429

430

432

434

436

70

443

444

446

447

449

451

453

75

457

458

460

461

463

464

466

80

468

469

470

471

473

474

476

85

474

475

476

478

479

480

482

90

476

477

478

480

481

482

484

Azimuth.

18" lat.

19° lat.

20° lat.

21° lat.

22° lat.

23° lat.

24" lat.

Meridian.

C. 80217

6.80222

6.80226

6.80232

6.80237

6.80242

6.80248

5

219

224

228

234

239

244

250

10

225

230

234

239

244

250

255

15

235

239

244

249

254

259

264

20

248

252

257

262

266

271

277

25

265

269

273

277

282

287

292

30

284

287

292

296

300

305

309

35

305

308

312

316

320

324

329

40

327

330

334

338

341

345

350

45

350

353

357

360

364

367

371

50

373

376

379

382

386

389

392

55

396

398

401

404

407

410

413

60

417

419

422

424

427

430

432

65

436

438

440

443

445

448

450

70

453

451

456

459

461

463

465

75

466

468

470

472

473

476

478

80

476

478

479

481

483

485

487

85

482

483

485

487

489

490

492

90

484

485

487

489

490

492

494

GENERAL INSTRUCTIONS FOR FIELD WORK.

Azimuth.

24° lat.

25° lat.

26° lat.

27° lat.

28° lat.

29° lat.

30" lat.

Meridian.

6.80248

6.80254

&80260

6.80266

6.80272

6.80279

6.80285

5

250

36§

262

268

274

280

287

10

255

261

267

273

279 i 283 292

15

264

270

276

282

288 294 300

20

277

282 288

293

299 30ft

311

25

292

297

302

308

313 310 325

30

309

314

31»

324

330 335 340

35

329

333

338

343

34* 353 358

40

350

354

358

362

367 372 377

45

371

375

379

333

387 391 396

50

392

396

399

403

407 411 415

55

413

416

420

423

42B 430 434

60

432

435

4:-,S

442

445 448 451

05

450

453

455

458

461 464

467

70

465

468

470

473

475 478

481

75

478

480

482

484

487 i 489

492

80

487

480

491

493

495 : 498

50C

85

492

494

496

498

501 503

505

90

494

496

498

500

502 504

507

Azimuth.

30° lat.

»l "tot.

32° lat.

33° tot.

34° tat.

35° lat,

36' lat.

• Meridian.

6.80285

ti. 80292

6.80299

6.80306

6. 803 U

6.80320

6.80327

5

287

294

300

307

314 322

329

10

292

298

305

312

319 326

333

15

300

306

313

320

326 i 333

340

5

311

317

324

330

337

343

3.^0

25

325

331

337

343

349

355

362

30

340

346

352

35g

364

370

376

35

358

363

369

374

380

385

391

40

377

382

386

392

397

402

407

45

396

400

405

110

414

419

424

50

415

419

423

428

432

436

441

55

434

437

441

415

449

453

457

60

451

455

•I/IS

462

465

469

472

65

467

470

473

476

480

483

486

70

481

4S4

486

4*9

492

495

49S

75

492

494

497

500

502

505

508

80

500

502

.->05

507

510

512

515

$5

505

T,07

510

512

514

517

519

90

507

500

511

514

516

518

521

ELEVATION S BY VERTICAL ANKLES.

69

A cimtith.

36° lat.

37' lat.

38° lat,

39' lat.

40* lat.

41* lat.

42' lat.

Meridian.

6.S0327

6.80335

6.80342

6.80350

6.80357

6.80365

6.80373

5

329

336

344

351

359

366

374

10

333

340

348

355

363

370

378

15

340

348

355

362

369

376

384

20

350

357

364

371

378

385

392

25

362

368

375

382

388

395

402

30

376

382

388

394

401

407

413

35

391

397

402

408

414

420

426

40

407

412

418

423

429

434

440

45

424

429

434

439

444

449

454

50

441

445

450

454

459

464

4GS

35

457

461

465

469

474

478

482

60

472

476

480

484

487

491

495

65

486

489

493

496

500

503

507

70

498

501

504

507

510

514

517

75

508

510

513

516

519

582

525

30

515

517

520

523

525

528

531

35

519

522

.•524

527

529

532

634

90

521

523

526

528

531

533

536

Acimnth.

42° lat.

43 Mat.

44" lat,

4.Vl:it.

46° lat.

47* lat.

48" lat.

Meridian.

6.80373

6.80380

6.80388

6.80896

6.S0404

6. 8041 1

«. 80419

5

374

382

389

397

404

412

420

10

378

385

393

400

108

415

423

15

384

391

398

406

413

420

428

30

392

399

406

413

420

427

434

35

402

408

415

422

429

436

442

30

413

420

426

433

439

446

452

35

426

432

438

444

450

456

462

40

440

446

451

457

462

468

474

45

454

459

464

470

475

480

485

50

468

473

478

482

487

492

496

55

482

486

490

495

499

503

.508

60

495

499

502

506

510

514

518

65

507

510

514

517

520

524

528

70

517

520

523

526

529

532

536

75

525

528

531

534

536

539

542

80

531

534

536

539

.542

544

:>47

35

534

537

540

542

545

548

550

90

536

538

541

544

546

540

551

70

GENERAL INSTRUCTIONS FOR FIELD WORK.

Azimuth.

48° lat.

49° lat.

50° lat.

51° lat.

52° lat.

53elat.

54° lat.

Meridian.

6. 80419

6.80426

6.80431

6.80442

6.80449

6. 80457

6.80464

5

420

428

435

443

450

458

465

10

423

430

438

445

453

460

467

15

428

435

442

450

457

464

471

20

434

441

448

455

462

469

476

25

442

449

456

463

469

476

482

30

452

458

465

471

477

484

490

35

462

468

474

480

486

492

498

40

474

479

485

490

4%

501

506

45

485

490

495

500

505

510

515

50

496

501

506

510

515

520

524

55

508

512

516

520

524

528

533

60

518

522

526

530

533

537

541

65

528

531

534

538

541

545

548

70

536

539

542

545

548

551

554

75

542

545

548

551

554

557

559

80

547

550

553

555

558

561

563

85

550

553

555

558

560

563

566

90

551

554

556

559

561

564

566

Azimuth.

54° lat.

55° lat.

56° lat.

57° lat.

58° lat.

59° lat.

60° lat.

Meridian.

6.80464

6.80471

6.80479

6.80486

6.80493

6.80500

6.80506

5

465

472

479

486

493

500

507

10

467

474

481

488

495

502

509

15

471

478

485

492

498

505

511

20

476

483

489

496

502

509

515

25

482

489

495

501

508

514

520

30

490

496

502

508

514

519

525

35

498

503

509

515

520

525

531

40

506

512

517

522

527

532

537

45

515

520

525

530

534

539

543

50

524

528

533

537

542

546

550

55

533

537

541

545

548

552

556

60

541

544

548

552

555

558

562

65

548

551

555

558

561

564

567

70

554

557

560

563

566

.569

572

75

559

.562

565

568

570

573

575

80

563

.566

568

571

573

576

578

85

566

568

570

573

575

578

580

90

566

569

571

574

576

578

580

GRAVITY.

71

'

Azimuth.

60° lat.

61° lat.

62° lat.

63° lat.

64° lat.

65° lat.

66° lat.

Meridian.

6.80506

6. 80513

6.80520

6.80526

6.80532

6.80538

6.80544

5

07

14

20

26

32

38

44

10

09

15

22

28

34

40

45

15

11

18

24

30

36

42

47

20

15

21

27

33

39

44

50

25

20

26

31

37

42

48

53

30

25

30

36

41

46

51

56

35

31

36

41

46

51

56

60

40

37

42

46

51

56

60

64

45

43

48

52

56

60

64

68

50

50

54

58

62

65

69

73

55

56

60

63

67

70

74

77

60

62

65

68

72

75

78

81

65

67

70

73

76

79

82

84

70

72

74

77

80

82

85

87

75

75

78

80

83

85

87

90

80

78

80

83

85

87

89

91

85

80

82

84

86

88

90

92

90

80

83

85

87

89

91

93

Azimuth.

66° lat.

67° lat.

68° lat.

69° lat.

70° lat.

71" lat.

72° lat.

Meridian.

6.80544

6.80550

6.80555

6.80560

6.80565

6.80570

6.80575

5

44

50

55

61

66

70

75

10

45

51

56

62

66

71

76

15

47

53

58

63

68

72

77

20

50

55

60

65

70

74

78

25

53

58

62

67

72

76

80

30

56

61

65

70

74

78

82

35

60

64

69

73

77

81

84

40

64

68

72

76

80

83

87

45

68

72

76

79

83

86

89

50

73

76

79

83

86

89

92

55

77

80

83

86

89

91

94

60

81

84

86

89

91

94

96

65

84

87

89

92

94

96

0.80598

70

87

90

92

94

96

6.80598

6.80600

75

90

92

94

96

98

6.80600

01

80

91

93

95

97

6.80599

01

02

85

92

94

96

98

6.80600

01

03

90

93

95

97

98

00

02

03

GKAVITY.

139. A publication on modern methods for measuring the In- tensity of gravity is in course of preparation. When published it will contain detailed instructions for the determination of gravity.

72 GENERAL INSTRUCTIONS FOR FIELD WORK.

TRAVERSE.

140. Instructions for precise and secondary traverse are given in Special Publication No. 58.

PRECISE LEVELING.

141. For general instructions for this work, see Special Publi- cation No. 18, entitled, " Fourth General Adjustment of the Pre- cise Level Net in the United States and the Resulting Standard Elevations " and also Special Publication No. 22, entitled, " Pre- cise Leveling from Brigham, Utah, to San Francisco, California.'*

EECOJTSfOISSANCE FOR TRIANGTJLATIOHT.

142. The reconnoissance preliminary to precise triangulation is made under special instructions suited to the work to be under- taken. On tertiary triangulation the lines are usually of com- paratively short length where the intervisibility of the points is easily tested, but for the occasional instances where such is not the case, the methods briefly indicated below may be employed.

Where the intervisibility of the points desired can not be deter- mined by trial, or where it is necessary to estimate the heights of towers which will make them intervisible when the line is obstructed elsewhere than in the immediate vicinity of one or both of the points, the map method of reconnoissance may be used. The degree of precision obtained by this method will depend upon the accuracy of the map itself, which is usually indicated by its source, date, and method of compilation, as well as by internal evidence.

The difference between the apparent and true difference in ele- vation of two points is affected by two factors, the curvature of the earth's surface and the refraction of light by the earth's at- mosphere. These factors are of opposite sign and of an approx- imately fixed relation to each other, so that the combined effect can be applied as a single factor. The effect of refraction is about one-seventh as much as the curvature; the formulas for the separate effect of each can be found in various works on geodetic surveying, but the formulas below give the approximate resultant :

h (in teet)=K* (in miles) times 0.574,

K (in miles) = Vh (in feet) times 1.32.

AZIMUTH.

73

143. Below is a table, condensed from the one given in Appendix 0, Report for 1882, which gives the distance K (in statute miles) ;it which a line from the height h (in feet) will touch the horizon, taking into account terrestrial refraction with a mean assumed coefficient of 0.070.

Correction for earth's curvature and refraction.

Dfet.

Con. Dist.

Corr.

Dfet.

Coir.

Milr*.

Feet..

Mil**.

Feel.

Mil<«.

Feet.

1

0.6

21

253.1

41

964.7

2

2.3

22

277.7

42

1012.2

3

5.2

23

303,6

tt

1061.0

4

9.2

24

330.5

44

1111. 0

5

14.4

25

358.0

45

1162.0

6

20.6

26

388.0

W

1214.2

7

28.1

27

418.3

47

1267.7

8

36.7

28

449.9

48

1322. 1

9

46.4

29

482.fi

49

1377. 7

10

57.4

30

516.4

50

1434. 6

11

«9.4

31

551.4

51

1492. 5

12

82.7

32

587. G

52

1551.6

13

97. 6

33

624. »

53

1611.9

14

112.5

M

•63.3

54

1673.3

15

129.1

35

703.0

55

1735.8

16

146.9

36

743.7

56

1799.6

17

165.8

37

785.fi

57

IS64.4

18

185.9

38

H28.6

58

1930.4

19

207.2

3S

872.8

59

1997.5

28

229.5

40

918. 1

W

3065.8

To determine how much tl^ line of sight between two stations will clear or fail to clear an intervening hill, either the table above may be used or the following formula employed:

-<i.r>so3<f,<f»,

where

k =height of lin« at obstruction,

fei=height of lower station,

fe2— height of intervening obstruction or hill,

**= height of higher station,

di— distance from lower station to intervening obstruction,

ds=ldistance from intervening obstruction to higher station.

AZIKTTTH.

144. General remarks. — Parties engaged in general coast surveys should make azimuth observations at one station for every 20 or

74 GENERAL INSTRUCTIONS FOR FIELD WORK.

30 figures of the triangulation. The azimuth may be measured at .any convenient station of the triangulation, but preferably at some station at which the deflection of the plumb line is not large in the prime-vertical, therefore avoiding, if possible, points having near-by mountain masses to the east or wesi.

Observation and computation of azimuth. — For examples of observations with the repeating theodolite and also with the direction theodolite, see Special Publication No. 14, entitled, " De- termination of Time, Longitude, Latitude, and Azimuth " (5th edition).

Observations on the sun for azimuth. — It is occasionally desir- able in reconnoissance for triangulation or in magnetic determina- tions to have an approximate azimuth. For methods of obtain- ing such an azimuth by observations on- the sun, see Principal Facts of the Earth's Magnetism.

TOPOGRAPHY.

145. Use of the plane table. — Full details regarding topographic- surveys with the plane table will be found in A Plane Table Manual, Appendix No. 7, Coast and Geodetic Survey Report for 1905, which may be obtained bound separately.

146. Control of topography. — The most satisfactory way of mak- ing detailed topographic surveys is to first complete and compute a systematic triangulation, and plot the points determined on a projection. This will not always be feasible in charting new regions, on account of both time and expense, and the topography will sometimes liave to be executed at the same time as the tri- angulation.

147. Where topography is carried on simultaneously with tri- angulation and other work, if practicable, the triangulation will be kept sufficiently in advance so that the distances (not necessarily the geographic positions) may be computed and plotted on the sheet before filling in the topography. In all cases where this will cause too great a delay or is not practicable from other causes, the topographer must check the distances on his sheet by the computed distances as soon as they are available, and where there are important discrepancies must correct the error by examining the portion affected.

Upon combined operations in Alaska the specifications for the standard control of topography in Alaska shall be as follows :

In general, main-scheme triangulation stations for control of hydrographic and topographic work should be distributed along

TOPOGRAPHY. 75

the coast at intervals not greater than about 5 miles. This tri- angulation should be of the tertiary grade. To supplement the main scheme, intersection stations of the triangulation, or stations located by plane-table triangulation, by transit and tape, or transit and stadia, should be distributed at intervals not greater than about 2 miles. In localities where triangulation is impracticable traverse with transit and tape may be used for control, provided that the accuracy of the traverse is equal to that of tertiary tri- angulation.

When the details of improvements along the water front, such as docks, prominent buildings, etc., are to be located, a control point should be established at a distance not greater than 500 meters from such improvements.

Control stations shall be marked and described in accordance with paragraphs 76-96. All triangulation signals must be cut in with the plane table and shown on the topographic sheet ; those falling off the limits should have direction lines drawn on the sheet. All traverse lines run must, if practicable, be checked by closing circuits, and small errors adjusted ; if large errors appear, the lines must be rerun. On a 1-20000 scale the closing error shoiild not exceed 8 meters per mile of traverse, and an error of half that amount will usually be obtainable.

Unless otherwise specified, the standard of accuracy for the location of shore line in Alaska shall be equal to that for the Atlantic and Pacific coasts of the United States.

The position of any part of the well-defined and permanent shore line shall not be in error more than 10 meters, where the shore line is less than 1,000 meters from a triangulation or other con- trol station ; not more than 20 meters for distances between 1,000 and 4,000 meters from such a station; and not more than one- half of 1 per cent of the distance from a control station where the distance is greater than 4,000 meters from such a station.

The descriptive report for each sheet must give the closing errors of the traverses run and state how and between what points (ho discrepancy was distributed. If proper care is exercised, it will be possible to fit a projection to the plane table sheet by the triangulation points, so that there will be little or no error in the result ing chart

148. The magnetic meridian should be drawn on the sheet in the field from at least one point, by means of the declinatoire. In regions of large local disturbance, additional determinations of the magnetic meridian should be made.

76 GENERAL INSTRUCTIONS FOR FIELD WORK.

In order to utilize all the available force at the beginning of a season, it may be advantageous to make a plane table survey of a harbor and fill in the hydrography on this, the signals to be de- termined by triangulation later.

148. When former triangulation stations are searched for and not found, or when stations are recovered which are insufficiently marked or described, the deficiency in marking or description must be remedied and u report made to show existing conditions.

150. Scale. — For all general coast topography in new regions, unless otherwise specified, a scale of aofoa will be used. Larger scales, as 10000 (and in exceptional cases r^nny), are to be used for special harbor surveys where the amount of detail or the im- portance of the locality warrants, but smaller scales than ?*&$-» will not be used unless specially authorized.

151. Contour intervals for ordinary coast topography should be either 20, 50, or 100 feet; 40-foot intervals should not be used. The choice of intervals should depend on the nature of the coun- try and the scale of the sheet. Only one specific interval .should be used on a single sheet.

152. Contours. — Within the continental limits of the United States, the relief will be indicated by contours. In Alaska mid the Philippine Islands " form lines " instead of " contours " will be used for this purpose, except in the case of a large scale survey. The term " contour " is generally understood to mean lines of equal elevation located by a sufficient number of determined points along its course, so that in open country on slopes of o° or less m» part of it shall be out of position more than one-half the horizon- tal distance between each successive contour.

In Alaska this standard of accuracy is not practicable as it is important for the benefit of the navigator to include a large area of rugged country back from the shore, so that its general con- figuration will l>e indicated and the peaks and hilltops charted to serve as landmarks.

For this purpose the relief will be shown by " form lines," the sketching of which will be controlled by as many points (refer- ence points) of determined heights and location as can be secured from the shore as the survey proceeds along the coast.

In general, these reference points, together with the elevations of prominent summits, shall be distributed over the area so that there will be at least one reference point for every 4 square inches of field sheet, with such additional elevations as can be obtained without unduly delaying the progress of the work.

TOPOUIIAPHY. 77

Form lines developing the slopes and summits of points and headlands which may he of UM- in determining a vessel's position from seaward should closely approximate the value of contours, as they are often use* I when the immediate shore line lies below the observer's horizon.

In order that the relative value of the form lines may be known, the pc>' -it ion and height of each reference i>mnt shall be inked on the sheet.

153. Interior elevation. — Und<T the head of triangulation, pro- vision is made for determining important elevations visible from the coast and beyond the limit of the plane table sheet. Informa- tion indicating the relation of these elevations to the surrounding country is of value, especially on small-scale general charts. While it is impossible to obtain correct detailed information with- out going over the country, yet it is suggested that by plotting on a small scale (as on a pteee of a general chart) the points determined, a sketch may be made showing the trend of the ridges ami the lew areas as far as visible. This will to some extent avoid the false idea which. is given of a mountainous country by showing on the chart only detached summits.

154. The plane of reference for elevations is mean sea level and must be used unless otherwise instructed. Elevations are to be stated in feet. All elevations given either by figures or contours should represent the elevation of the ground ; when for any reason the elevation of the top of trees or vegetation is given, a note to that effect should be added, with an estimate of the height above ground in each case.

155. Elevations may be read from the hypsograph (see Appen- dix No. 4, Report for 1902), or they may be scaled from a graphic diagram. They can be obtained by using the '" Table of factors for computing differences in elevation " and " Table of corrections for curvature and refraction," pages 338 and 339 of Plane Table Manual (also printed separately).

156. Laying out sheets. — Plane table sheets should, in general, be laid out to run parallel with the coast to cover as great a length of coast line as convenient, and to include the signals necessary for its control. Sheets containing small detached fragments of topog- raphy should be avoided as far as possible; this can. sometimes be done by placing a subplaii on an adjacent sheet. Where the topography permits, sheets should preferably be laid out with the two sides parallel to the meridian.

78

GENERAL INSTRUCTIONS FOR FIELD WORK.

Iii order to improve the field sheets and field records pertaining to topographic and hydrographic surveys, it is directed that when- ever drafting facilities are available for the field parties, pro- jections shall be made in the field, thus obviating the errors due to the distortion of a projection constructed under climatic con- ditions which differ materially from that in the field.

157. Table of dimensions of standard topographic sheet, 30 inches by 52 inches, expressed in nautical and statute miles, for different scales :

Qnala

Nautical miles.

Statute miles.

Width.

Length.

Width.

Length.

TlhtTS

2.06

3.56

2.37

4.10

4.11

7.13

4.74

8.21

8.22

14.25

9.47

16.41

16.45

28.51

18.94

32.83

TTT&oOO

41.12

71.27

47.35

82.07

158. The features to be included in ordinary coast topography are the following :

159. The careful location of average high-water line and the low-water line so far as it may be determined or estimated without waiting for low tide.

160. Rivers and streams for a reasonable distance back from the coast, according to their importance ; large streams should be surveyed to the limit of the detailed topography, while small and unimportant creeks need be shown only as far as rowboats can ascend ; navigable streams should be surveyed to the head <>f tidewater or ship navigation.

161. Off-lying islets, reefs, and rocks, including elevations of all prominent rocks and islets. Off-lying reefs should be designated as bare, awash, or covered at high or low water, as the case may be.

162. Towns, settlements, roads, and important trails within a reasonable distance of the coast. The individual buildings in a town must not be shown except those of sufficient prominence to be useful as landmarks. When there is no street system and it is desired to indicate a settlement a group of small buildings can be used as a symbol.

TOPOGRAPHY. 79

163. Objects along the shore either natural or artificial that may be useful in future liydrographic work should be located with care and so named or described that they may be identified without difficulty. In coastal topography, even where the hydro- graphic survey follows closely, it is essential to mark permanently a sufficient number of points to make it unnecessary to redeter- mine them by theodolite or plane table should later hydrographic work become necessary- On a rugged, rocky coast this is easily accomplished by placing patches of cement on the rocks, having embedded therein a large nail or other object to make identifica- tion more certain. Under other topographic conditions different durable marks may be used to give to the survey a more permanent value. Descriptions of the marks and locations in duplicate must accompany the descriptive report of the sheet on which they are located.

164. The location and elevation of hills or mountains within the limits of the sheet, so far as may be obtained from the vicinity of the coast.

165. The nature of the coast line and of the low-water line, as sand, coral rock, mangrove, etc., must be indicated by symbols, and the general vegetation along the shore must be shown.

166. Features not fully surveyed, as the fast land back of the mangroves and large areas of swamp land, also the extension of a stream beyond the limits actually run, may be indicated by broken line or appropriate note.

167. Use of sextant and theodolite in topography. — While the plane table is the most valuable instrument for topography, the surveyor should not regard himself as restricted to its use. Where located signals are in sight and the shore is lined with swamp or mangrove, and in other situations presenting no suitable locations for the table, the sextant may be used to advantage in filling in topography by locating each principal feature by two or preferably three sextant angles, with additional angles to tangents of points and other objects. A continuous sketch should be made in a sketchbook, with the angles written opposite the corresponding points on the sketch. Cases may also arise where the topography may be obtained advantageously with a theodolite traverse line (the transit and stadia or chain method).

A rapid and sufficiently accurate method of mapping rivers which are comparatively unimportant yet navigable by small boats is by a modification of the stadia method, in which the distance

80 GENERAL INSTRUCTIONS FOR HELD WORK.

readings are made on a stadia rod by a plane-table alidade, sup- ported on a board nailed on top of a pole throat into the river bottom at the side of the boat, the angles being measured by a .sextant.

168. When any of these auxiliary methods are used the work .should be plotted and combined by the topographer and added to the general topographic sheet, and the descriptive report should slate what portions are so surveyed.

169. Stadia errors. — The source of tlie largest systematic errors in stadia measurements lies in tl\e different refractive power of !he air strata at the bottom of the rod as compared to thos.- ai the top. All stadia readings •within 1 meter of the ground should be avoided, especially in hot climates, as readings above this limit arc practically free from error. When necessary to use the full length of the stadia rod, attach an extension piece without any graduation.

170. Stadia rods should be carefully tested before beginning a season's work, even though it is practically certain that they were used with the same alidade and diaphragm «n the previous season, or have come direct from the Washington otBce.

171. Approximate locations.— If from any trhmgulation or i>lune- table station breakers or other indications of off-lying dangers not previously located are noticed, directions should at once be deter- mined, and also, if practicable, vertical angles. From the latter and the elevation of the instrument approximate distances may lx> computed, which will aid in identifying the objects from other sta- tions. Another method quite useful in rapidly getting approsi- mate locations of objects so as to permit of their future identifica- tion is to take cuts on them from a plane-table station and then from another station nearby. Of course such locations are to be confirmed by good intersections from other points.

172. Revision. — In revision of the shore line and adjacent areas where changes of moment have occurred it is more economical to make an entirely new outline survey rather than to select places where changes are thought probable and then to work each way from such spots to junctions with unchanged portions of the coast. Thi9 applies principally to stretches of coast where intermediate triangulation points are lacking.

When it is not deemed advisable to make an entirely new outline survey, bromide copies of the original topographic sheet will be furnished. From the bromide copy the chief of party will transfer in pencil to a plane-table sheet the data covering the area to be

TOPOGRAPHY. 8*

revised. This will permit erasing where objects shown on the bromide no longer exist or when changes have occurred since tho original survey. The new topography will then be inked and ihe sheet forwarded to the office in the usual form of a completed topographic sheet.

173. In the revision the following features heretofore shown on tin original sheets and the charts are not deemed of importance to navigation and will be disregarded, viz:

Individual buildings.

(V.Kceptions: Those of large size close to water front or a detached group of small ones along shore which would serve as a landmark ; also individual buildings back from the water front which are conspicuous and will serve as navigational aids, such as church spires, factory chimneys, water towers, etc., and the principal building of the life-saving stations. These navigational aids and life-saving stations should be well determined and listed in the descriptive report.) Woods.

iL.n-cittioHis: \Vhere they will he of navigational impor- tance, such as si conspicuous clump of trees or where the growth along shore is an exceptional and distinguishing feature.) Minor roads.

(Exceptions: Those leading up from the wat< r. All fences.

In the case of the water front of cities and large towns the details of the wharf line and adjoining streets should be carefully located and drawn, using all available accurate information. Back of this the street system will be compiled in the office from local maps obtained by the field party. Such maps should have sufficient points and an azimuth in common with the plane- table sheet to insure location and orientation. In general, the inclusion of three streets back from the water will be sufficient for the chart.

174. Plans of towns and local maps, if available, should be obtained. These must be inspected in the field and marked to distinguish between details that exist and those that are pro- jected only. They will be used in the office for filling in details, especially of towns, but not for the positions of important objects which must be determined by tbe topographer. Copies of maps of value obtained should be forwarded to the office with the topographic sheet.

175. Symbols and lettering. — The standard topographic symbols are to be followed.

13027°— 21 6

82 GENERAL INSTRUCTIONS FOR FIELD WORK.

176. The high-water line, being one of the most important fea- tures on the sheet, should be drawn with sufficient strength to make it clearly distinguishable. The use of a full line for defining the limits of vegetation outside of the high-water line or the limits between marsh and fast laud should be avoided.

177. Time need not be taken for the elaborate covering of u sheet with topographic vegetation symbols, but limits may be shown with words in the center to show the area covered. Words may be used to indicate vegetation features for which there is no special symbol.

178. The field topographic sheet is a survey record; it should show all useful information plainly, neatly, and correctly, but time that can be more usefully employed should not be expended in endeavoring to make it a handsome drawing.

179. Valuable information, useful notes, etc., should not be omitted for fear of marring the appearance of the sheet ; nor should the topographer hesitate to place the necessary informa- tion on the sheet because he is not expert at lettering.

180. In lettering topographic and hydrographic sheets, names applying to the land should be in vertical letters, the names applying to the water, including objects covered at high water, should be in slanting letters. All geographic names are to be in black ink, and names solely for surveying use, as of signals and stations, are to be in red ink.

181. Care must be taken not to confuse the symbols for sunken rocks (a simple cross), rocks awash (three lines crossing), and rocks above high tide (heavy dot or shape). Brief notes are desirable clearly indicating the nature of important reefs and rocks, as " awash at low water," " awash at high water," " coral heads bare at low water," " breakers at low water," etc.

182. The following remarks apply particularly to Philippine topographic sheets. The cocoanut palm being usually a distinctive feature on the coast, should be shown by the special symbol. Mangrove growing in the water should be limited by a very light line to preserve the detail and correct position, and yet to repre- sent it differently from the strong black line used for the high- water line. Sometimes where there is mangrove the solid shore may not be seen, and it may not be practicable to locate it. Its approximate position should be shown by broken lines sketched on the sheet. The ordinary coral-reef symbol should be used only to represent the limit of reefs bare or awash at low water, and should not be used to represent reefs covered to some depth at low tide. When not developed by the soundings the limits of

TOPOGRAPHY. 83

submerged reel's should be indicated by the sunken-rock symbol. Rice paddies may be represented conventionally by .small irregular quadrilaterals bounded by slightly irregular lines and a little grassing.

183. Inking of sheets should be done by the topographer him- self or by a member of the party under his supervision, and as soon as practicable after the field work is completed on each sheet. Intervals of delay in field operations may be utilized advantageously for this purpose. The inking of a sheet can not be considered as finished until all essential notes and names are inked.

184. Accuracy, neatness, and clearness are necessary in inking sheets ; beyond this fine drafting is not essential.

185. When Tor any reason an uuinked sheet is transmitted to the office, the greatest care must be exercised by the chief of party that every feature, fact, and name is clearly and distinctly shown. Tin- vnpographer must also make it a point to see and verify the sheet at some rime after it is inked, examining every detail.

186. li is particularly important in such case that small de- tached rocks along the shore, and other features that might be mistaken for accidental markings, should be made clear, and in general such objects should be inked by the topographer.

187. The elevations of summits should be distinctly marked on the sheet, and care must be taken that they are not rubbed or lost before inking. Red ink is to be used for numerical elevations.

188. Triangulation stations should be marked by small black circles inclosed in red triangles, with names in red ink, but in no case should this symbol be permitted to obscure an essential topo- graphic feature; for instance, in case of an offshore rock or islet used as a triaugulation station, the rock or islet should not be obscured by the station symbol, but the latter may be omitted if necessary and an explanatory note may be added as to the station.

189. I'hme-tahlp positions should be marked on the sheet with small red circles when the positions are recoverable and likely to be of future value; otherwise such positions should not be inked.

190. Titles should not be inked on original sheets in the field, but should be furnished on Form 537a and pinned to the sheet. The information must include general locality, special locality, names of chief or party and of officers making survey, date (months and year), and scale, together with a list of all data forwarded with the sheet. In the Philippines the stamped title

84

GENERAL INSTRUCTIONS FOR FIELD WORK.

form should be filled in with ink on the sheet, or on a slip pinned to the sheet.

191. In preparing and inking original sheets, north shall be taken as the top, and titles, names, numbers, and symbols shall be put on normal to the meridian regardless of the direction of the borders of the sheet, except where it is desirable thai name* be lettered to conform to geographic features. In such eases the names shall be inked so as to be read when looking north. N.-i should by their direction and proximity clearly indicate the object designated.

192. Photographs or tracings of sheets. — When there is reason to believe that the mode of forwarding a sheet is not secure, it should, if practicable, be photographed, or if photographic- facili- ties are not available, an outline tracing of the more importan; features of the original sheet may be made. Otherwise, no trac- ing of an original sheet should be made in the field. Bromide en- largements from photographs of sheets should not be made except at Washington, unless specially ordered.

193. When sheets are photographed the plates must be pre- served until the sheets have been received at the otlice. Prims should not be made unless the sheets are lost.

.194. List of plane-table positions. — Before transmitting topo- graphic sheets to the office, chiefs of parties will prepare a list of the prominent objects on the sheers that have been determined by the planetable, namely, spires, chimneys, cupolas, flagstaffs, trees, etc., and such natural objects as sharp, well-defined moun- tain peaks, rock cliffs, and other objects thai might be recovered and utilized, and particularly such objects as will In? useful in hydrographic work; and indicate the position of each object listed by scaling the D. M. anil 1). P. from the sheet in the following form, giving the height, if determined :

Plane-table volitions.

\

Object and description. ' Latitude.

D. M.

Lonp!- tude.

D.P.

Height.

Remarks.

0 ,

Jfetrr*.

0 ,

Miter*.

/•••'..

Cupola, Harrison's house. . . 42 21 Cupola, Blackwell's barn... 42 22

•A:A S45

72 40 72 39

SOS

724

146

138

Top. Weather

vane.

Chimney, souare house, 42 25

632

72 37

395

lo?

Top.

Smith's.

North chimney, Rodger's 42 26

981

72 38

1,023

12.-,

Top.

house.

Episcopal Church spire 42 25

63

72 40

875

250

T op of

|

-.

cross.

Murray Mountain 42 27

426

72 46

125

3,256

The north

peak.

HYDROGRAPHY. 85

195. This list should be attached to the descriptive report. The exact TM>sition of the objects referred to should, of course, be distinctly indicated on the sheet. Where space permits, the more important objects, and especially those landmarks which should appear on the chart, should be named directly on the sheet itself, either close to the object or by reference letter and note elsewhere on the sheet. Brief legends descriptive of impor- tant landmarks may also, where practicable, be conveniently placed on the sheet.

196. Landmarks for charts. — A list of the objects which are of sufficient prominence for use on the charts must be furnished. The selection, determination, and description of these points is of primary importance. When placed upon the published charts with brief descriptive legends they are little less than indis- pensable for —

(a) Alongshore navigation, especially at difficult entrances or those subject to frequent and considerable changes ;

(&) The original location and determination of aids to naviga- tion and subsequent verification of their positions;

(c) Hydrographic examination of features subject to change, to serve as the base for more complete surveys, such as entrance approaches, bars, and channels. Also the verification of reported shoals or other features incorrectly or incompletely charted.

197. Tn relinquishing charge of a topographic sheet, the chief of party will inspect and approve each sheet before1 it is trans- ferred to the office or to another chief of party for completion. When circumstances are such that a departure from this rule is unavoidable or when any parr of the provisions of the instructions for completing these sheets are omitted, an explanation shall be forwarded promptly to the office for approval ; a full explanation of the circumstances must also be entered in the descriptive report accompanying each sheet.

HYDROGRAPHY.

198. Data to start survey. — When the information is available from previous work, and the locality of the work and conditions are such as to require it, the following will be furnished with the instructions from the office, and the chief of party should at once examine the information to see that it is complete and understood: 1'rojections on which have been plotted triangulation points, shore line, and all objects or features located by plane table or otherwise which may be useful in the hydrography ; list of geo-

£6 GENERAL INSTRUCTIONS I (tit FIELD WORK.

graphic positions; descriptions of stations; tidal plane of refer- ence ; description and relation of tidal bench marks ; copies of previous charts or surveys: information as to dangers reported or other special features to be examined, and, in the case of continu- ous surveys along the coast in a new region, a copy of the progress sketch of the previous season.

In regions where survey work has not previously been done the triangnlation may have to be accomplished and the tide plane determined by the party charged with the hydrography, and the projection will then be made in the field. In some cases it may be desirable to carry on the hydrography simultaneously with the triangulation or topography in order to save time or utilize the services of all of the party at the beginning of a season. In such cases preliminary locations of the signals should be plotted graph- ically on the boat sheet, but all the work must be planned with the view of ultimate control by the triangulation, and the more important stations should be carefully marked.

199. The lists of geographic positions and descriptions of sta- tions furnished to field parties must be returned to the office upon the completion of the work. When former stations are recovered that are found to be insufficiently marked or described, or the marks partially effaced, or the witness marks gone, the defects should be remedied and an amended description forwarded (see pars. 84-96). Stations should not be reported as lost unless an exhaustive search has been made. When building signals over stations care should be taken not to disturb the station marks.

200. Shore line. — When there is reason to suppose that the shore line has changed materially since the previous survey the im- portant features should, if practicable, be located in connection with the hydrography, either with the plane table or by deter- mining prominent points by sextant angles (preferably three at each point) and sketching in the intermediate shore line. Shore line so located should be drawn in broken line. The same course should be followed when the hydrography precedes the topography and it is impracticable at the time to obtain the complete topo- graphic information desirable.

201. Scale. — Unless otherwise directed, inshore hydrography should be plotted on scale not less than ^nnhnF, and must be done in sufficient detail to fully develop recommended sailing lines, ap- proaches, channels, and anchorage areas and remove doubt as to dangers. Anchorages, harbors, and channels may sometimes re- quire scales of nj^nr or even Winy- Offshore hydrography may usu- ally be plotted conveniently and economically on smaller scales,

HYDROGRAPHY. 87

88 4oooo> 'eoooo> soooo) or Twoinr- Where there are no dangers or details either of the last two may be sufficient for charting pur- poses.

202. Location of signals. — It is desirable that in advance of the hydrographic development a reconnoissance be made, the best loca- tions for signals chosen, and the whole work systematically planned.

203. If the hyxk'ographic work is to be based on triangulation and topographic points previously determined, these should be first recovered, as far as practicable, and if necessary additional points located from them.

204. For triangulation methods, instruments, and records which should be used for the extension of the triangulation beyond the limits already execuied and to supply the place of points lost, see paragraphs 12-1.11. The sextant should not be used for this purpose nor for the location of important hydrographic signals or of permanent objects, such as lighthouses, beacons, buildings, and other useful landmarks.

205. When in the course of the hydrographic work it is desir- able to locate new signals by sextant, three angles should be taken, if practicable.

206. In some cases it may be necessary to locate a subordinate signal or object by angles from several positions of the boat, the latter determined from other signals. Where recourse to this de- vice is necessary at least three positions should be used as a check.

207. The officer in charge should make sure while yet on the ground that the position of every signal or object used in the hydrography is determined with sufficient accuracy for the scale of the projection, and this must be tested by actually plotting or computing in the field.

208. Ureat care must be taken that ample information for the correct plotting of every hydrographic signal accompanies the record. A list of such as depend on plane-table locations and a list of such as depend on sextant angles should be given in the " Description of stations " and in the " Descriptive report."

209. In connection with the triangulation and plane-table work along the coast in a new region special attention must be given to determining suitable objects for hydrographic work, each of which shotild be described and marked when necessary, so MS to be avail- able for future use. (See paragraphs 73-75 and 194-196.)

210. A signal erected exactly over an old station should bear the name of that station. If for any reason a signal is located near, but not exactly at a previous station, it must have a dis-

88 GENERAL INSTRUCTIONS FOR FIELD WORK.

anguishing name, or may be given the old name followed by " \.i. 2 " or the year.

211. Names of signals. — For convenience short words of not more than three or four letters should he used for names of hydrographic signals. Avoid using in the same locality two n.'imes that resemble each other in sound.

212. List of permanent positions determined. — Before transmit- ting hydrographic records or sheets to the office chiefs of parties will prepare a list of prominent objects or positions of a perma- nent character that may be useful in future work that have been determined in connection with the hydrographic work. Indicate the position of each object listed by scaling the D. M. and D. P. from the sheet, in the form given under " Plane-table positions " (paragraph 194). This list should be attached to the "Descrip- tive report."

213. Character of signals. — It will materially facilitate hydro- graphic work to have a sufficient number of conspicuous signals which may be readily picked up by the sextant observers. For convenience as well as economy natural objects, such as bowlders, cliffs, and lone trees, and artificial objects, such as towers, flag- staffs, lighthouses, gables of buildings, etc., should be used as sig- nals when available. It Is considered that the success of a party engaged on offshore hydrography depends largely on the type of tall signal and signal buoy allowing of the longest range of visi- bility. Specifications for the tall type signal and different classes of signal buoys may be had upon application.

214. A good form of hydrographic signal is a tripod with slats across two of its sides, or a pole with banners of cloth stretched be- tween cross pieces so that the banners will show in different direc- tions. Driftwood, small trees, and other material on the ground and in the Tropics bamboo poles fastened with wire or rattan, may be used economically. Signals near each other or similarly situated should be varied in form or color to avoid likelihood of confusion. The directions from which they will be viewed should, of course, be considered in building signals. Natural as well as artificial objects may readily be made conspicuous by whitewash. Against a dark background white signals show best; against the sky black is preferable. For general use white is the better color, and cloth the better material, other conditions being equal.

215. A tripod made of lengths of iron pipe, wired together through crosses at the top, makes a simple signal that will stand in a moderate depth of water, and if wrapped with cloth and

HYDROGRAPHY. 89

with flags set in top may be seen at a long distance. In exposed situations in the water such signals may be made more secure by pumping the legs into the bottom by means of a water jet; long- poles and saplings have also in this manner been pumped in on ocean bars and have withstood storms.

216. Indefinite objects, such as tops of round hills and centers i.i' islands, should never be used for critical or inshore hydrog- raphy, but for offshore hydrography it is .sometimes necessary !o use the summits of mountains which have been determined 1/y triangulation : of course, for this purpose, definite and conspicuous points are to be selected as far as practicable.

217. When sounding from boats, it may sometimes be desirable to use the foremast of the vessel as a signal ; in such case the vessel should be anchored with a short scope, and her position determined whenever there is any change, due to change in the direction of the \vind or tidal current. The angles determining the positions and the time they were taken must be noted, and this information should also be put in the record book of the sounding party.

218. Plan of development. — The plan of development should be carefully considered in advance, so as to cover properly the whole area, with suitable allowance for the relative importance of the different parts ; the closeness of development should vary from a maximum in channels and anchorages having depth near the draft of the vessels to be accommodated to a minimum on exten- sive flats of much less depth and in clear areas of much greater depth. Careful attention must also be given to the development of shallow channels and waters that are likely to be used by light- draft vessels, such as motor boats. The soundings upon the chart in addition to indicating to the mariner dangers, channels, and anchorages, also enable him by casts of the lead to recognize his position, and this requires a development of the material and relief of the bottom within the limits of ordinary sounding depths.

219. In working on the general scale along a coast in new re- gions, closer development should be made of all parts where ves- sels are likely to be compelled to approach land, as in possible anchorages or off promontories, even though present conditions do not warrant special large-scale surveys. The lines should also be closer off projecting points of land or reefs. In regions where con- tinual changes are going on the development need not be so de- tailed as in regions where changes do not take place.

220. The order of the development of the hydrography should depend on an economical management of the party. Much val-

90 GENERAL INSTRUCTIONS FOR FIELD WORK.

uable time may be lost in sending boats to sound a long distance from the ship or headquarters, and whenever circumstances per- mit the anchorage or the shore quarters should be shifted to keep near the working ground.

221. Systems of sounding lines. — Systems of parallel lines cover an area most evenly and economically, and zigzag lines, except as provided for to supplement wire-drag work, should, in general, not be used. The development should usually be by straight lines perpendicular to the general trend of the coast, though the direc- tion will depend somewhat on currents, wind, and vessel.

When there is a strong irregular current in a thoroughfare or river, lines run normal to the channel, owing to the nonuniform progress of the boat over the bottom, will not afford reliable means for plotting soundings unless position angles are observed fre- quently. Under such circumstances the greater part of the de- Telopment should be made by lines run with or against the cur- rent. For a certain mileage of sounding lines, a system of close parallel lines will develop a given area more thoroughly than if the same mileage was laid out in two systems, at right angles, of wider-spaced lines. However, cross lines furnish a valuable check on the accuracy of the work, and it is therefore recom- mended that for inshore hydrography the first system of parallel lines be crossed by lines approximately perpendicular and spaced several times as far apart as the first system. Outside of the 10-fathom curve, such cross lines will be run as will insure that no important changes of depth remain undeveloped. When, however. a system of lines extends seaward for a considerable distance be- yond where it can be checked by observations on fixed objects, a few cross lines should be run to enable the draftsman to detect gross errors. In many localities the submarine relief is char- acterized by a succession of more or less continuous ridges which trend in a common direction, such as the submerged glaciated areas in Maine and Alaska, the fringing coral reefs of the Florida Peninsula, and the common sand waves and banks of rivers and coastal waters.

Where such areas are surveyed and developed by means of the hand lead, the trend of the ridges should be ascertained by means of a general system of lines and final development completed with lines run at an angle with the direction of the axes of the ridges. Lines making a more or less acute angle with the axis of the feature are necessary also in the development of steep slopes, nar- row channels, and crests of bars.

HYDROGRAPHY. 91

222. The spacing of lines will have to depend largely on the character and relief of the bottom and the importance of the re- gion. In general coast work with flat and sandy bottom and without indication of danger, inshore lines may be spaced 200 to 400 meters apart, but this interval should be diminished for steep slopes, broken, uneven, or rocky bottom such as are found in the Philippines and Alaska. In important anchorages and channels lines as close as 50 meters may be required. Between the 10 and 100 fathom curves about four, or even less, lines to the mile should be sufficient in regions like the South Atlantic and Gulf coasts where there are no indications of dangers. In general the mini- mum requirements will be included in the instructions, and the chief of party should not hesitate to increase the number of lines for the development of the area, as the survey may require, re- porting the necessary change of details to the office. All areas with depths up to 100 fathoms, including detached lumps outside the 100-fathom curve, should be developed sufficiently for the pur- poses of navigation. On an abrupt coast, outside of the 100- fathom curve, lines from 5 to 10 miles apart should be run off- shore at least to the limits of visibility of the mountain peaks, or the 1,000-fathom curve.

223. For the sake of economy care must be taken not to extend the close inshore system of development into open and deep areas where it is unnecessary, as a serious loss of time and energy may result. The system of lines must be varied to suit the conditions. Ordinarily the close inshore work will be done with launch or boat, and the more open offshore work with ship, the latter system slightly overlapping the limit of the former.

224. Sounding interval. — The interval between soundings should depend on the nature of the bottom and the depth of the water. In depths of critical importance to navigation it should be made as short as is consistent with good work, and it should always be less than the interval between lines. Generally in moderate depths of water more soundings will be taken than can be plotted on the sheet.

225. Time interval. — The time interval should usually be uni- form, the recorder indicating the time by the order " sound " to the leadsman. For very irregular bottom the time soundings should be abandoned and the leadsman should sound as rapidly as possible. Under normal conditions and with a single leads- man the following time intervals have been found to meet the requirements :

92 GENERAL INSTRUCTIONS FOR FIELD WORK.

Depths under 2 fathoms 15-second interval.

Depths from 2 to 4 fathoms 20-second interval.

Depths from 4 to 7 fal houis ;W-seeond interval.

Depths from 7 to 10 fathoms 40-second interval.

Depths from 10 to 15 fathoms 1-minute interval.

226. Sounding speed. — The speed of the boat should be varied a* may be necessary for efficient and economical work. It may be increased in very shoal water when soundings can be made rapidly, and also in deeper open water where a close interval is unnecessary. But it should never be so great as to interfere with getting correct soundings. It is impossible to obtain up-and-down casts when the vessel is running at high speed. About 5 knots should be considered as the maximum speed through the water for sounding with a hand lead under favorable conditions.

227. Precautions in case of danger indications.— When the i«»t- tom is rocky, or when detached rocks are known or suspected to exist, the precautions in sounding should be much increased.

228. In all cases of shoals, suspicious soundings, and indications of dangers, whatever additional work is necessary to develop the bottom thoroughly and to determine the least depth of water must be done regardless of any prearranged system of lines. It mu>c not be assumed that the regular lines of soundings show the least depth. A sounding showing even very little less than the average depth should be regarded as the indication of a possible shoal, much more so when two such shoaler soundings are found on con- tiguous lines, and in such case very careful investigation should be made of the vicinity to obtain the least depth.

229. Depth curves. — A valuable test of the completeness (if tin* data from a hydrographic survey is to draw the curves for all depths. The data are adequate when no doubt exists of the location of any portion of a curve.

230. Additional development. — All channels, sailing lines, un<I anchorages should be sounded thoroughly and dragged if i- sary ; additional lines in the direction of the axis of the channel or of the sailing lines should be run if they are not parallel with the system of sounding lines adopted for the general development Sailing lines should not be recommended without actual test by running lines of soundings over them.

231. Ranges for running lines. — Sounding lines are ordinarily run on compass courses. Ranges of natural objects on sh >re should be picked up when practicable and will be especially use- ful when there is any wind or current. Usually, however, it will

HYDROGRAPHY. 93

not be desirable to delay the work to select ranges or for the pur- pose of getting the boat in the exact position to start a proposed line, and this must not be done unless there is special reason for it. When essential to select a range, the angle between some signal and the line proposed to be run may be taken off the sheet with a protractor, and with the sextant set to this angle search made for suitable objects ashore in the direction of the line.

232. Running lines by compass. — A proposed system of parallel lines spaced as directed should be laid out in pencil on the h<>;ii sheet. In following a course indicated by a pencil line, when a position plots off to one side, position angles should be taken at

i he moment of changing course. No time should be wasted, how- ever, in attempting to follow closely the pencil lines on the boat sheet.

233. la close development with parallel lines, soundings should not be taken between the last position on one line and the first position on the next line.

234. In eases of exposed shoals with breakers it may be im- practicable to do more than run a line just outside of the breaker* and to note the distance of the sounding boat off the breaker** at numerous points.

235. Special development of reefs, shoals, bars, and channels.— In surveying a reef with a single high point or surface a buoy ;>• generally placed on the highest point and radial lines run from this; but this may give an inj{>eriect idea of the shape of the reef, as the lines diverge rapidly from each other. New lines should be introduced, therefore, between the first radial lines as they recede from the buoy, or preferably the area in question should be devel- oped by a system of close parallel lines and cross lines. On shoals or rocks that are bare at some stage of the tide the depth should be obtained if practicable.

236. If the reef has more than one high point, several buoys placed upon them will give the means of laying out upon a dia- gram and of executing by sounding a regular plan of work which will show the i>eculiarities of the reef, increasing the soundings where the slopes are steep or the irregularities great. It is very desirable to visit rocks and shoals at extreme low water, when an examination may show how near the surface any portion ap- proaches.

237. In the development of areas remote from shore signals, water signals (or buoys with signal superstructures) must be established so that details mav be studied in their true relative

94 GENERAL INSTRUCTIONS FOR FIELD WORK.

position and results matte conclusive; these water signals must he connected with the remote shore signals. (See par. 275.)

238. In harbors lines should be run to the outer face of quays and wharves to show that water can be taken to them.

239. When convenient, shoals and flats bare at low water may be sounded over at or near high water. When reduced for tide these soundings will show the height above the plane of reference. These h rights will be plotted on the sheet as "minus soundings," that is, the heights in figures will be plotted with the minus sign before each. In general, whenever a sounding is less than the amount of the tide reduction at the same moment, the difference should be plotted as a minus sounding. All minus soundings are, of course, to be included within the low-water line.

240. locating reefs in heavy weather. — On a field of work ex- posed to the sea, reefs and shoals may be discovered, located, or verified during heavy weather by occupying two or more stations, and with an instrument cutting in the breakers, or by cutting them In from a vessel. The depths can be ascertained during fair weather.

241. When the survey of a shoal or rock is finished, care must be taken to note upon the spot all useful ranges, bearings, and marks which lead over it or close to it on every side.

242. Examination for adequate development. — The development of channels having moderate depths in the fairway, and that of liars, if there are any, which obstruct the fairway, is of the ut- most importance and should receive the close personal attention of the chief of party. After the lines are plotted and the curves drawn in he should carefully trace out each channel and assure himself that no soundings are wanting to show exactly how much water can be carried throughout its whole extent, and extra lines should be run where there is the least room for doubt. Should he find indications of a bar, a further examination must be made to develop its form and extent and to make sure of having found the least depths upon it.

243. Dragging for dangers should be resorted to in cases of im- portant channels and anchorages where obstructions have been re- ported and not found or where the nature of the bottom and sur- roundings indicates a likelihood of dangers which might be missed in the ordinary sounding lines. Even in the closest development with the sounding lead pinnacle rocks may be missed, and a thorough sweeping of a doubtful area is necessary to prove that it is clear. Experience indicates that this precaution is well war- ranted in important areas.

HYDROGRAPHY.

95

The wire drag is the only sure and effective means for this purpose. It is described in the Coast and Geodetic Survey Special Publication No. 56, with general directions for its use.

In plotting a large area of drag work the method described in Special Publication No. 56 should be followed. For small areas the positions may be plotted on the regular hydrographic sheet, but the connecting lines should not be drawn, as this will inter- fere with the legibility of tlie soundings when plotted. A piece of vellum should be used to show the details.

A drag made of pipe and intended for use with a surveying vessel is described in Appendix No. 6, Coast and Geodetic Survey Report for 1903. For surveying operations this apparatus has been superseded by the wire drag.

Where special apparatus is not available a drag of some sort should be improvised to search for an important reported obstruc- tion which can not be found by the lead. Two pulling boats may be used, and the principle of the wire drag should be followed in keeping the drag taut by means of weights at each end and the boats towing on courses somewhat divergent. Wire should pref- erably be used, or, in its absence, rope or light chain, or an iron pipe or bar suspended horizontally may be towed beneath a launch or between two boats.

244. The length of drag will depend on the nature of the work and the amount and quantity of material available. With stand- ard equipment lengths of drag under 3,000 feet are rarely used except in channels with less width than this. The following table gives information relative to drag lengths in ordinary use:

Length of drag.

Length of section.

Effective width.

Conditions.

Less than 3 000 feet

Feet. 300

Feet.

Narrow channels.

3,000 feet

300

2,700

Very broken bottom.

4,000 feet

400

3,600

Broken bottom.

5,000 feet

500

4,500

Fairly clear bottom .

6,000 feet and over

600

Deep water.

Lengths of drag for deep-water work are commonly 9,000, 12,000, and 15,000 feet, depending on the area to be covered and the cur- rent velocity.

245. Drag depths shall be referred to the plane of mean low water unless otherwise instructed. It is considered that an examination to a depth of 50 feet at mean low water is sufficient

96 GENERAL INSTRUCTIONS FOR FIELD WORK.

to insure safe navigation for surface vessels, while an examination to 100 feet is necessary to safeguard submarine navigation. Therefore, unless otherwise instructed, the following will be tli<» standard drag depths: Deep-water areas' to KM) fed or over : area- with depths bei we.-n ">() and 100 feet to within 10 or 20 feet from the bottom ; and areas with depth* less than 50 feet to within ahout 3 feet from- the bottom.

When the drag is lowed through the Avater the bottom win* will usually lift slightly. The amount of this lift, which is rarely over 2 feet, shall he determine* 1 by tests usually conducted from tin- tender . For this purpose a tester should be used which may consist of a J-inch metal rod about 3 feet long attached to one end of a small chain. This rod and chain is graduated in the same manner as a lead line, the chain being used to insure an invariable length. The tender should stop a short distance ahead of the drag opposite the point to be tested and lower the tester to a depth about equa 1 to the upright length. When the wire strikes the rod the tester is lifted until it clears the wire and the difference between the upright length and the reading of the tester wlien it clears gives the lift.

To obtain the upright setting for a certain effective depth add to this depth the lift correction and the height of the tide above mean low water as shown by predictions. White predicted tides are used for setting the drag, the final reduction is to be made by using observed values obtained during the course of the work- on a near-by gauge.

In channels and in deep water the drag is usually set to one depth throughout, and for the latter work it is customary to avoid depth changes by setting the drag for the maxim-urn height of tide that will occur during the day. In shoal water fairly long drags can be used by setting the drag at different depths to con- form to the bottom contour as shown by soundings. In this class of work frequent depth changes are necessary, in order to allow for rise and fall of the tide, to conform to changing bottom contours and to avoid shoals previously discovered. In this class of work it is not good practice to have the difference in length between adjoining uprights greater than one-fortieth of the distance between them.

246. In dragging areas whore soundings of previous surveys are widely spaced and where additional information relative to the depths is required soundings taken at stated intervals at each alternate buoy during the progress of the drag will give a staggered line of soundings over the entire path. The soundings

HYDROGRAPHY. 97

are plotted at the position of the buoy by using the time interval on the normal path of the buoy. When the position of the drag varies from its normal curve the position of the buoy at which the sounding is taken should be fixed in order to plot its position on the curve of the drag. This method of sounding should be used only when specially instructed.

247. All operations and angles in wire-drag work are to be re- corded for final preservation and for later work on the smooth sheet. A separate smooth sounding record and a wire-drag record are to be kept for each sheet. The end launch officer records, Cor future comparison, each angle that he signals, together with the time. Each tender records all data obtained on shoals; depth changes, giving the time that the change started and ended, the new depth and the buoys involved in the change; drag tests, etc. All dnta in regard to shoals are to be copied from the tender records into the smooth sounding record on the guide launch, while other information is transferred to the wire-drag record. Soundings taken during the progress of wire-drag work shall be recorded in a separate sounding volume. When cuts or bearings are taken from the end or guiding launch to locate the position of a sounding at an intermediate buoy they shall be recorded in the same volume with the sounding.

On the first page of each record are to be entered the names of objects used for control and the manner in which their loca- tions are obtained, together with the shore names assigned to them for convenience in recording. On the second page the party organization shall be given, with the name and duties of each member. Rubber stamps are provided for insertion of data at the beginning and end of each day. A stamp may be obtained for insertion of initial lengths of upright at the beginning of the day and whenever a depth change is made during the day. At each position the time, position angles, buoy angles, distance angle, signaled angle, and distance shall be entered in the order named. A buoy angle is to be considered as plus if the buoy is to the right of the object, and minus if to the left. When the drag catches on a shoal, an excellent check on the shoal position is obtained by observing and recording a bearing to the indicated position, with a note as to the number of the buoy nearest the shoal.

Successive days are to be lettered in order and corresponding days in the wire-drag and sounding records are to be given the same letter. Explanatory notes should be entered, when neces-

13027°— 21- -7

98 GENERAL INSTRUCTIONS FOR FIELD WORK.

sary, in the wire-drag record, and every care taken to make it :i clear and complete record of each day's work.

For long-drag work positions are to be recorded on the end launch and later transferred to the right-hand angle column of the guide-launch record.

248. To reduce the records, the upright length is to be entered in the proper column at the top of each page and where it is changed by a depth change. The correction, as shown by tests, is entered and subtracted from the upright length to obtain the drag depth. If there is a correction for swell it shall be noted by the officer in charge and added to the correction. For deep drag work a factor of safety may 1)6 introduced, at the discretion of the chief of party, by adding a foot or two to the lift shown by tests. Tidal reducers are to be entered in the same manner as for ordinary hydrographic work and applied to the drag depth to give the effective depth. If the tidal change occurs between two positions, it is shown at the preceding1 position if it decreases the effective depth, and at the succeeding position if the contrary is true. All distances must be checked by recomputation.

249. At the end of each day in the record an effective depth diagram will be entered. This diagram, which is simply a sum- mary of all effective depths obtained during the day is to be entered in the following form :

Position Remarks

43 45

1 N 6 F B

34 -> 2.8-3.4 2 F

44 35

8 N 2 F Tide.

The first entry shows the initial effective depths, the letter B indicating that the line begins. On an inclined section be- tween two different upright lengths, the lesser depth is to be considered as extending horizontally to the first upright set at a greater depth. Thus at the beginning of the day buoy No. 5 is set at 43 feet and buoy No. 6 at 45 feet making1 the path of buoy No. 6 the dividing line between depths. The second entry shows a depth change of 34 feet made from buoy 3 to buoy F in the direction of the arrow. As the depth is decreased the change1 extends automatically to buoy 2 as soon as buoy 3 is changed. The fractional position numbers show that the change started

HYDROGRAPHY. 99

between positions 2 and 3 at a time when buoy No. 3 had trav- ersed 0.8 of the distance between the two positions, and that it t-iided when buoy F had traversed 0.4 of the distance between positions 6 and 7. At position No. 8 an increase of 1 foot in the effective depths, due to tidal decrease, is indicated.

250. When the drag parts, care should be taken to eliminate uncertainty by the rejection of a sufficient number of positions.

251. For plotting, the smooth sheet is protected by tracing cloth held securely in place, with small holes cut through over each control object position on the sheet. A number of boat positions are plotted, after which the buoy positions are plotted and pricked through on the smooth sheet. The successive buoy positions are connected by straight lines, using a pencil hard enough to indent the smooth sheet. Care must be taken to plot the buoy positions within u reasonable time after the boat positions, lest the tracing change its position with relation to the sheet. The tracing is then removed while the path lines are drawn in pencil on the smooth sheet. Every fifth position is indicated by its number and the letter of the alphabet assigned to the day, using ink of one certain color. These numbers should be entered only on the guide-launch side of the strip. The curved line of the drag is drawn at the end of each strip, using the buoy spacer. The positions of all shoals discovered during the day are plotted, either immediately before or after plotting the day's work, and numbered as for the drag positions. When a drag strip ends on a shoal, care must be taken to extend the line of the drag back of the shoal. I

After the various strips are plotted in pencil they are subdivided to sliow effective depths. For changes due to tide the line of the drag is drawn with the spacer at the proper point. Depth changes are shown by connecting, with a line, the position of the first buoy Involved at the time the change started and the similar position of the lust buoy changed. If the change affects less than half the drag, the uvo positions are connected by a straight line. If more than half the drag is changed, it is best to locate the middle buoy involved at the time it was changed, assuming a uniform rate of change, and to connect the three points with a smooth curve. With a drag set at different depths, the dividing lines are obtained hy plotting the positions of the dividing buoys at each drag position and connecting succeeding buoy positions with straight lines.

A fter a strip is subdivided each subdivision is outlined with colored ink in accordance with the following color schemes, and with the rule that deeper areas are completely surrounded with a line of the proper color, while areas of less depth are surrounded

100 GENERAL INSTRUCTIONS FOR FIELD WORK.

by the proper color, exivpt when- they adjoin an aiva of greater depth :

19 feet and under Brown.

20 to 29 feet Yellow.

30 to 39 feet Blue.

40 to 59 feet Red.

60 to 79 feet Purple.

80 feet arid over Orange.

Each area has one or more light lines extending across it, with a space for a numeral representing the difference between the effective depth and the color base. Thus an area dragged to 94 feet will be surrounded with an orange-colored line and contain the numeral 14. When the strips are inked, corresponding posi- tions of N and F are indicated by short lines drawn from each toward the other. Each fifth position is indicated by slightly longer lines.

In shoal localities, where an area may be covered several rimes by drags set at different depths, the subdivision described above may be simplified by tracing each strip as it is plotted, subdivid- ing the strip on the tracing and then transferring the subdivi- sions to the smooth sheet.

252. All records of dragging operations should be kept in wire- drag record books, and the work clearly explained.

253. Position angles. — For locating position of sounding boat the two methods generally used are by theodolite angles on the boat from two stations ashore, and by sextant angles from the boat on three shore signals, or a combination of the two. The former is the most precise, but is not well adapted to surveys of extended areas.

254. The second method is employed in nearly all the coast work, the principles involved being the same as in the location of a plane table in topographic work by the three-point problem. The strength of a determination of position depends directly on the relative positions of the three fixed points and the position sought. There are usually a number of objects from which to select in taking the sextant angles, and good judgment is re- quired in making this selection; some positions of the objects with respect to the observer give strong conditions and some very weak conditions for the angles.

255. Strength of position angles. — A single angle between two fixed points gives as a locus of the vertex part of the circum- ference of a circle through the two fixed points in which the

HYDROGRAPHY. 101

given angle may be inscribed. Two angles measured between three fixed points determine the position as at the intersection of three such loci passing through each two of the points, respec- tively. The strength of the position depends in part on the angle at which these circles intersect ; as they approach tangency the position becomes weak, until the limiting case is reached, when the position is on the circumference of the circle passing through the three fixed points. In this case the three position circles coincide and the position is indeterminate and can be plotted only as somewhere on the circle.

256. Whenever the distance between any two of the fixed points is small as compared with the distance from them to the observer, the con-esponding position circle will be poorly determined and the position will be weak.

257. Based on the two preceding paragraphs, the following should be observed in selecting objects for angles:

258. Avoid any selection in which the boat's position is on or near the circle paasing through the three fixed points. This is commonly called a " revolver " and is to be constantly guarded against. In case there is no choice of signals and a "revolver" is expected, as may sometimes occur inshore near the end of a line, a third angle should, if practicable, be taken to a point of land or other defined object.

259. Avoid a selection in which two of the fixed points are close together as compared with their distance from the observer.

260. A strong position will be obtained with the three objects •nearly in line or with the central object nearer than the others

and no angle less than 30°.

261. Small angles should generally be avoided, as they give weak positions in most cases and also are apt to be inconvenient to plot.

262. Tbere is one rase, however, in which a small angle will give a strong position, and that is when two of the objects are nearly in line and not close together and the third object is so located as to give a good angle of intersection with them. The limiting case is where the position sought is in range with two ot" the objects. Only a single angle need then be observed, but a second angle on a foxirth object may be taken as a check. A range should be taken when there is opportunity, but the range points should not be relatively close together.

263. As slight errors in angles affect a position more with dis- tant signals than when near objects are observed, preference should always be given to the latter, other conditions being favor-

102 GENERAL INSTRUCTIONS FOR FIELD WORK.

able. The uncertainties of plotting due to paper and instruments also make it preferable to use near objects. Thus for inshore hydrography it is desirable that signals on the adjacent shore be used, and not very distant signals, as for instance, those on the opposite side of a bay.

264. When the central object is very close and the other two objects distant, the whole angle between the latter should be ob- served if practicable, or the two separate angles should be taken from the same spot, to avoid the error in position that will other- wise result from angles taken by observers at points! slightly apart, if the two angles are not taken at the same instant.

265. If practicable, avoid angles between signals having con- siderable difference of elevation, when either is near the observer.

266. If in running the sounding line both angles change slowly, the position will be weak. In plotting it should be noted that the position is strong if a slight movment of the center of the pro- tractor throws the arms away from one or more points, and that the position is weak if such movement does not appreciably dis- turb the relation of the arms to the three points.

267. The time interval between positions will depend on the scale and the character of the hydrography, but on large scale work should seldom exceed three or four minutes. For con- venience in plotting and spacing soundings, positions should ordi- narily be taken on the full minute, and when possible at uniform intervals. Position angles should, however, be observed when there are sudden changes of depth and at all changes of course and of speed.

268. Where the change of course is considerable, positions should be taken both at the time the change is made and as soon as the boat is on the new course, and in such case the track of the sounding boat should be plotted as a curve and not as a sharp angle.

269. In addition to the position at the beginning of the liu<>, position angles should again be observed when the boat gains full headway (to be noted in the record) in order to avoid the serious errors in spacing soundings on the plotted sheet as a re- sult of the variable speed of the boat. The same holds true when the speed is slowed down on the approach to shoal water at the end of a line; that is, position angles should be taken when the boat is slowed down as well as at the end of the line. The irregu- lar and improbable depth curves sometimes seen on plotted sheets near the shore are generally due to a failure to take account of the

HYDROGRAPHY. 103

changes in speed of the boat near the beginning and end of sound- ing lines.

270. Positions may conveniently be recorded in the following form, the signals being named from right to left :

4 Bet 70° 40' Cat Dog 41° 14'

271. The position number is to be placed immediately to the left of the time at which position was taken, being careful that there is no uncertainty as to which time is referred to. It is important that the time recorded should be that at which the position and sounding were actually taken ; discrepancies in the hydrography will result from lack of care in this respect.

272. A range is indicated by zeros with u line drawn through them, thus:

4 Bet 61° 27' Cat Dog 90

273. Buoys and other aids to navigation within the field of work should be determined by special sextant angles. If found to be out of position or unfavorably located, this should be promptly reported, as well as any recommendations as to desirable posi- tions for aids to navigation. (See par. 401.)

274. The method of locating positions by two theodolites ashore should be used when extreme accuracy is demanded, as in harbor improvement surveys. Although not often employed in general coast work, it may be convenient in some cases. For instance, the signal at the masthead of a vessel may sometimes be dis- tinguished at a greater distance offshore than the shore stations can be seen from the vessel. The two theodolities are set up at suitably situated triangulation stations. All the directions are referred to a known direction as zero, which it will be convenient in plotting to have to the left of any position of the vessel, when the theodolite is graduated clockwise. This zero should be veri- fied, say at the beginning of each page of the record, by recording a pointing on the reference object.

275. A time ball or flag is shown from the vessel each time a position is required, and the instant it is dropped the direction of the foremast of the vessel will be observed at each station, and the time recorded at the two stations and on board. Or observa-

104 GENERAL INSTRUCTIONS FOR FIELD WORK.

tions made by a prearranged time schedule, in which case occasional signals should be made, if possible, for the comparison of clocks. The clocks should be set to agree and compared at the end of the day.

276. Positions for offshore hydrography. — In developing offshore areas along the Atlantic and Gulf coasts, survey bu^ys are placed two or three miles beyond the limit of visibility of the tall signals on the coast. The positions of these buoys are determined by intersecting cuts taken from the Survey vessels while at anchor at various points within the range of visibility of both shore signals and the buoys whose positions are to be determined. With this control the fixed positions on sounding lines are carried from o to 7 miles beyond the limit of visibility of the shore stations. For the survey of an important bank offshore out of sight of ob- jects on land, a sextant triangulation should be carried out from the short to locate several buoys or beacons placed on the bank to serve as signals during the hydrographic development. For the intermediate stations between shore and bank sailboats may prove convenient, as they can be readily shifted from point to point in a scheme which requires several figures to make the connection. If, owing to rough seas or other causes, this method is found impracticable, the use of two ship logs and a record of the engine revolutions, previously standardized, and the compass to determine a position on the bank by the adjustment of outward runs from a known position combined with that of return runs to a similar position in sight of land is recommended. The relative positions of the control signals on the bank can then be determined by courses and log distances, as well :is sextant angles. When the signals are short distances apart, a run between any two by compass and log should be immediately repeated in the reverse direction to eliminate the effect of current and other sources of error. For long distances the two runs should begin and end respectively with the same phase of tide. The record should be complete as to the compass deviations, log corrections, cur- rents, wind, and apparent drift. The sounding lines should be plotted and adjusted by the field parly. In all coast hydrography where the lines run offshore out of sight of signals, current ob- servations shall be made while on the sounding lines, about once every two hours or at intervals of not over 10 miles. Each course is to be corrected for leeway. Having an annemometer available, a table should be prepared giving a factor for the wind at each 45° from ahead or astern on either side of the vessel. When possible, astronomic observations at the current

HYDROGRAPHY. 105

stations shall be taken for ship's position in addition to all of the dead reckoning data obtained, making also full use of wireless time comparisons. Complete adjustment of the positions mtist be shown.

277. A reconnoissance of a bank offshore, where signals can j^ot be seen from a boat, may be made by anchoring the ship and sounding with a boat, obtaining the distance from the ship by measuring the vertical angle from the water line to the mast- head and taking bea rings on the boat with the ship's compass. Th<> height of the mast above the water furnishes a vertical base for plotting the distance of the boat.

278. Soundings with lead and line. — The leadsman should be trained to estimate the probable depth for the next sounding in order that he may pay out an adequate amount of spare line; too much may be more objectionable than too little. The effort should be to have the lead draw the line taut as it reaches the bottom ; also to have the lead reach the bottom as the leadsman .nets over it, or just before the line becomes plumb. The leadsman si ion Id then quickly lift the lead off the bottom, and as it touches again read the depth. This is an important precaution for the pur- l>ose of straightening the line and keeping the lead vertical. When there is a swell or the surface of the water is agitated the leads- man must be careful to make an allowance for the height of the waves, so that the reading of the lead line will give the depth from the mean surface.

(a) The following sounding leads and hand lines are in gen- eral use:

For hand lead in depths up to 8 fathoms, a 6 to 8 pound lead is used.

For hand lead in depths over 8 fathoms, a 10 to 12 pound lead is used.

For all hand lines No. 7 or No. 8 Silver Lake sash cord or Sampson spot cord is used.

For trolley soundings in depths up to 20 fathoms, a 20-pound lead with No. 9 cord is used.

For trolley soundings in depths over 20 fathoms a 30-pound lead with No. 12 cord is used. (See pars. 354 and 360.)

For the sounding machine in depths up to 500" fathoms leads from 30 to 40 pounds are used with stranded wire. In greater depths a shot of 30 to GO pounds is used with piano wire.

Where subsurface currents exist an extra heavy lead should lie used to permit a straight stretch of the leadline from bottom to surface.

106 GENERAL INSTRUCTIONS FOR FIELD WORK.

279. Soundings with vessel underway. — When working in mod- erate depths (from 20 to 60 fathoms), and yet beyond those in which it is practicable to sound with a hand lead (over 20 fath- oms), there is considerable saving of time and of wear on ma- chj,nery by using methods which permit the soundings to be taken without stopping the vessel.

280. Trolley rig. — A satisfactory and often-used method is that of dropping the lead near the bow and reading the depth as the lead line comes vertical under the leadsman stationed on the quarter-deck. With a sounding lead of from 20 to 30 pounds weight up-and-down soundings can thus be obtained rapidly in depths up to 50 fathoms, with speeds up to 4J knots, without stopping. Various methods are used for carrying the lead forward and automatically releasing it. A trolley wire may be rigged along one side of the vessel, with a grade downward toward the bow. The lead is suspended from a traveler hung from two grooved wheels which carries it forward until a projecting bolt on the traveler strikes a rubber surface on a boom, pushing back the catch holding the lead and releasing it. The lead drops to the bottom, and the traveler is hauled aft again. Another device is described and illustrated in Wharton's Hydrographical Sur- veying.

281. Deflection scale. — A system of sounding underway with sounding machine and wire (piano wire, No. 21 B. & S.) has been used in moderate depths (under 10 fathoms). An iron weight of 30 to 60 pounds, attached to sounding wire, is employed, the amount of wire out read on a registering sheave, and the angle of deflec- tion from the vertical of the wire noted on a horizontal scale pro- jecting from the deck. Soundings are made rapidly without stop- ping, the weight being lifted only a short distance off the bottom and not brought to the surface. The weight dragging near the bottom will develop the'presence of shoal spots between the sound- ings. An occasional sample of bottom may be brought to the sur- face. The correction for deflection of the wire is — Z(l — cos a) where I is the inclined length of wire and a is the angle of deflec- tion from the vertical, supposing the wire to be straight. This method of sounding has been used to advantage only in moderate depths (10 to 3X) fathoms) and at moderate speeds (4 to 6 knots). In greater depths (over 40 fathoms) the angle of deflection will become too great, and the curvature of the wire will introduce difficulties in the correction.

282. A modification of this method has been used in depths from 30 to 50 fathoms. Soundings were taken when the headway of the

HYWtOUKAPHY. 107

vessel was reduced sufficiently to keep the correction for inclina- tion of wire small. As soon as the angle is reduced to the desired limit It is read, the lead is dropped, and the instant it strikes bot- tom the registry dial is read and the reading recorded. The ad- vantage of this over up-and-down soundings is that less reversing of machinery is required, and that the vessel, retaining some head- way, is under better control and the proposed sounding lines can he more easily followed.

283. In machine sounding in moderate depths where vertical casts are obtained there may be some saving in simply lifting the lead a short distance off the bottom and going ahead without reel- ing in, except where a sample of bottom is desired. In using the heavier sounding leads for trolley rig, a proportionately heavier grade of sounding line should be used, as a pendant between the lead and 20-fathoin mark, as noted in paragraph 278 (a). Over an extended area with depths greater than 20 fathoms a section of wire of that length may be used to eliminate a portion of the stretch incident to the use of a long hemp or cotton line.

284. Pressure tubes. — Pressure tubes are designed for use in sounding when a vessel is under way in depths up to about 90 fathoms. Pressure tubes with appliances employing the overflow device, and others with springs and pistons, are also used. Pres- sure tubes, while satisfying the requirements of navigation, should not be used in depths less than 20 fathoms or when very accurate hydrographic survey work is required on account of errors due to temperature and other causes. On off-shore work where sound- ing tubes are used in the course of a sounding line, every fifth sounding should be checked by a vertical measurement of the depth with wire and registering sheave.

285. Sounding machines. — The Cosmos hand-sounding machine may !*• used successfully for soundings to depths of 500 fathoms, using No. 24 standard Brown & Sharpe gauge and about a 35- pound lead. When sounding in greater depths, steel wire should be employed. Other small sounding machines may be used when available, such as the Kelvin navigational machine or the Tanner machines. In all cases it is preferable to use a sepa- rate registering sheave, such as the Tanner, for reading the length of wire out, instead of the dial on the reeling drum, which is subject to correction, depending on the amount of wire on the drum. In using the Sigsbee sounding machine the scale attached to the upright carrying the leading sheave will show the strain on the wire when heaving in. A 90-pound strain is the approxi- mate limit in using the 21 Brown & Sharpe gauge wire. When

108 GENERAL INSTRUCTIONS FOR FIELD WORK.

sounding in depths over 1000 fathoms the speed in paying out and reeling in should not exceed 100 fathoms per minute.

(a) In splicing stranded wire, a lay of 16 inches with neat tucks at each end will suffice.

For piano wire, a splice 3 inches long will suffice. In making the splice, caution should be observed not to give the lay at the cross or middle of the splice so short a nip that it will afterwards be straightened out under strong tension. The splict- to be wiped with solder, giving a long taper to each end.

To cover the splice completely with solder, which must be done, several layers of felt, ticking, or moleskin cloth with tallow coating in the palm of the hand will serve to wipe the- splice as the solder is dropped or poured over it. In this opera- tion care must be taken that the soldering iron, ladle, or flame from the blow torch does not come hi contact with the wire.

In preparing the splice for soldering a flux of muriatic acid with zinc dissolved in it till it will take no more should be used before the solder is dropped or poured on.

Pure tallow, sperm candle or sweet oil only should be used in greasing the wiping cloth.

An electrician's soldering torch with soldering wire facilitates the operation. Stranded wire No. 24 B. & S-. gauge is furnished in sealed tins containing 300 fathom lengths. Piano steel wire No. 21 B. & S. gauge is furnished in sealed tins containing 2000 fathom lengths.

(&) The ordinary sounding-record books may be used for work with these machines; the time required to reach bottom should be recorded for the deeper soundings as a useful check. For de- scription of the Sigsbee deep-sea sounding machine and explana- tion of its use, reference should be made to Tanner's Deep-Sea Exploration (tT. S. Commission of Fish and Fisheries, 1897) and to Sigsbee's Deep-Sea Sounding and Dredging (Coast and Geo- detic Survey, 1880).

286. Sounding records. — All sounding records must be complete and intelligible, and the chief of party must personally see that the record is being kept in a systematic and careful manner. Give description of sounding apparatus (whether hand or ma- chine), and state size and kind of line or wire, whether register- ing sheave is used. etc. Many things which are perfectly clear to an observer, having the work fresh in his memory, may not be so to a stranger; hence the necessity of making complete notes with each day's work and record ing everything essential to a

HYDROGRAPHY. 109

complete understanding of the record. All uncertainties and doubtful places should be carefully investigated before leaving the field.

(a) Sounding-record volumes must, as far as practicable, be kept separate for each hydrographic sheet and numbered in sepa- rate series. It is inconvenient in plotting and tiling records to have in one volume soundings that go <>n dill'erent sheets; to avoid this, where projections arc not furnished, the scheme of sheets should l>e planned in advance as far as circumstances will permit. Boat sheets should conform to the limits of the smooth sheets.

A special form of record, " Soundings with wire." is now avail- able and should be used for deep-sea sounding.

287. Identification letters and numbers. — In order to aid in the identification of sounding records, hydrographic projections sent from the office will be designated by a temporary number, and those made in the field should be assigned a letter, and these field numbers or letters, market 1 plainly in pencil, should form a part of all sounding records, descriptive reports, etc., pertaining to each sheet, respectively. At Manila, Philippine sheet numbers will be assigned to each field party at the beginning of the season, and the records and reports must be systematically marked in ink with the corresponding sheet numbers.

288. Information notes. — At the beginning of each day's work enter in the sounding book the time the party left the vessel, or the vessel left the anchorage : the distance to the field ; the fact that the sextants, clock, »nd lead lines have been examined and were correct, or the corrections, if any ; describe sounding appara- tus used — if machine give weight and form of sinker and kind and size of wire used, also any departure from ordinary methods ; the names of the observers, recorder, and leadsmen, and should any of these be relieved during the day a note should be made in the column of remarks at the time it occurs. If there are two ob- servers, state which takes the right and which the left angle, also the one in charge. Should there be any correction or fact recorded later, which should be known before commencing the plotting of the day's work, a note calling attention to it should be inserted at the beginning of the day's record, also the name and location of the tide gauge or staff to be used in reducing the soundings.

289. At the close of the day's work note again the examination of sextants, clock, and lead lines, and their corrections, if any, the time of returning to the vessel, and the distance from the working ground.

110 GENERAL INSTRUCTIONS FOR HELD WORK.

290. In the division of work between the two observers it will be well for one to supervise the steering of the boat and the

plotting and the other to watch the correctness of the leadsman and the recorder.

291. Standard time is to be used in all records and so noted in the column " Time " at the beginning of each day.

292. Any information that will be of value in plotting the sheet or in explaining the hydrography should be noted in the remark column, as, for instance, the force and direction of the wind, the state of the sea whether rough or smooth, the force and direction of the current, the bearing and estimated distance of any object passed by the boat and which is or should be plotted on the pro- jection, and the time of crossing the range of two well-defined objects. The time of changes in wind or current should be noted, as well as eddies, tide rips and their trend, whirlpools, etc. When, owing to surf or other dangers, a sounding line can not be run to the shore, explanation should be given in the record, with estimate of distance to the shore or danger.

293. Special care should be taken that sounding records are complete in the following respects :

(a) In remark column the relation of beginning and end of line to some object should be given approximately, as " line begins about 300 meters 30° from A Tree; " " line ends 25 meters from reef, 0° O Run." Also, for every line beginning or ending near the shore, the estimated distance in meters to the shore, reef,' or breakers must be stated, and for every important object passed on a sounding line, as rock awash, breakers, buoy, etc., the estimated distance and bearing must be noted, or when not otherwise deter- mined an additional sextant angle should be taken to it from two or more positions.

(&) The course should be noted at beginning of each line, and when changed the time of change and direction of the course should be indicated, as C. C. to 56°.

In the new form of sounding record the ship's ox- boat's head as read by compass should be entered in the first column on the right- hand page, and the course intended to be made good should be written in the remark column as an indication of leeway.

In offshore work, the course, corrected for variation and deviation, should be entered in the remark column, and the ship's deviation card should be entered on page 1.

(c) A reference mark should be made against every sounding or time to which any note refers.

HYDROGRAPHY. Ill

(d) When stops are made, the '' ahead " time should be noted, as well as any change of speed.

294. Courses, bearings, and directions .should be stated in de- grees (from 0°, at north, through east, south, and west to 360°) and not in points, and degrees should be used instead of points for all purposes on board vessels of the Survey. Whenever there is a possibility of confusion, a statement should be made as to whether the course is magnetic or true.

295. Name and location of tide gauge to be used in reduction should be entered at the heading of each day's work.

296. The second page of a volume of soundings should contain an index of signals determined and an index of currents noted ; also a special reference to any other important information con- tained in that volume, giving in each case the page reference.

297. Duplication. — Sounding records should not be duplicated, except when specially directed, or when there is considered to be an unusual risk in forwarding records. A good security against loss will be to forward the sheet and records at different times ; the records to be sent by registered mail.

298. Soundings will in general be recorded in fathoms and in- tegral feet; only in such cases as in developments less than 40 feet at critical or controlling points, in channels, across bars, and in fairways, need fractions of feet be recorded, or, in other words, this will depend upon the locality and depth of water.

299. " No bottom " soundings are not satisfactory, and where practicable the depth should be obtained. They are quite objec- tionable in harbor surveys.

300. Character of bottom. — The sounding record should show the character of the bottom at the top of each page and at each change reported by the leadsman, by the usual abbreviation used on the charts, which are as follows : M, mud ; S, sand ; G gravel ; Sh, shells; P, pebbles; Sp, specks; Cl, clay; St, stones; Go, coral; Oz, ooze ; bk, black ; wh, white ; rd, red ; yl, yellow ; gy, gray ; bu. blue; dk, dark; It, light; gn, green; br, brown; hrd, hard; sft, soft; fne, fine; crs, coarse; rky, rocky; stk, sticky; brk, broken; Irg, large; sml, small; stf, stiff. The occurrence of grass, kelp, hyacinth, or other growth should be noted; also where kelp Is towed under, and at what stage of the tide it is covered.

301. It is particularly important that information as to the bot- tom be given for harbors and anchorages. The information given by the sounding lead may be somewhat superficial, and when con- venient a useful check is furnished by the actual experience in

112 GENERAL INSTRUCTIONS FOR FIELD WORK.

anchoring and the material brought: up by the anchor, which should be noted.

302. In the record of soundings, one line should !*> omitted after the sounding on which a position was taken, and about four lines between the end of one line of soundings and the beginning of the next line.

303. The times of soundings and positions should be carefully recorded, as they are used in spacing the soundings. The time th^ boat starts or stops is required, although the angles may he la ken earlier or later. When under way, ii' no sounding is taken on the position, leave that part of the line blank in the record.

304. Corrections. — Erasures should not he made hi words. Mistakes discovered may be crossed out and corrected by writing above or to one side, with explanation, if any. Full explanation must be written in the record if any work is rt-jeeted, using Min- or red pencil.

305. The recorder shou.d promptly call attention to any un- usual sounding ; if it is confirmed it should be marked O. K.

306. The success of the hydrographic work depends directly on the correctness and clearness of the record ; the recorder must make sure that he hears and records every fact properly and that the record is complete, and must not hesitate to ask for repetition when necessary. He should call back the figures as entered.

307. To save space in plotting upon the sheet, each day's work is known by a letter. The vessel and each boat should have a separate series, distinguishing them by using capitals of one color for the vessel and lower-case letters of another color for each boat, these distinctions to be preserved in the books, on the sheets, and in the table of statistics. For convenience of reference the letters used in each book should be given on the outside of the covers in the proper colors.

308. When the alphabet has been exhausted for day letters, use double letters or primes, as AA or A'. Rod, blue, and green are the best colors to use: black should not be used, as this would ob- scure the soundings.

309. When a sounding maehine of any kind is used the record should clearly state the kind of machine, manner of making sound- ing, and correction to machine or registering dial, and bow cor- rection was obtained. (See also par. 288.)

310. Reduction of sounding. — The plane of reference having been established and related to the graduation of the staff, the reducers, or tide corrections, to he applied to the soundings are

HYDROGRAPHY. 1 13

derived by taking the difference between the tide-gauge reading at the time of sounding and the tide-gauge reading of the plane of reference. If the tide-gauge reading at the time of sounding is greater than the reading of the plane of reference the correction to be applied to the sounding will be minus. If less the correction will be plus.

311. The reducers, or tide corrections, for open ocean areas or for depths over 7 fathoms, will be entered in the sounding record in integral feet On bars at entrances and over inside water areas for depths less than 7 fathoms and more than 3 fathoms the reducers will be entered to the nearest half foot, and for depths of 8 fathoms or less to the nearest tenth of a foot. The correc- tion for the lead line, also to tenths of feet, must be applied at the same time as the tide reduction, but the lead-line correction may be omitted if not exceeding one-half of 1 per cent of the depth. The reduced soundings will be entered in integral feet in the column headed " Reduced soundings field " (see pars. 335-337) except in developments in less "-ban 40 feet depth, at critical points s sco par. 298) the reduction shall be made so as to take account of the fraction of a foot. In verifying the reduction of soundings discrepancies of two or three tenths of a foot may be disregarded.

Lead-line correction. — The correction for lead line or sounding apparatus, when necessary, will be entered in the sounding record in feet and tenths for depths of 7 fathoms and less, and in integral feet for depths over 7 fathoms, using the minus sign for correc- tions to be subtracted and the plus sign for corrections to be added to the soundings. When integral feet are used, a fraction of 0.8 foot or more in a positive correction, and a fraction of 0.3 foot or more in a negative correction will be counted as an extra foot, but the smaller fractions will be neglected in each case. The correction for lead line or sounding apparatus may be omitted if not exceeding one-half of 1 per cent of the depth.

312. The record must show, by initials at the end, by whom reducers were entered and soundings reduced, and by whom each of these operations was checked. It can not be too strongly im- pressed upon the commanding officer and chief of party and their subordinates as well that good results in hydrographic surveying can not be expected unless attention is paid to details. It is, therefore, the duty of the officers engaged upon Survey work to see that the records conform in all respects to these instructions.

13027°— 21 8

114 GENERAL INSTRUCTIONS FOR FIELD WORK.

313. Planes of reference. — The planes of reference adopted for the reduction of soundings and the publication of the charts of the Coast and Geodetic Survey are as follows:

314. For the Atlantic and Gulf coasts of the United States and Porto Rico, the mean of the low waters.

315. For the Pacific coast of the United States, Alaska, the Ha- waiian Islands, and the Philippine Islands, the mean of the lower low waters (except for Wrangell Strait, Alaska, 3 feet lower than the mean of the lower low waters).

316. For the derivation of the above planes, see under "Tidal observations."

317. Plotting hydrographic sheets. — On boat sheet, smooth sheet, or tracing of either, positions should be plotted, and sufficient soundings should be plotted in pencil to keep track of the work and to make sure that the area is properly covered. All sound- ings showing unusual or dangerous depths at critical places should be plotted so that immediate examination can be made, before leaving the locality, of doubtful points and spots that give indica- tion of danger to navigation. Approximate plane for tidal reduction should be used where tide observations are available, getting the plane by comparison with predictions. Where ob- servations are not available use predictions in the form of tide curve prepared at the office on the tide-predicting machine in the form of a tide roll or marigram. This is recommended for preliminary hydrographic plotting and for wire-drag work. Ap- proximate depths should be plotted on the boat sheet only.

318. Field parties must plot all sounding lines on the smooth hydrographic sheets, plotting the positions in ink and indicating them by pen dots instead of small circles. See paragraph 327 and the following paragraph for instructions. The protractor, par- ticularly if it is a metal one, should seldom be permitted to touch the face of the smooth sheet. Before beginning the pro- tracting, stretch a piece of tracing vellurn over the entire sheet and cut circular holes one-fourth inch in diameter over each signal. Letter the names of the signals legibly on the tracing. In pricking the plotted positions apply sufficient pressure to mark through the vellum onto the smooth sheet. After plotting a few positions lift up the vellum enough to expose the area just protracted, and number the positions and connect them with hard-pencil lines. After the sounding lines are fixed on the sheet and there is opportunity for further office work, due to unfavorable weather for field work or other causes, the sound- ings should be entered in pencil after they have been reduced

HYDROGRAPHY. 115

for tide as noted in the preceding paragraph. It is important that in so far as practicable the hydrographic sheets should be completed in the field. In no case should the soundings be inked in by the field party.

319. Necessary details on completed sheet. — Every original hy- drographic sheet when sent in from the field must contain the following :

(a) Projection in black ink, fine full lines, the latitudes and longitudes on each end of each parallel and meridian; a note at bottom giving the latitude and longitude, with seconds in meters, of some one triangulation station.

(6) Trianguhuion, plane table, and such other points as may have been determined or established by the hydrographic party must be plotted, each with its distinctive symbol and name. The standard symbol of triangulation point is a black circle 2 milli- meters in diameter with red circumscribed triangle, the name of the point lettered in black. The symbol of plane-table position is a red circle 3 millimeters in diameter with name lettered in red. The symbol of hydrographic signal is the same as for plane- table symbol except that blue ink is used. The positions of all signals should be accentuated by fine black dots in the needle holes to assist plotting. Large buildings and prominent land- marks determined in connection with the hydrography should be indicated on the hydrographic sheet and designated by appropriate legend ; if necessary, a reference letter may be used and the legend placed where there is more room. (See also par. 196.)

(c) The shore line must be drawn on the sheet in a continuous black line if it has been surveyed by a plane table; if sketched in by a hydrographic party, it is to be indicated by a broken line. The high-water line and all information outside of it should be transferred from the topographic sheet ; the low-water line and other features outside of high-water line should, however, be left in pencil until the hydrography is plotted, when the information should be combined, in general giving greater weight to low-water line as developed by the soundings. The low- water line should be indicated by dotted line, as far as determined. The area be- tween high and low water should not be sanded.

(d) The soundings on the finished sheet should be plotted in pencil by the field party with the positions, letters, and numbers in colored ink. Minus soundings, which represent the heights above the plane of reference of areas bare at low water, should be given with the minus sign and inclosed within the dotted low- water line.

116 GENERAL INSTRUCTIONS FOR FIELD WORK.

(e) Rocks, reefs, coral, and shell banks, sunken or awash, must be marked with the proper symbols. Where the least depth over a submerged rock is obtained, the depth should be shown, with the word " Rock " or " Rk." Do not use symbol for sunken rock in such case.

(f) The positions of all buoys, light vessels, etc., must be given with their proper symbols, and depths at same determined.

(g) Bottom characteristics should be noted on the sheet at moderate Intervals, to give information contained in the record. The standard abbreviations are to be used. (See par. 300.)

(h) The limits of grass, kelp, etc., and the conditions of tide or current when these show must be indicated. If the bottom is grassy, it must be so written. Kelp must be marked with its proper sign.

(i) The names of islands, points, rocks, reefs, shoals, banks, channels, creeks, etc., must be given on the sheet. Care must be taken to obtain these names correctly. Names should, as far as practicable, be placed on the land area, leaving the water area clear. Lettering should not be allowed to obscure soundings.

(;) All ranges, bearings for dangers, etc.. and sailing lines on courses or ranges should be given and drawn as follows : The range in black lines broken with long dashes; the bearings in black dotted lines; and the sailing lines in black lines broken with short dashes, with the positions of the objects for ranges and bearings determined, marked, and named, and the names of the objects and the purpose of the range or bearing written along its line.

(fc) Current stations and tidal stations must be plotted in posi- tion.

(1) Titles should not be inked on original sheets by the field party, but must be furnished on Form 537 and pinned to the sheet. The information for the title must include the general locality, special locality, names of persons actually in charge of sounding, and of chief of party, vessel, dates of beginning and ending, and scale, together with a list of all data forwarded with the sheet. The title of a hydrographic sheet must clearly indicate the limits of the hydrography, and the same title must be given on the- record books pertaining to it.

320. Table of statistics. — A table of statistics should be made as the -sheet is plotted and transmitted with the sheet. This table may be written on computing paper and should be in the following form :

HYDROGRAPHY.

Statistics sheet No. _

117

Date, 1903.

Letter.

Vol- ume.

Posi- tions.

Sound- ings.

Miles,

statute.

Vessels.

January 28. .. ...

a

1

164

1 309

24 8

Launch

Total

7,488

53,981

950.8

There must be a note stating the unit for soundings (fathoms or feet) and the plane of reference. Also a tidal note giving the location of the gauge, and if there was more than one title gauge, for what parts of the sheet each was used; also the follow- ing information :

I'htiie of reference, reading on gauge. Lowest tide observed, reading on gauge. Highest tide observed, reading on gauge.

321. Depth curves. — The depth curves must be drawn on Hie sheet, and each curve should include the outer soundings of the depth -represented by the curve. When curves run so close to- gether as to confuse the sheet, the less important, or those rep- resenting greater depths, may be dropped. Curves must not be completely drawn where the information is insufficient, but parts of curves or curves with broken line may be put in.

The field party should leave the curves in pencil. When the sheet is verified at the office the curves will be inked with full colored lines, in general according to the following scheme:

Zero or mean sea-level curve .Yellow.

6-foot or 1-fathom curve _..Grocn.

12-foot or 2-fathom curve -_Red.

18-foot or 3-fathom curve _Blue.

24-foot or 4-fathom curve Yellow.

30-foot or 5-fathom curve . Red.

36-foot or 0-fathom curve ---—Green.

60-foot or 10-fathorn curve Yellow.

120-foot or 20-fathom curve- - Blue.

300-foot or 50-fathom curve Red.

600-foot or 100-fathom curve Green.

1,200-foot or 200-fathom curve Yellow.

6,OuO-foot or 1,000-fathom curve. —Blue.

GENERAL INSTRUCTIONS TOR FIELD WOUK.

(The 24 and 36 foot curve will be omitted except in special cases. )

Depth curves are of much value in interpreting and examining the results of the field work. The depth curves will often indi- cate areas of shoaler depths requiring further examination. Also abnormal and improbable curves are a strong evidence of prob- able uncertainties or inaccuracies in the hydrographic survey. Depth curves correspond to contours on land, and in nature are therefore generally of graceful sweeping form, free fioni sudden changes in direction and from corners; ordinarily they can not cross or abruptly run into each other ; on approaching they tend toward parallelism ; any departure from probable natural condi- tions is an indication of error either in field work or in plotting, or it may be an indication of shoaling that will require further examination. A study of the characteristic bottom forms in any region is of value in the interpretation of hydrography, as such forms are apt to repeat themselves under similar conditions.

In relinquishing charge of hydrographic sheets and accompany- ing records the chief of party will inspect each record and sheet and approve each sheet before it is transferred to the office or to another officer for completion. When circumstances are such that a departure from this rule is unavoidable or when any part of the provisions of the instructions for completing these records and sheets are omitted, an explanation shall be forwarded promptly to the office for approval and so noted in the descrip- tion report accompanying each sheet.

322. Comparison with previous surveys. — In plotting comparison should be made with the results of all previous surveys and with charts covering the same region, if available, especially as to all dangers or less depths shown on previous surveys. Develop pre- vious dangers and verify their location and extent.

323. All remarks, comments, etc., in sounding records should be carefully noted in plotting; abrupt changes in depth should be verified by checking tide reduction, etc. ; boat sheets and descrip- tive reports should be examined and compared to see that all essential information is on the smooth sheet.

324. Character of drafting. — The drafting work on the finished hydrographic sheet requires accuracy, neatness, and legibility, and of course good judgment and knowledge of the work, but it does not require expert penmanship.

325. Marking positions. — As each position is plotted on the sheet a point should be pricked through to show its exact posi tion, and this point should be marked with a light dot of colored

HYDROGRAPHY. 119

ink (small circles should not ho made). The successive positions on the lines will be connected by lines drawn with a hard pencil.

326. Each position must be numbered and the number placed .iust below and to the right or left of the position; the position numbers must be small and so placed as not to interfere with thi- soundings.

327. The letter of the day's work must be placed at the begin- ning and end of each line, at about every fifth position on the line, and at the point of any decided change of direction in lino.

328. The color of the position, day letter, and position num- ber must be the same as the color {riven the vessel or boat in the sounding record.

329. Style of numerals. — Vertical block numerals (no hair lines) have been adopted for soundings on hydrographlc sheets. The penciling, as well as the inking, should follow this style, using a pencil hard enough to avoid smudging, but not KO hard that ir will unduly cut into the paper.

330. Distinctness of important features. — It is important in plot- ting hydrographic sheets that the more important features, snch as rocks and least depths on shoals, shall be perfectly clear and distinct, and great care must be taken not to obscure them by at- tempting to plot all of the numerous soundings that may have been taken for the development of such a feature. »If for any reason an important feature is not clear on the finished sheet, or is so shown that there Is a likelihood of its being overlooked, a note should be added calling attention to it.

331. Selection of soundings. — Where the number of soundings taken is greater than can be plotted on the sheet, as many sound- ings should be plotted as is consistent with clearness ; those show- ing the least depths on shoals, greatest and least depths in chan- nels, and changes of slope must be shown, the selection being such that a cross section could be drawn from it showing all important features; in no case should a mere mechanical selection be made, as, for instance, every third or every fourth sounding.

332. Enlarged scale for complicated areas. — It is sometimes diffi- cult to properly plot the soundings to show the development of a complicated area on the scale of the general hydrographic sheet- In such cases an enlargement of the plotted positions should be made and the soundings plotted on the enlargement, which may appear on the sheet as a subplan. The enlargement should be to some even decimal scale, and the scale should be stated on the plan. The curves at the margin of the subplan should be reduced

120 GENERAL INSTRUCTIONS FOR FIELD WORK.

and transferred to the main sheet to make sure that the work is consistent.

333. Overlap of sheets. — For adjacent hydrographic sheets the curves and soundings should be common for a narrow strip, JUKI this overlap should be in accord on the two sheets.

334. Dangers and stage of tide. — Definite information should be given on the sheet as to dangers which show at various stag' the tide, as so many feet above low water, awash at low wat<jr. awash at high water, breaks at half tide, breaks in heavy weather only, and the like. "Awash " should always be qualified by the stage of tide at which it occurs, and the mere use of the symbol for rock awash will not be sufficient for any important danger. (See par. 235.)

335. Tide rips should be indicated on the sheet by words, quali- fied as heavy, moderate, or light.

336. Depth units. — The unit to be used in plotting the soundings will depend upon the locality, the character of the body of water, and the closeness of detail to be shown. Extensive inclosed waters and inside routes frequently have but from 2 to 5 feet of water or even less, and of course should be plotted in feet and frac- tions (see par. 337) at critical points. Sheets in generally deep water will be plotted in fathoms and sixths of fathoms to a depth of 6f fathoms, -| being plotted as \ ; in fathoms and quarter fathoms from 7 to 8f fathoms, f being plotted as \ ; and for greater depths fractions will be omitted. But one depth unit must be used for the whole area of any sheet

337. On sheets plotted in feet no fraction of feet will be shown (fractions of less than 0.8 being omitted, and those of 0.8 or more being written as the next whole foot), except that in critical places (under 40 feet in depth) on navigable bars, in channels, and shallow inclosed waters and inside routes fractions (i, \, and f) shall be shown where important; but on outlying dangers all fractions shall be omitted and the next lower foot shall be given.

338. In converting fractions the following will in general be ob- served : When plotting in even feet omit all fractions of less than 0.8, and those of 0.8 or more write as the next whole foot ; when plotting in quarters take 0.1=0, 0.2=i, 0.3=4, 0.4=$, 0.5=4, 0.6=4, 0.7= J, 0.8=|, 0.9=1; when plotting hi halves, take 0.1 to 0.3 as 0, 0.4 to 0.7 as i, and 0.8 to 1 as 1; when converting from feet to fathoms and quarters, take less than 1 foot as 0, 1 foot and less than 2.5 feet as \ fathom, 2.5 feet and less than 4 feet as £ fathom, 4 feet and less than 5.5 feet as f fathom*, and

HYDROGRAPHY. 121

5.5 feet and over as 1 fathom; when converting from feet to fathoms, for less than 4.9 feet drop the fraction, for 5 feet and over take the next whole fathom.

339. Defining reef limits. — The limits of reefs as located by the hydrography should be fully marked on the sheets in the field. The danger limit of rocky bottom having some depth of water, but. which can not be investigated in detail, should be indicated by the sunken rock symbol. The coral-reef symbol should be used to indicate the extent of coral reefs either bare or awash at low water.

340. Errors and omissions. — Where from any reason but a single angle is available (as when a mistake has been made in reading one angle) a line of position may be plotted by setting the angle on a protractor and plotting several points in the vicinity of the work. The boat must have been at some place on the line drawn through these points, and its location can be fixed by the intersec- tion of this line with the course made good, or by plotting on it the distance from either the preceding or succeeding position ac- cording to the time interval. If two angles have been observed, but without a common object, the two lines of positions may be plotted separately and their intersection will be the position of the boat.

Mistakes in angles or record may sometimes be detected by estimating the position from time and course and testing the angles with the protractor. No arbitrary deviation from the record should be made, however, unless it is reasonable and sup- ported by other evidence. Such cases, or rejection of any portion of the record, should be noted in the column of remarks with rea- son therefor, and this statement must be signed and all defects corrected before leaving the working ground.

341. North the top of sheet. — In plotting and inking original sheets, north shall be taken as the top, and names, soundings, and signals shall be put on normal to the meridian, regardless of the direction of the borders of the sheet, except where it is desirable that names be lettered to conform to geographic features. In such cases the names shall be inked so as to be read when looking north. Names should by their direction and proximity clearly indicate the object designated.

342. Very large sheets should be avoided in plotting hydrogra- phy, being inconvenient to handle both in office and field. The standard size of topographic sheet is 30 by 52 inches. Somewhat larger sheets may sometimes lie necessary for hydrography, but they should not exceed 42 by 60 inches.

GENERAL INSTRUCTIONS FOR FIELD WORK.

343. For smooth hydrographic sheets, Whatman's paper is fur- nished mounted, of size 30 by 52 inches. When larger sheets are required backed drawing paper of the best available quality should be used.

344. A multiplicity of sheets should be avoided as far as practi- cable by completing each sheet in its entirety. Fragmentary sheets for small pieces of work should be avoided ; such information can often be placed as a subplan on another sheet covering the vicinity, separated by a border and with subtitle.

345. For boat sheets a good quality of mounted paper should be used, and a paper with brownish tint has been found very satis- factory.

346. Thin transparent celluloid has been used advantageously for boat sheets ; one side of this material should have a dull finish so that it may be written upon with a pencil. The celluloid is laid over the smooth sheet and the signals marked. In the boat the celluloid is used over a sheet of paper.

347. The boat sheet, if one is used, should always be forwarded to the office, to assist in the final verification.

348. The distances that will be included on a sheet of given size and scale may readily be obtained from the following table of scale equivalents, by dividing the length or width of the sheet by the length of 1 mile on the given scale. For instance, a sheet 42 inches by 60 inches on scale ^^5 will include an area 11.5 by 16.5 nautical miles.

Nautical mile. Statute mile.

Scale.

Inches.

£&. ***-

Centi- meters.

T»'oS

14.S93

37.08

12.672

32. 19

7.296

18.53

6.336

16.09

4.864

12.36

4.224

10.73

zoOoT?

3.648

9.27 1 3.168

8.05

>il4oi

2.432 ' 6. 18

2.112

5.36

1.824

4.63

1.584

4.02

1.459

3.71

1.267 3.22

1.216

3.09

1.056 > 2.68

0.912

2.32

0.792 2.01

0.730

1.83

0.634 1.61

0.365

0.93

0.317 0.80

0.182

0.46

0.158 i 0.40

TvoSoou

0.073

0.18

0.063

0.16

349. Manipulation of protractor. — In plotting positions it is well for the sake of rapidity to have a uniform practice in placing the protractor. It is usually preferable to place the central arm on

HYDROGRAPHY. 123

the central object, with the right and lefl arms about equally distant from the corresponding objects; keeping the central ob- ject on, push the instrument up, reducing the distances on either side equally until all three arms are on. Handled in this manner the clamped arms of the protractor are not touched by the hands. The protractor should be examined occasionally to see that it is in good adjustment and has no lost motion in any of its parts. A protractor may be tested by measuring with it several angles which have been accurately constructed geometrically on drawing paper.

350. For plotting angles where the three-arm protractor can not advantageously be used, either because the angles can not be set off or the positions fall under the frame, the Court celluloid pro- tractor should be used. This is more convenient than using tracing paper.

351. Spacing soundings. — In plotting soundings the space be- tween the plotted positions should be divided (using the con- venient standard spacing dividers) according to elapsed time and the soundings placed at positions indicated by their times. Where there is any distinction the more reliable system of lines should be plotted first. The center of a numeral, or group of numerals, representing a sounding is the position of the sounding.

352. In starting a sounding line from a position determined when the boat is at rest, another position should be determined after an interval of one minute or when the boat has attained the sounding speed. Where considerable change of course is made and soundings are continued with the vessel or boat under way, allowance must be made in plotting for the curve made in turn- ing and the fact that there is an appreciable interval before the vessel is on the new course. In such a case a position should be determined just before changing the course and another as soon as the boat is on her new course.

353. Sheets should be carefully examined for differences in depths* when sounding lines cross one another.

Discrepancies at crossings should be recognized as evidence of some fault in apparatus, method, or record which requires a study to discover its source and indicate the most probable correction, and possibly a reexamination in the field. The following typical errors are likely to produce large discrepancies and which are most readily detected by such a study. Careless protracting or spacing of soundings; errors In applying lead-line correction: confusion of numbers, such as 7 for 11; miscalled sounding; reversed angles, left for right or right for left : misreading sex-

124 GENERAL INSTRUCTIONS FOR FIELD WORK.

tant 5 or 10C ; confusion of signals; sextant badly oui of im ment ; erroneous tide reduction.

The following are typical of errors which will require u ; careful analysis of all available data: Spacing of soundings when affected by unrecorded variations of speed and course ; unrecorded errors in length of lead line; large clock errors; plane of refer- once, when the soundings of one line at a crossing depend on a tide gauge blocked by shoals from the free accesss of the tide; tide gauge too distant, or othewise not well located in relation to the hydrography ; abrupt changes of slope, especially those due to the existence of ridges formed by wave action on bars; dif- ference in state of the sea, when the soundings of one line are more affected by rough water than those of another; sound- ings affected by the existence of a bight in the lead line when running with or against wind, sea, or current ; very soft or ooze bottom; a condition which, permits of a considerable latitude of judgment as to what is the bottom.

Most of the errors in the first list will have the effect of displacing the sounding line from its true location, and a study of the data should bring out the fact that the divergence was inconsistent with the record, and lead to the discovery of the error.

Unrecorded variations of speed most frequently occur at the start or finish of a line — a comparison of time intervals and dis- tances between positions should indicate the trouble. A faulty plane of reference will produce discrepancies where lines of sound- ings reduced by it cross other lines of soundings reduced from a different gauge.

Curves of equal depth afford useful evidence of the source of several discrepancies, among which is that of a tide gauge poorly located with reference to the hydrography. Under this condition curves located by means of adjoining parallel sounding lines, run at different stages of the tide, will have a jagged unnatural appearance.

The existence of sand ridges on a bar should be apparent from an Inspection of the whole area of the bar. A slight difference In the position of the vessel at a crossing might result in a sounding being taken on the crest of n ridge and one on the side or bottom of the steep Inward slope. The possibility of an underwater bight hi the lead line should be capable of inference from the notes in the record. And here it is pertinent to once more stress the importance of full notes In the record. It should be obvious from the foregoing how necessasry they are in clearing up discrepancies.

HYDROGRAPHY.

125

Sufficient notes may save from rejection an apparently unreliable sheet.

If the study does not result in an actual correction of one of the Hues, yet it may plainly show good reasons for the rejection of one of the lines and, in consequence, warrant the adoption of the other. When the data do not afford a reasonable explana- tion of the difference, and the latter amounts to as much as 5 per cent of the depth in critical parts of the water area, the work should be revised in the field.

354. Lead-line corrections. — To avoid large corrections to sound- ings it is desirable and convenient to have the lead line as nearly correct as practicable. The following method has been found to give a fairly constant lead line : First, each lead line should have its own sized lead and not be subject to different tensions from leads of different weights ; second, before marking, let the line, with lead attached, drag after the vessel for several hours a day for two or three days, and afterwards keep the line soaked in salt water; third, mark the fathoms with line under a tension equal to the weight of the lead, laying off the marks with a steel tape; the intermediate marks can be put in with line extended on the deck, averaging the spaces.

355. Verification. — The load line must be verified by the officer in charge at the beginning and end of each day's work, and the corrections in feet and tenths recorded in the sounding record or a statement entered that lead line is correct. In verifying the line care should be taken to apply a pull equal to that of the lead in water.

356. Permanent marks may be placed on a deck or a wharf with copper tacks, and the verification of lead line can then be quickly accomplished.

357. The record in the sounding book of the comparison of lead lines should be so explicit as to avoid any possibility of error in applying the correction to soundings, and the following form is recommended :

Mark on lead line = M.

True length on tape or standard= L.

Correction to soundings = L-M.

Urn. 2fm. 3 fin.

5.8ft. 11.9ft. 18. 1 ft.

•-0.2ft. -O.lft. +0. 1 ft.

126 GENERAL INSTRUCTIONS FOR FIELD WOKK.

358. The minus sign indicates that the lead line is too short, so that the depths obtained with it appear too large and the correc- tion to the soundings is subtractive. The plus sign indicates that the lead line is too long, so that the depths obtained with it appear too small and the correction to the soundings is additive.

359. The lead-line correction may be neglected if not exceeding one-half of 1 per cent.

360. Lead lines are marked as follows:

1 fathom-. — A piece of leather with one strip.

2 fathoms. — A piece of leather with two strips.

3 fathoms. — A piece of leather with three strips.

4 fathom*. — A piece of leather with four strips.

5 fathoms. — White rag.

6 fatJioms. — A piece of leather with one strip.

7 fathoms. — Red rag.

S fathoms. — A piece of leather with three strips. 9 fathoms. — A piece of leather with four strips.

10 fathoms. — A piece of leather with a hole in it.

11 fathoms. — A piece of leather with one strip.

12 fathoms. — A piece of leather with two strips. 18 fathoms. — Blue rag.

14 fathoms. — A piece of leather with four strips.

15 fathoms. — Same as 5.

16 fathoms. — A piece of leather with one strip. 11 fathoms. — Same as 7.

18 fathoms. — A piece of leather with three strips.

19 fathoms. — A piece of leather with four strips.

20 fathoms. — Two knots.

361. Up to 5 fathoms the line should be marked with small white cord for every foot, the half-fathom mark being distin- guished by a cord with a knot, and this designation for half fathoms- should continue to 10 fathoms.

362. Sounding poles instead of lines may be used in shoal depths.

363. Sextant glasses. — A sufficient supply of spare sextant glasses should be kept on hand. When the glasses become unserviceable they should be returned to the office. Sextant glasses are ex- pensive, and precaution should be taken against their being lost, broken, or scratched.

364. In case of emergency sextant glasses may be resilvered in the field by the following method: The necessary requisites are tin foil and mercury. Lay the tin foil, which should exceed the surface of the glass by a quarter of an inch on each side, on a

HYDROGRAPHY. 127

smooth pad of paper ; rub it smooth with the finger ; add a drop of mercury about the size of a small shot, which rub gently over the tin foil until it spreads itself and shows a silvered surface ; gently add sufficient mercury to cover the leaf, so that its surface is fluid. Prepare a slip of clean tissue paper the size of the tin foil. Brush the surface of the mercury gently to free it from dross. Take the glass, previously well cleaned, in the left hand and the paper in the right. Lay the paper on the mercury and the glass on it. Pressing gently on the glass withdraw the paper. Turn the glass on its face and leave it on an inclined plane to allow the mercury to flow off, which is accelerated by laying a strip of tin foil as a conductor to its lower edge. The edges may he removed after 12 hours, and in 24 hours give it a coat of var- nish made from alcohol and red sealing wax. Spare sextant glasses are now furnished with each sextant.

365. The mercury-tin amalgam, while less readily affected chemi- cally, is more liable to mechanical injury than silver, and caution is therefore necessary in handling the sextant glasses.

366. Dangers previously reported. — Existing charts and publica- tions must be carefully compared with the development of the Held work. Should a rock or shoal previously indicated on a chart or mentioned in a publication not be found during the progress of the work, the locality must be so carefully searched and the records must be so complete as to show beyond doubt that the rock or shoal does not exist. It must be specially mentioned in the descriptive report, and in this report must be given; if pos- sible, the evidence of anyone who may be deemed an authority in the matter. No rock or shoal which has found a place on the publications is removed unless it is proved beyond any doubt that such rock or shoal no longer exists.

367. Information must be obtained from all available sources. Pilots, fishermen, shipmasters, boatmen, and others living in the vicinity or acquainted with the locality, must be consulted, and every place credited with a rock or shoal, even if only by rumor, must be examined. (See par. 374.)

368. Blank areas on charts. — Surveying vessels when proceeding to or from the field of work should take opportunity, when it will not materially delay more important duties or interfere with their instructions, to obtain occasional soundings in areas on the charts where no information is at present given, particularly in the ordinary tracks of vessels.

369. Ranges for compass deviations. — Report should be made of ranges of prominent and easily distinguished objects that would

128 GENERAL INSTRUCTIONS FOR KIKIJ) WORK.

be suitable and useful for the purposes of determining the com- pass deviations of vessels in the vicinity of important harbors or anchorages.

370. Information affecting navigation, reports of dangers, and changes in aids to navigation. — All persons in the service of the Coast and Geodetic Survey should communicate to the Director any valuable information obtained affecting the interests of navi- gation along the coasts. Special reports should promptly be made of any information of the following classes, giving in each case the authority and such recommendations as may seem desirable: rocks, reefs, shoals, or sunken wrecks (with depth of water over same), either riot shown or incorrectly shown; aids to navigation differing- in any respect from the data given on the charts or in the light or buoy lists; important errors or omissions on charts or in Coast Pilots or sailing directions; changes in depths or di- rections of channels, changes in coast line, currents, etc. (See also par. 401.)

371. Determination of aids to navigation. — All aids to naviga- tion in the area of the field of work, not already located, should be determined. Even outside of the limits of proposed work, when practicable, lights and buoys established by proper authority should be determined in position and descril^ed when they are not shown on the charts or have not previously been determined by this Survey.

372. Vessels en route from one port to another, when weather and other circumstances will permit, should verify the positions of lightships and seacoast buoys. The positions of all buoys and lightships on the field of work should be accurately determined.

373. Care of property. — Reasonable and proper care should at all times be taken of property, boats, and vessels employed in the sur- vey work.

COAST PILOT.

374. The following outlines briefly the topics on which informa- tion shall be sought for publication in the Coast Pilot. It is a general guide for those whose special assignment is coast-pilot work. All officers, when in a position to do so, shall collect such information and forward coast-pi lor notes as herein directed, which shall include all the data obtained on any or all of the subjects mentioned.

(a) The Coast Pilot aims to supply all information not fur- nished on charts or in other readily available forms, which may

COAST PILOT. 129

be of use to the navigator of any craft whatsoever, regardh>s< of draft, size, or service.

(6) Inquiries shall be made of local authorities, commercial organizations, yacht clubs, and others interested, for the purpose of ascertaining their needs so far as they can be supplied by this service, or through reference to other bureaus.

(c) Relations shall be established, when possible, whereby the Survey will be advised at all times of any matters which should receive consideration in the oflice or attention in the field.

(d) Preparation for this work shall be made by collecting all data in the office or elsewhere available bearing upon the region which is the subject of investigation, such as reports of aids and dangers uncharted or incorrectly charted; examine previous pub- lications and note omitted, incomplete, or erroneous information; make study of United States Army Engineers' blue prints of surveys of improved areas in order to determine necessity for extension of their surveys to cover indicated changes beyond the limits of their work ; note localities requiring examination on account of the incompleteness of surveys or increased im- portance of locality; ascertain fmrn the office in what localities our data on tides and currents are incomplete and should be siipplemented in the field ; consult Senate and House documents on examinations, surveys, reports, and improvements in regions under consideration.

Bromides of original sheets may be required where a chart is inadequate by reason of scale or lack of detail for purposes of field examination.

(e) In the field, data will be collected from all available sources ; offices of the United States Army Engineers should be visited to obtain results of their surveys and examinations, pro- gram of proposed operations, and information on subjects useful to the Survey.

(f) Application for similar information shall be made, per- sonally if possible, to municipal engineers in charge of water fronts, engineer departments of railways controlling deep-water terminals, State authorities or others engaged in the development or operation of waterways, mariners and other individuals in- terested in shipping.

(g) Travel to and from the field of work and movements by members of the party while on the working ground shall be by steamer as far as possible in order to collect data from local masters and pilots making the runs. On the working ground,

13027°— 21 9

130 GENERAL INSTRUCTIONS FOR FIELD WORK.

visits should be made to lightships, tenders, and stations of the Bureau of Lighthouses, and officers and employees interviewed. No general rules can be laid down, but the following points will be suggestive so far as applicable to any particular region. The amount of detail to be given requires much judgment, as over- minute details tend to obscure the most useful facts. Obviously certain classes of information may be useful as in a new country previously unsurveyed which may not be necessary to give in con- nection with a well-known coast.

375. General description of the coast, following the geographic sequence of the published Coast Pilots, and including the aspect or appearance of the coast on making the land ; describing promi- nent objects, as, on a bold coast, the headlands, peaks, etc., with their form, color, and height ; or, on a flat coast, the water tanks, spires, beacons, etc. Especially describe the first landfall and objects useful as guides to navigation.

376. Outlying dangers and islands, the limits of tide rips and breakers, and their relation to wind and tide.

377. Landmarks. — Description of all prominent landmarks likely to be useful to navigation or to future surveying operations. If mountains, state whether summits are often clouded. Give measured or estimated heights of mountains, hills, cliffs, islets, or rocks referred to. Describe ranges in use by pilots and means of identifying them. Suggestions should be made as to other ranges that would be useful or as to artificial marks that it would be de- sirable to erect. (See par. 194, "Topography.")

378. Directions for passing the outlying dangers.

379. Refuge. — In case of stress of weather the best anchorage or the nearest harbor of refuge to run for ; or in extreme cases of damage the best place to beach a ship. Locate and describe life- saving stations and houses of refuge. Give character of beach and behavior of vessel in breakers.

380. Pilots. — Information as to their station or cruising ground, any special regulations or signals, their charges, the possibility of obtaining tugs, etc. ; anchorage while awaiting pilot or tug.

381. Approaches. — General remarks, usual course from along- shore or from sea, dependence on lead, approaching in thick weather.

382. Bars. — Describe principal marks and aids. Give directions for approach, with description of outlying and other dangers and how to avoid them. Least depth and width at best place for crossing bar; most favorable time to enter. Does bar break in ordinary or only in heavy weather? How far out do breakers extend?

COAST PILOT. 131

Give velocity and direction of wind and stage of tide producing these conditions. Can entrance be made while bar is breaking^ and, if so, for what draft? Give character of bottom, and usual allowance made for squat, pitch, and tides under different condi- tions on the bar. To what change in depth and position is the bar channel or approaches subject ; if any, give magnitude and direction of change. (See Currents.)

383. Channels. — Give minimum available depth throughout and where necessary the minimum width. Give character of bottom and describe all aids and natural objects. Are channel banks defined by grass or other growth, color of slioals, or in any other readily recognizable manner?

State maximum draft possible and greatest draft entering or leaving. Where maximum draft differs from minimum depth in channel state reasons for same, as swell, squat, tide, and rocky or soft bottom. Note depth and character of approaches to wharves, piers, dry docks, marine railways, and coal stations. Manner of approaching them and why. Are channels permanent, subject to considerable or frequent change, under improvement, or maintained?

384. Description of the shore, with characteristics (as height, color, wooded, cultivated, bold, sandy) of each important head- land, point, island, and rock. j

385. Inshore dangers. — Extent and nature, least depth over them ; whether visible ; if breaking, at what stage of tide ; how much, if any, is bare at low water; marks or ranges for clearing them by day or night. In regions where dangerous shoal {ireas or pinnacles are marked by kelp or other growth state the ordi- nary significance of such growths, at what stage of tide they show at surface, and when, if ever, they are towed under.

In regions where bowlders, ledge, coral heads, or similar dangers probably exist it is very desirable to examine the sus- pected areas at extreme low water, at which time important fea- tures may show above or near the surface.

In the examination of entrances and approaches for off-lying dangers, advantage should be taken of heavy weather to locate any shoals marked by breakers. The existence of rocks or other shoals in localities of considerable current is often indicated by rips and swirls ; such disturbances should be rioted at strength of current and investigated.

386. Ports. — Commercial importance, character, and magnitude of trade, chief exports and imports, facilities for coaling and watering vessels, supplies and provisions obtainable, facilities for

132 GENERAL INSTRUCTIONS FOR FIELD WORK.

repairs to hulls and machinery, marine railways or dry docks ,( length, draft forward and aft, and tonnage hauled), wharves, piers, and docks, and depth of water and character of bottom alongside and in approaches, whether public or private, and rules for use, harbor regulations, means of communication.

Locate and describe customary anchorage, custoinhou.se and landing, time ball, station for reporting vessels, storm warning and small craft warning display stations, quarantine stations. hospitals for mariners ; and obtain copies of all published pilot, harbor, and anchorage rules and regulations where possible. Note harbor improvements in progress or projected.

387. Sailing directions for approaching, entering, and leaving channels and harbors ; such directions should be actually tried under different conditions and verified before they are adopted or recommended for use.

Verify ranges and determine and describe any natural ranges or leading marks, defining sailing lines, points of change of course, dangers, and other features.

Check bearings or obtain azimuth of dredged channel axes. Wherever possible locate aids by means of ranges, bearings, or angles for use as checks on their position. Add any useful details not given in light list, reliability of lights and buoys, visibility of lights and audibility of fog signals. Note localities of unusual sound reflection. Locate and describe marks and aids, whether natural objects or others, used in connection with works of im- provement which may serve as navigational guides. Locate and describe fish weirs, oyster stakes, and similar constructions, also day marks and lights maintained on them. Give rules and regu- lations relating to them.

388. Aids. — Lights, lighthouses, buoys, beacons, and other aids shall be verified on the ground for location, description, depth alongside, and in relation to the features they are intended to mark. Note buoys which tow under or do not watch properly. Where the distinctive characteristic of an aid is its color state whether it is generally clearly distinguishable.

389. Currents, tidal or nontidal. — General conclusions from ob- servations or other information. Give velocity, direction, dura- tion, and relation of time of slack to that of high water or low water. Note set with reference to axis of channel and openings through bridges and at other contractions of the fairway, across bars and in entrances; approaching docks and piers; occurrence of rips, swirls, and eddies. Effect of wind and freshets on cur- rents, and if flood current is ever entirely overcome. Describe

COAST PILOT. 133

fully all abnormalities in currents or marked variations from usual phenomena. The notes shall cover the entire field of oper- ations, both inshore and offshore, and include all horizontal move- ments of surface waters, whether tidal or nontidal, or both. Where the currents are due to winds or other meteorologie cause or are greatly modified in velocity, direction, and duration thereby, the variations produced shall be determined and their relation to the conditions that produce them shall be developed if possible, such as velocity and direction of wind.

390. Tides. — Collect all available data bearing upon tides, ex- cessive or abnormal rise and fall, duration of stand, and time of high and low water. Where gauges are in operation make special effort to obtain records of such conditions; whether due to wind, flood, or other causes. Note rate of progress of the tidal wave and the variation of amplitude along its course.

In shoal areas, especially inclosed waters, give variation of. surface level due to storms and, if possible, develop relation of velocity and direction of wind to locality and magnitude-of result- ing changes in surface elevation.

391. Bridges, contractions, and obstructions. — Note kind of bridge, horizontal and vertical clearance at high water of open- ings through fixed spans, draws when closed, and at other con- tractions; also depth in openings. Length, beam, and draft ac- commodated by locks and similar structures. Vertical clearance of aerial cables and trolley wires. Which side of draw or open- ing of bridge should be used, and if only one, state for what reason. Obtain copies of rules and regulations governing the operation of bridges and locks and signals in use.

392. Ice. — Season during which it is encountered. Its form of occurrence; movements under influence of winds and currents; extent to which it affects navigation. Season of navigation as affected by ice, flood, fog, low water, storms, and in addition in the case of canals and other artificial waterways give period of navigation fixed by law or regulation.

393. Bivers. — Give draft and class of vessels which can enter ; point to which tide reaches; depth on bars and permanency of channel ; strength of current ; effect of freshets ; distance to head of navigation for steamers and other craft.

394. Canals. — Describe location and approaches, give total length of each lock, capacity of locks, controlling vertical clear- ance under overhead structures, passing points, tie-up points, local contractions, variations of surface elevation, period open to navi- gation, rules, signals, and regulations governing operation.

134 GENERAL INSTRUCTIONS FOR FIELD WOKK.

395. Anchorages, with descriptions relative to their capacity, holding ground, amount of protection, and circumstances of weather under which tested. Character of bottom, marks, and rules and regulations for use, if any.

396. landing places, especially on a coast exposed to swell.

397. Watering places for vessels — Rivers, streams, or springs. — At ports state whether water is piped to wharves or supplied by water boats, and charges; convenience for watering ships. Give distance upstream that salt water extends at different seasons or under other varying conditions; state also when overboard water is sufficiently fresh for drinking or boiler purposes.

398. Weather. — Under this head state briefly only new and im- portant facts, as prevailing winds and their seasons, directions from which gales come and how they affect anchorage, land and sea breezes, rainy seasons, fogs, and freshets, and seasons or conditions when prevalent.

399. Wrecks, where usually occurring; tendency of wrecks to break up or remain in position. Information bearing upon the occurrence of wrecks is exceedingly desirable. It is of first im- portance that inquiries be made and investigations instituted for the purpose of developing the various causes contributing in any way to marine disasters, such as little known currents, addi- tional aids required, misleading or deceptive bottom relief in approaches, shifting shoals and channels, imperfect or inadequate charts.

400. Change of coast line or depths. — Mention any reliable evi- dence as to recession or growth of shore line or change of depths. Note any important facts regarding changes observed. Give evi- dence, if any, of subsidence or emergence of shores. Locate and outline limits of dumping ground for dredged or other materials. Give location of submarine cables and water mains. Define limits and give regulations for forbidden anchorages. Report all obstructions and temporarily obstructive operations. Note espe- cially localities where changes of any sort affecting navigation are likely to occur and report those which should receive frequent attention in order to keep publications up to date.

401. Information of importance affecting navigation, such as rocks, reefs, shoals, sunken wrecks, aids omitted or incorrectly charted, errors or omissions on charts or in Coast Pilots, changes in depth, channels, coast line, and currents shall be forwarded to the Director without delay for insertion in the Notice to Mar- iners or other immediate publication. Where aids are involved a report shall be forwarded, without recommendation, to the

COAST PILOT. 135

lighthouse inspector and a copy sent to the Director. Sugges- tions or recommendations in regard to aids shall be sent only to the Director.

(a) To avoid delay, inspectors and chiefs of parties on the Pacific coast and Alaska are authorized, whenever the Survey develops rocks or other important information that should reach the public promptly, to issue a notice furnishing such information. A copy of such notice, with full details, will be mailed to the Washington office at once.

402. Inside route pilot. — (a) Special effort shall be made to col- lect information which will contribute toward the completeness of our publications covering the inside routes through inland waters and all entrances and inlets affording access to such routes and with special reference to the needs of motor boats.

( 6 ) Note extent of routes and period of navigation ; draft that can be accommodated at low water and at high water; width of opening and vertical clearance under both fixed, spans and draw- bridges ; rules and regulations for operation of drawbridges ; draft in canals; length and width of locks; obstructions; tolls; period of navigation ; attended or unattended bridges ; rules, regulations, und signals.

(c) Navigability of tributary rivers, creeks, and other water- ways, and distance from mouth to falls, rapids, dams, or other head of navigation, including depths and distances to various points.

(d) At points where boats must wait for the tide the time of local high water referred to some known point should be given. Character of bottom shall be determined generally, especially at local shoalings such as cross-overs, bars, oyster reefs, etc. Outline :md describe snng infested and stump infested areas.

(e) The extent to which the tide affects the inland waters and variation of surface due to winds and floods should be noted. Localities of strong currents should be mentioned with at least approximate velocities and relation of time of occurrence to that of local or other high or low waters. Note points where naviga- tion depends upon freshet conditions or high water due to rain.

(/) Locate and describe, in relation to the channels or other features they are intended to mark, all aids to navigation, includ- ing private aids such as pointers, poles, bush stakes, beacons, buoys, etc. Note buoys which tow under or do not watch properly. Where the distinctive characteristic of an aid is its color, state whether it is generally clearly distinguishable.

136 GENERAL INSTRUCTIONS FOR FIELD WORK.

(0) Give points where services of pilots are necessary, where they can be obtained and rates, extent of traffic, development or settlement of region, and mention places where supplies, provi- sions, gasoline, oil, water, ice, and hotel accommodations can be obtained; also facilities for watering vessels and coaling, where small craft can be repaired, hulls, machinery, or both; give loca- tion of marine railways and the length, beam, draft forward and aft and tonnage they can haul.

(h) In remote districts telegraph, telephone, railroad, steam- boat, or other lines of communication and postal facilities should be described.

(t) Occurrence of obstructive growths such as hyacinth, grass, kelp, etc., and season in which they are found, occurrence of ice and to what extent and for what period navigation is affected.

(j) Note especially localities where changes of any sort affect- ing navigation are likely to occur and which should receive fre- quent attention in order to keep publications up to date.

(k) Entrances and inlets. — Describe principal marks and aids: directions for approaching entrance with description of outlying or other dangers and how avoided; least depth and width in entrance, across bar in channel, and locate same. To what change in depth and position is entrance subject; if any, give rate and direction.

(1) Does bar break in ordinary or only heavy weather? Give direction and velocity of wind producing this effect. How far out do breakers extend? Can entrance be made while bar is break- ing and, if so, for what draft?

TIDES.

403. Purpose. — Tide observations are made in connection with hydrographic work in order to furnish data for computing the plane of reference, for reducing the soundings to that datum, and for use in making predictions and giving tidal information for the Tide Tables, Coast Pilots, and Charts. Tide observations also furnish determinations of mean sea level for use in connection with precise leveling, and give information valuable for other en- gineering and scientific purposes. One or more tide gauges must be maintained in connection with all hydrographic work, and the tide observations should be made as complete as circumstances will permit.

404. location of gauge. — The corrections necessary to refer soundings to the adopted plane of reference shall be in error not

TIDES. 137

more than one-fifth of the allowable error in the determination of depth ; where the allowable error in depth is one-half foot or less the correction shall not be in error more than one-tenth foot. Gauges shall be located so as to approximate the conditions on the working ground within the limits stated. Should doubt attach to any gauge in use, a temporary staff shall be established in the im- mediate vicinity of the work and the .results at the gauge in ques- tion verified. In connection with hydrographic work, it will in general be desirable to set up an automatic tide gauge at some cen- tral point, and continue it there throughout the season; subsidiary tide staffs are also to be established in the immediate vicinity of the work, as may be necessary. In selecting a site for a tide gauge, existing facilities and the accessibility of the location to the observer must generally be taken into account. It will be conven- ient to place an automatic tide gauge on a wharf if a substantial one exists. It is important, however, that the location shall be such that there is free communication with the sea, shelter from storm waves, and deep water close to the position at low tides. For use in connection with the hydrography on the outer coast it is advisable to avoid a location for a gauge well inside of a river mouth or shallow estuary, or in any body of water having only a narrow connection with the sea. For surveys of offshore bars and exposed channel approaches, where especially accurate soundings will be required for which the record of an inshore tide staff will not be sufficient, a temporary tide staff should be established by pumping down a scantling, or otherwise.

405. Abnormal tides due to configuration of shore. — In straits connecting two areas having tides of different ranges and epochs of occurrence, it will usually be found that there is a portion of the strait in which the tide varies rapidly from place to place. For instance, within a single mile at Hell Gate. East River, N. Y., the time of tide changes about an hour, and the mean range varies about 1 foot. Similarly, in the channel north of Vancouver Island, British Columbia, there is a difference of about 2 hours in the time of tide, and of about 5 feet in the mean range, within a few miles. There may be an appreciable difference in both time and height of tide on the different sides of the same island in an archi- pelago, for it often happens that rapid changes occur in the tides and currents of such groups. Sometimes the occurrence of a shoal near one end of a rather small detached island will cause the tides to differ considerably on opposite sides of the island, depending upon the location of the shoal with reference to the approach of the tide wave.

138 GENERAL INSTRUCTIONS FOR FIELD WORK.

406. Abnormal tides due to wind and shallow water. — lu large shallow bays, in broad stretches of rivers or along shores where the water is shoal, the wind sometimes has considerable effect upon the level 'of the water surface, and two tide stations dif- ferently exposed to the wind may be affected unequally.

Failure to detect the conditions that cause abnormal tides, and to adopt measures that will permit the elimination of these ab- normalities, has caused sounding lines to fail to cross by several feet, and, in some instances, has seriously vitiated the survey.

Where the conditions described prevail, an additional staff should be established in the immediate location of the work, and comparison of the tide observations made with those of the stand- ard gauge. The observations at the auxiliary gauge should in- clude one complete range of the tide on each day that the gauge is used, unless its bench mark is connected with the bench mark of the automatic tide gauge by a line of levels. In that case the observations at the auxiliary gauge may cover only those hours during which sounding dependent upon the gauge is being done.

407. Tide gauges. — There are four types of tide gauges, known as tide staffs, box gauges, pressure gauges, and automatic gauges. The plain tide staff is the simplest and most common form of tide gauge, and whenever possible should be used in connection with the other kinds of tide gauges in order to refer the tidal results to permanent bench marks. The box gauge is sometimes employed when the water is too rough to obtain satisfactory readings from a plain staff. The pressure gauge may be used from a vessel at anchor where the water is too deep to erect a tide staff. The automatic gauge is used when a continuous record of the tide for a long period is desired.

408. Tide staff. — The simplest form of the gauge is a plain board, about 5 inches wide, 1 inch thick, and the length should be based on the extreme fluctuation of the water surface in the locality in which it is to be used. It should be graduated to feet and tenths (not inches) with numbers increasing upward, the lowest gradua- tion being zero (0). It must be securely fastened in a vertical position to a pile or other suitable support. The zero should be placed lower than the lowest known tide, and a temporary bench mark established as close to it as practicable, in order that the staff may be readily returned to its original position if displaced by accident. For reference to permanent bench marks see para- graphs 514-520. It is desirable that the tide staff be painted, espe- cially if it is to be used for a considerable length of time. In the

TIDES. 139

latter case the graduation should be cut into the wood on the edge of the staff.

409. Glass tube. — When the water is rough it is a great advan- tage to have a glass tube, partially closed at the submerged end by a. notched cork or otherwise, secured to the face of the staff. Some floating object should he introduced into the tube in order to give definition to the water line within, such as a ball cut out of cork and scorched to blacken it, or a little colored oil. This ball should be somewhat smaller than the bore of the tube, as it otherwise tends to increase the capillarity in the tube and indi- cate heights A little above the true.

410. Portable staff. — It will sometimes be preferable to use a movable tide staff, which may be taken out and cleaned or re- painted or removed for use in another locality. A guideboard is secured in a vertical position to a pile or other suitable support, and pieces nailed to it so as to form grooves in which to slide the tide staff, exposing only its graduations. This guideboard need not extend lower than mean tide level. A metal plate is screwed to the top of the guicleboarcl, forming a stable support for a metal shoulder which is fastened upon the back of the movable staff. This metal plate serves as a temporary bench mark, whose rela- tion to the zero of the graduation must be stated.

411. Portable staffs are recommended for use especially in con- nection with automatic tide gauges at stations where the observa- tions are to be continued for several years. An ordinary fixed staff left in the water soon becomes discolored and the gradua- tions illegible, necessitating frequent removals for painting. A poi'table staff that is placed in the water only when it is to be read and is kept in the tide house at other times remains in a good condition much longer. For convenience in storing in the tide house the staff may be in sections fastened together with a hinge, which should usually be placed on the face of the staff so as not to interfere when the latter is lowered in its guides. As it may sometimes be convenient to use the same portable staff at different stations a standard size is desirable. It is recom- mended that such staffs be made 5 inches wide and 1 inch thick or slightly less when dressed. The guides should be constructed with the sliding space about 5* inches by 1J inches to provide sufficient margin for the staff to be lowered without binding.

412. Multiple staffs. — When the range of tide is too great to be measured by a single staff, a succession of staffs may sometimes be used along shores with gentle slopes. A field glass will usually enable the observer to read the outer staff. When possible,

140 GENERAL INSTRUCTIONS FOR FIELD WORK.

these should be arranged so that the graduations will be con- tinuous from one staff to the other, so that the readings on all the staffs will be referred to the same zero.

413. Box gauge. — This form may sometimes be employed wli<m more accurate readings are required or when the swell renders the use of a simple staff inconvenient. It consists essentially of a float that rises and falls in a vertical box to which the tide has access; and to the float is attached either an index that moves over a fixed scale or a graduated rod that moves over an index. The float is usually a copper cylinder about S$ inches in diameter and 3 inches high, the bottom being weighted to give it steadiness in the water. The top may be provided with u socket for a graduated rod or with a ring for an index wire or cord. The float box is similar to that used for an automatic gauge (see pars. 441 and 442 for description), but the opening in the bottom may be somewhat larger. A hole 1$ inches in diameter in a float box that is 12 inches in diameter will permit the water to enter freely enough to give a perceptible slow motion to the float when the water outside is rough. Such a motion is desirable in order that the observer may be assured that the box is not clogged with sediment or other obstruction.

414. A convenient form of box gauge, where the range of tide is not too great, is to have a light wooden rod fixed in a socket on top of float and steadied by cross pieces at the upper end of the float box, so as to rise and fall in a vertical line. The rod should be numbered from above downward, zero being at the upper end. The top of the box, or of a board or a piece of sheet metal placed at a convenient height for the eye, is used as the reading point.

415. In another form of box gauge one end of a small flexible wire is attached to a ring in the top of the float, while the wire passes over one or more pulleys and terminates with a counter- poise. An index on the wire passes over a fixed scale, which may for convenience be either vertical or horizontal. The scale is usually that of nature, but sometimes it is desirable to either increase or diminish the scale, which can be done by passing wiivs over drums of different sizes or by means of movable pulleys.

416. A form of box gauge adapted especially for use in the rough waters on shoals offshore consists of a white-pine pole staff, cross section 1 by 1 inch, with rounded edges, graduated on each of the four sides in feet and two-tenths with the zero (0) at the top, and set in a hollow cylindrical white-pine float 1$ inches

TIDES. 141

outside diameter and seven-eighths inch inside diameter. The float should be thoroughly covered with shellac and liquid par- affin. The length of the rod will depend upon the range of tide in the locality where it is to be used, and the length of the float should be about four-tenths that of the rod. The float well con- sists of a 2-inch iron pipe, the bottom of which is set in a 1000-pound concrete block to serve as an anchor. The pipe should be long enough to reach above the ordinary waves at high tide, and a one-fourth inch hole should be drilled in the side several feet above the concrete anchor. A cap with a square hole for the .staff is to be .screwed on top of pipe after the float staff has been placed inside. Just below the cap, a 2-inch flange for the at- tachment of guy wires may be screwed on the pipe, and four small sheaves, one for each guy wire, secured to this flange by wire loops. The top of the pipe is to bo secured by four guy wires of No. 6 wire with leads making an angle of 60° of more with the vertical. The end of each guy wire is to be anchored with concrete blocks, giving a total weight of about 2000 pounds to each anchor. For convenience in handling, each concrete block may be cast with wire-rope loops projecting. After the anchors have been set the guy wires are led through the sheaves at the top of the float pipe and drawn taut, a fence-wire stretcher- being convenient for this purpose.

417. Reference of box gauge to bench mark. — The reference of a box gauge to a bench mark should be such as to indicate clearly the elevation of the mark above the water surface when the icauge reads -zero (0), the position of the water surface at this time being the true datum of the box-gauge readings. This refer- ence can generally be most satisfactorily obtained by erecting a plain fixed staff near by, the zero of which is referred to the bench marks by spirit levels. Sirmiltaneous readings of the water on both gauges are taken when the water is reasonably smooth. A comparison of these readings will give the difference between the datums of the two gauges, which difference applied to the eleva- tion of the bench mark above the zero of the fixed staff will give the elevation of the bench above the datum of the box gauge. The difference between the box-gauge readings and the fixed-staff readings should be frequently checked, as a small leak in the float of the box gauge might change the line of flotation and conse- quently modify the datum.

418. The relation of a bench mark to the box-gauge datum may also be obtained directly as follows : (a) When a graduated rod,

142 GENERAL INSTRUCTIONS FOR FIELD WORK.

with scale inverted, is attached to the float and moves over an index known as the reading point. In this case obtain the eleva- tion of the bench mark above the reading point and add the length of the float rod as measured from the zero (0) graduation to the water line on the float. In case the bench mark is belo\\ the reading point, this distance should of course be subtracted from the length of the float rod. (6) When an ungraduated rod with an index is attached to the float and moves over a fixed vertical scale, the graduations increasing upward, the elevation of the bench mark above the zero of the scale should be added to the length of the float rod as measured from the index to the water line on the float, (c) When a cord or wire connects the Hoat with an index, a direct measurement is usually impracticable. In this case the elevation of the bencli mark above the water sur- face should be obtained when the water is smooth, and to this should be added the box-gauge reading taken at the same time. In every case the separate measurements must be entered in the record in order that the results may be properly interpreted and verified.

419. Pressure gauge. — This is an instrument for determining the tide by measuring the variation in pressure at the bottom, due to the rise and fall of the water. Although the results are very rough as compared with those obtained by a plain tide staff, the gauge is serviceable in obtaining the approximate tides on shoals where the water is too deep to erect an ordinary tide gauge but is sufficiently shoal to anchor a boat. A simple form of this gauge formerly used by this Survey consisted of a strong rubber bag, holding about 6 gallons, connected with a flexible, air-tight tube, having an inside bore about one-fourth inch, and made in sections like garden hose. The upper end of the tube has a stopcock and a steam gauge. The bag is incased in an iron box, which is nearly water-tight, so as to exclude the influence of short-period waves, the same as for a box gauge. The iron box containing the inflated bag is lowered to the bottom, the gauge on board the vessel being read at intervals, the rise and fall of tide being indicated by change of pressure.

420. The aero-mercurial gauge and manometer, two other forms of pressure tide gauges, have also been used and description of them can be furnished when desired.

421. Automatic tide gauge. — This machine, which is known a!*> as a self-registering gauge, traces a curve that graphically repre- sents the rise and fall of the tide, the abscissse indicating time

TIDES. 143

and the ordinates the height of the tide. The essential parts of the machine consist of a clock that moves paper forward at a uniform speed and a float that is free to rise and fall with the tide, and is so connected with a tracing pencil that the latter moves perpendicularly to the motion of the paper and propor- tional to the rise and fall of the tide. The combined motion of the paper and the pencil produces a tide curve, sometimes called ; marigram, from which the height of the tide for any desired time can be read by means of a scale. The automatic tide gauge now in use by this Survey is illustrated in figures 18 and 19 and is described in the following paragraphs:

422. Clocks. — There are two clocks, as illustrated in figure 18. The one on the right-hand side, No. 1 in the figure, is the motor dock, and the other one, No. 2, is the time clock. The motor clock turns the main cylinder which regulates the motion of the paper. The cylinder, which is 12 inches in circumference, turns once in 12 hours, moving the paper forward at the rate of 1 inch per hour. The motor clock lias two mainsprings, both being connected with the driving apparatus, and in case of one of these breaking by accident it is sometimes possible to operate the machine with the remaining one until there is an opportunity for repairs. The purpose of the time clock is to mark the hours on the record. It is similar to an ordinary striking clock; one spring runs the clock und the other operates a device that trips the recording pencil, making a short horizontal mark on the record each hour.

423. Each of the clocks will run eight days with one winding, but it is recommended that they be wound twice a week. The clocks may be regulated and corrected as similar clocks in ordi- nary use. The minute hand of the time clock must not be turned backward when it is between 10 minutes before and 5 minutes after the hour mark " XII," in order to avoid injury to the hour- marking device.

424. Rollers. — There are three rollers; the xuintly roller (Jig. 19. No. 3), a round brass rod with flanges at each end, one of which is removable for putting in the paper; the main cyliwltr (figs. 18 and 19, No. 4) ; a hollow drum made of brass tubing about 1 foot in circumference, with 12 sharpened steel pins set at equal intervals around the cylinder near each end. designed to prevent the paper from slipping over the smooth surface of the cylinder as it is revolved by ihe motor clock; and the receiving roller (fig. 18, No. 5), a hollow tube of sheet brass, with a small slit running its entire length, and two flanges, one of which is removable, so that the completed tide roll may be removed.

144 GENERAL INSTRUCTIONS FOR FIEIJ) WORK.

425. Connection between clock and gauge. — The axle of the hour hand of the motor clock extends through the back of the case and has a toothed or carrier wheel upon it (fig. 19, No. 6). The main cylinder has a hinged carrier arm attached to its axis, which can be thrown into or out of the teeth of the carrier wheel, thus making or breaking the connection between the motor clock and the gauge.

426. Paper. — The paper used on the machine is about 13 inches wide and is furnished in rolls about 66 feet long, which is sufficient for one month of record. This roll is placed on the supply roller, fed over the main cylinder, where it receives the tidal record, and is then mechanically wound upon the receiving roller.

427. Tension weight. — This is the smaller of the two weights furnished with the machine. It is attached to a cord (fig. 18, No. 22), which is wound around the tension ivciglit pulley (figs. 18 and 19, No. 13). This pulley is provided with a pawl and ratchet for winding up the tension weight from time to time. The action of the tension weight winds the tide roll on the receiving roller, keeps the paper on this side of the machine taut, and also assists the motor clock in turning the main cylinder.

428. Tension spring (fig. 19, No. 21). — This spring presses against the supply roll of paper and keeps the paper on that side of the machine taut. As the paper is prevented from slipping over the main cylinder by a set of sharpened pins (par. 424), an excess of tension on either side of this cylinder is likely to cause the paper to tear, especially if it is damp.

429. Float. — The float furnished with the title gauge is a copper cylinder, 8* inches in diameter, 3 inches high, and weighted so as to float with about one-third of its height above the water surface. It is connected with the float pulley of the machine by phosphor-bronze wire, No. 23 American wire gauge. When in use the float rises and falls with the tide in a float box to which the water has access through a small opening. (See par. 441.)

430. Float pulley (figs. 18 and 19, No. 11). — A set of four inter- changeable pulleys with circumferences of 6 inches, 9 inches, 12 inches, and 16 inches is provided with each machine in order to adapt it to different ranges of tide. Special pulleys of other sizes are also sometimes used. These pulleys are about 1 inch wide and have threads cut in their faces to prevent the float wire, one end of which is attached near the edge of the pulley, from winding upon itself. There are from 18 to 24 turns of the thread on each pulley, For the removal or adjustment of the pulley there

TIDES. 145

are two clamp nuts (fig. 19, No. 23), which are set by means of a special wrench (fig. 19, No. 24). This pulley together with the counterpoise pulley is clamped to the pencil screw (fig. 19, No. 8), and causes the latter to turn as the tide rises and falls.

431. Counterpoise pulley (fig. 19, No. 10). — This pulley, which is threaded like the float pulley to which it is clamped, carries a wire or cord to which is attached a counterpoise weight.

432. Counterpoise weight. — This is the larger of the two weights provided with the tide gauge. Attached by a wire or cord to the counterpoise pulley it serves to take up all slack in the float wire, and rewinds the latter as the tide rises.

433. Sliding grooved pulley (fig. 19, No. 12).— This is free to slide on a long axle near the counterpoise pulley. It is designed to carry the counterpoise cord away from the float wire, and at the same time keep the wire as it winds or unwinds always oppo- site its proper thread on the counterpoise pulley. When the coun- terpoise cord is carried directly to a fixed pulley in the ceiling of the tide house, this sliding pulley is unnecessary.

434. Pencil screw (fig. 19, No. 8). — This is made of phosphor bronze about five-eighths inch in diameter, and has a square thread with a 1-inch pitch. For stations having a large range of tide, a pencil screw with a one-half inch pitch is frequently used. The threads at each end of the pencil screw are turned down to prevent the pencil arm from jamming.

435. Pencil arm (figs. 18 and 19, No. 9). — This arm carries the recording pencil. In its bearing is a nut that fits in the thread of the pencil screw so that as the latter is turned the arm moves along the screw, toward the clocks for a rising tide and in the reverse direction for a falling tide. If a very high or a very low tide moves the arm to either end of the pencil screw, the nut becomes disengaged from the screw thread and jamming is pre- vented. Springs are provided on each side of the pencil arm to force the nut back into the thread of the screw when the tide begins to reverse.

436. Datum pencil holder (fig. 19, No. 15). — This holds the pen- cil that traces the datum line. It may be clamped in any position on the datum pencil rod (fig. 18, No. 14), but it is desirable to have i; clamped near the middle of the rod.

437. Scale. — The height scale of the gauge depends upon the circumference of the float pulley and the pitch of the pencil screw. These should be chosen according to the probable range of tide at

13027°— 21 10

146

GENERAL INSTRUCTIONS KOK HELD WORK.

the station. The following table will indicate the proper pulleys and pencil screw to he used :

Extreme range.

Scale.

Float pul- lev circum- ference.

Pencil screw pitch.

Less than 6 feet

1:6

Inches. 6

Inch. 1

From 6 to 9 feet

1-9

9

1

From 9 to 12 feet

1:12

12

1

From 12 to 13 feet

1:16

16

1

From 16 to 18 feet

1:18

9

From 18 to 24 feet

1:24

12

]

From 24 to 32 feet

1:32

16

3

438. A metal scale (fig. 19, No. 16) is attached to the gauge by a clamping screw at each end. Both sides and both edges are graduated, making four different scales of 1:6, 1:9, 1 : 12, and

FIG. 20.

1 : 16. Any of these edges may be turned toward the main cyl- inder. To refer a point on the tide curve to the scale, there is a broad, two-pronged fork (fig. 19, No. 17), which may be slid along the scale.

439. Hour-marking device. — The time clock is like an ordinary striking clock, but instead of striking a bell it moves a lever (figs. 19 and 20, No. 18). connected with a tripping rod (figs. 18 ami 20, No. 19), which is supported on rocker arms about 2 inches above the metal scale. The pencil arm is jointed and provided with a hook which passes under or over the tripping rod. On the

TIDES. 147

hour the striking mechanism of the clock moves the lever outside, which in turn rocks the tripping rod. The latter engages the pencil hook and pulls the pencil arm, causing the pencil to make u short stroke parallel to the edges of the paper. The pencil, which is tracing the tide curve, is returned to position by a spring.

440. Installation of an automatic tide gauge. — When an auto- matic tide gauge is to be established at a station, a fixed or port- able tide staff should be set up first in order that a rough esti- mate of the mean range of the tide and of the reading for mean sea level may be obtained from a few observations. The hair' sum of mean high and mean low water for any four consecutive tides may be taken as mean tide level for the purpose of setting the automatic gauge. (See also par. 408.)

441. Float box. — This should be located where the water is not less than several feet deep at the time of the lowest tides. The box should be about 1 foot square on the inside, or 1 foot in diam- eter if cylindrical in shape, and long enough to reach several feet below the low««t tides. It must be set and maintained in a ver- tical position to prevent the float from scraping against the side^ as the tide rises and falls, and should be well braced. A single opening from three-fourths to 1 inch near the bottom of the box has been found to be most satisfactory. This opening should be below the lowest tides and should be so located that it can readily be cleared out if it becomes clogged. An opening in the bottom rather than in the side of the box has the advantage of being easily cleared by means of a stiff wire or slender gas pipe lowered down into the inside of the box. This enables the tide observer to do in a few minutes that which would often require the services of an extra man and a boat when the opening is in the side of the box. When necessary to remove the float attach a small clamp to the wire just beneath the table to prevent unwinding on the drum.

442. For a station that is to be occupied for only a short time a plain square wooden box made of boards or plank from 1 to 2 inches thick is sufficient. For stations that are to be occupied for several years a cylindrical cast-iron pipe, 1 foot inside diam- eter, is frequently used. To the bottom of such a pipe, if sup- ported several feet above the ground, is attached an inverted conical-shaped casting with a three-fourth-inch opening in the apex. The chief disadvantage that has been experienced with this kind of pipe is the formation of rust scales, which clog the opening in the bottom and are often difficult to remove. Prob- ably the most satisfactory form of float box that is used by the

148 GENERAL INSTRUCTIONS FOR FIELD WORK.

Survey at stations where the observations are continued for many years is a copper tube, 1 foot in diameter, protected by an outer wooden casing. Such a tube should be not less than one-sixteenth inch thick, as a thinner tube is not only too frail to handle but would also require more frequent renewals. A tube one-sixteenth inch thick, if not located too near other metal in the water, caus- ing electrolysis, might reasonably be expected to last for 15 years without renewal. For a longer series of observations a heavier tube would be more economical in the end. An inverted conical- shaped copper bottom with a three-fourth-inch hole in the apex should be soldered to the tube. If this bottom, instead of being a true cone, were made a little one-sided so that the apex came near the side of the tube, it would permit the hole to be cleaned by a slender rod lowered into the tube without removing the float. 443. To prevent freezing. — To prevent the formation of ice in the float box during cold weather and the consequent stoppage of the gauge, petroleum or kerosene has been frequently employed. A column of oil in the float tube of 2 or 3 feet in height would suffice for ordinary latitudes, and one of 4 or 5 feet in height, it is believed, would suffice for stations in Alaska. For a 12-inch tube it will require about 6 gallons of oil for every foot in height. If possible the gauge should be located where the depth at very low tides would be about 2 feet more than the length of the column of oil required. The amount of petroleum that can be used is limited by the depth of the small opening in the pipe below the lowest tides, for when a greater amount is used there will be a loss through the opening at extreme low tides. As the specific- gravity of kerosene is less than that of water, the surface of the oil inside the tube will be higher than the water surface outside. This height will be equal to about one-eighth of the whole column of oil. If a gauge has been working with ordinary sea water only, the introduction of petroleum in the float tube changes the line of flotation of the float and also the relation of the curve to the datum line. But this will not cause any inaccuracy of rec- ord if the observer furnishes readings of the staff, to which the curve can be referred. In locations where there is an extensive land drainage, such as near the mouth of a large river, the difference between specific gravity of the water at the end of the flood and at the end of the ebb may be sufficient to cause a sensible periodic oscillation in the height of the surface of the petroleum with reference to that of the sea outside the tube, thus introducing a periodic variation which is not tidal and which it would be difficult to get rid of. Therefore petroleum can not

TIDES.

149

he employed where there is much change in the specific gravity of the water.

444. The tide house (fig. 21). — A rough house is usually con- structed to protect the gauge from the weather and from being interfered with. This structure may be about 6 by 6 feet at

the base and 7 feet high at the eaves, with a door and win- dows. It should be well bolted down to the wharf. Sometimes it may be convenient to have the float box outside the house, es- pecially at permanent stations, as it permits of easy renewals of the box without hav- ing to remove the roof from the house; in this case a sloping cover must be pro- vided for the l»ox, which may be hinged and padlocked to per- mit of easy access to the float, while pro- tecting it from being interfered with. A float-tube opening in- side of the tide house should also be provided with a cover to prevent anything from being dropped down the tube. PIG. 21. Such u cover may be

made of two pieces

with a narrow slit in the center for the float wire. For the sup- port of the tide gauge, a strong table or shelf about 2£ feet high should be provided. If reasonably protected from molestation, an automatic tide gauge may be operated for a limited time without a house, but protected by a box with a hinged lid with padlock and a canvas cover.

150 GENERAL INSTRUCTIONS FOR FIELD WORK.

445. Setting up gauge. — When the top of the float box opens inside of the tide house, the gauge is -usually set upon the table or shelf so that the float pulley is directly over the float box. Otherwise a series of pulleys must be provided to lead the float wire to the float box. The gauge is to be adjusted so that at ap- proximately mean tide level the pencil arm will be near the middle of the main cylinder, the float pulley and counterpoise- pulley each about half filled with wire or cord, and the counter- poise weight arranged so that it can move freely between I lie limits of extreme high and extreme low water. The datum-line pencil is to be set near the middle of the main cylinder in order to reduce to a minimum the error resulting from hygrometric changes in the tide-roll paper. These adjustments are referred to in the following paragraphs.

446. Attaching counterpoise weight. — One or more fixed pulleys are placed overhead in such positions as to carry the counterpoise to one side of the house, or otherwise out of the way (fig. 21). The counterpoise, which is the larger one of the two weights pro- vided with the gauge, is attached to a movable pulley and placed upon a block of wood or other substance to raise it an inch or two above the floor. Pass the end of the varnished fish line, furnished for the purpose (the bronze float wire may be used if desired), through the small drill hole near the inner edge of the counterpoise pulley and tie a knot at the end. The other end of the line is then passed over the fixed pulley overhead, down through the movable pulley on the weight and up again to the ceiling where the end is fastened. After removing the block from under the weight it should hang just a little above the floor. The line will be wound upon the counterpoise pulley by the descent of the float to the water.

447. Attaching float. — To put on the float wire. The length of wire required for the float, in feet, may be obtained by the formula :

L=BC+D

where Z/=number of feet of wire required for float.

B=one-half the number of threads on float pulley. (7=circumference of float pulley used, expressed in feet. I>=distance from float pulley to mean sea level in feet, meas- ured by- the route the wire must take to the float.

448. Unclamp the float pulley by turning the nuts a very little, as too much slack will cause it to jam. Undo the end of the wire on the spool, holding a finger on the coils to prevent its spring- ing off the spool, pass the wire through the small drill hole near

TIDES. 151

the outer edge of the float pulley, twisting it a few times around itself to make it fast. Now revolve the float pulley, holding the spool of wire with the other hand and keeping sufficient tension to wind the wire smoothly in the threads cut in the pulley. The number of feet of wire which may be wound upon the float pulley is 2BC, and if this is less than L wind the pulley entirely full, clamp it tightly to the counterpoise pulley, and measuring off L—2BC feet of additional wire, cut it off and fasten the end to the float. Some one must keep tension on the wire or hold a finger on the float pulley ;while this is being done, or the wire will spring

* _ T

off the pulley. If L is less than 1BC, find , which is the

V

number of threads of the float pulley to be left unwound when the wire is cut and attached to the float. Place the float in the float box, letting it down gently until all the slack wire is used, and then hold the float pulley with friction enough to prevent any hasty descent of the float to the water. At the beginning of unwinding the wire from the float pulley by the descent of the float, take care that the counterpoise line enters the thread of its pulley, after which it will take care of itself. After the float is attached the position of the gauge should be adjusted so that the float hangs in the center of the float tube, and this position made secure by fastening the gauge to the table and the latter to the floor.

449. Adjustment of pencil arm. — The pencil arm should now be adjusted so as to bring the pencil about the middle of the paper at mean tide level. It is not necessary to wait until the tide is actu- ally at its mean level, for knowing the difference between that level and the actual stage of tide the pencil may be set accordingly by the metal scale provided with the gauge. If the arm is very far out of position, a first approximate adjustment should be made by running the arm to the end of the pencil screw nearest the clocks, thus disengaging it from the screw thread. This is done by turn- ing the float pulley, so that the float will rise and the counterpoise weight fall. The pencil arm should be held at this end and the float pulley turned either backward or forward until it lacks three turns of being filled with the float wire. The pencil arm should be then released and the float permitted to descend until it rests upon the water. This should bring the pencil arm within 1 inch of its proper position. To bring it closer to the position desired, press on the counterpoise pulley with one finger, to prevent the weight from turning it, a finger of the same hand resting upon the last coil of wire on the float pulley to prevent the wire from springing out of the grooves; then with the other hand slightly

152 GENERAL INSTRUCTIONS FOR FIELD WORK.

X

unclanip the two set screws within the float pulley. Next revolve the counterpoise pulley until the pencil is in the desired place, when the float pulley is firmly reclamped.

450. Attaching tension weight. — This weight is attached by a cord to the pulley at one end of the receiving roller. This pulley has a small hole in one flange, through which the end of the cord may be passed and knotted. As with the counterpoise cord, it is desirable that this cord also be led over a pulley in the ceiling of the tide house in order to provide as much space as possible for the falling of the weight. By suspending the weight by means of a movable pulley its motion is diminished one-half.

451. Starting the gauge. — The roll of paper provided for the record should be placed on the gauge, as described in paragraph 454, the tension weight wound up, and both clocks wound and set to the correct time. The datum-line pencil should be placed in the holder and the latter clamped near the middle of the main cylinder. The recording pencil should be adjusted in its holder. The tide observer who is to have charge of the station should be given the necessary instructions for the care and operation of the gauge.

452. Report on establishment of tide station. — When an auto- matic tide-gauge station is established, a separate report describ- ing the same should be sent to the office. Such a report should include the following information :

(a) Name of town or place, with latitude and longitude. (6) Name and location of wharf. A .sketch showing location is desirable.

(c) Name of owner of wharf and a statement of arrangements

made with him.

(d) Give position of tide staff and automatic gauge on wharf.

(e) Describe tide staff, giving dimensions, limits of gradua-

tions, and a statement whether it is portable or fixed.

(/) If a portable tide staff is used, state how it is supported and give the reading of the tide staff that corresponds to the fixed point of support.

(g) Give number and scale of the automatic gauge.

(h) Describe the float box, giving dimensions, method of se- curing it in position, exact position and size of opening in the bottom and the depth of the water below this opening. Give also the depth of this opening below the zero of the tide staff. A knowledge of the exact position of this opening is especially important when the float box becomes clogged and it is necessary to clear it

TIDES. 153

(i) State what precaution was taken to prevent freezing in the float box. If kerosene is used, give the quantity.

(j) Give a brief statement relative to the recovery of old bench marks and the establishment of new ones. Complete de- scriptions of the bench marks are to be given separately in connection with the leveling record.

(fc) Give the date when the installation of the gauge was coni- plated and the record started.

(I) Give the name and occupation of the tide observer left in charge of the station.

(in) Give any other information about the tide station that may be important.

453. Operation of automatic tide gauge. — The observer in charge of a station shall visit the gauge at least once each day for inspec- tion and comparison. Every effort must be made to secure a con- tinuous record by keeping the gauge in running order. When out of order and necessary repairs at permanent stations are pos- sible, they must be made at once and the bill sent to the office for payment or paid for by the observer and an account rendered. Details in such cases must be reported to the office without delay. When a self-registering gauge is maintained in connection with hydrographic operations in the field, a continuous record shall be secured by making staff readings every hour whenever the gauge is out of commission, and the conditions must be reported imme- diately to the chief of party.

Each day the gauge is visited the tide staff should be read and an entry made on the tide roll. A statement concerning the cor- rectness of the time clock should also be entered, and if tliis clock is in error it should be set correct. ( See par. 461. ) The condition of the wind and any other matter that may affect the record should be noted also. All notes should be initialed by the observer. The tension weight should be wound up each day the gauge is visited. The clocks are to be wound twice a week. The tide roll should be changed once a month. If the observations are to be continued for a year or more, the change in rolls should take place on the first or second day of each calendar month, excepting the February roll for common years, which should be removed on March 2 or 3. These duties are described in detail in the following paragraphs.

454. Placing paper on gauge. — The supply roller, which is the one with the solid rod, is removed from the gauge and the roll of paper placed on it, which may be readily done by removing one of the flanges and replacing it again after the rod has been

154

GENERAL INSTRUCTIONS FOR FIELD WORK.

passed through the central hole in the roll of paper. The roller is then placed in the gauge, turning it in such a way that the loose end of the paper may pass from below inward toward the main cylinder. It makes no difference which end of the roller has the movable flange. The main cylinder is then disconnected from the motor clock by throwing out the carrier lever from the toothed carrier wheel, using a slender stick or lead pencil to reach it, if necessary. Pass the paper over the main cylinder and insert the end about three-fourths inch into the slit in the receiving roller. Several turns of the paper should then be wound around this roller, the paper passing from the main cylinder over the top of the receiving roller. The main cylinder must be again connected with the motor clock, the tension weight wound up, and the pencils adjusted. Before placing paper on the gauge, it should have written on the inside, at the beginning of the record, the name of station, date, scale of gauge, kind of time used, name of observer, nnd, in hydrographic work, the chief of party.

455. Kemoving paper from gauge. — Place the tension weight on the gauge table or some other support, unwrapping by hand from the pulley as much cord as may be necessary. Disconnect the main cylinder from the motor clock. Wind up the remaining paper on the receiving roller, and take it from the gauge. Re- move one of the flanges, revolve the other flange so as to force a little more paper into the central groove, when the paper can be removed from the roller. One roll of paper is sufficient for ;i month of record.

456. After the roll has been removed it should be rewound on a wooden core to bring the record on the inside and to prevent injury in transit to the office.

457. Label. — The label, Form 489, should then be pasted on the outside of the roll. First, fold a few inches of the beginning of tlu> paper down on the side containing the record, thus making a square, smooth edge of double thickness. Then, on the side of the paper which has no record, paste the label parallel to this edge and about 2 inches from it, the bottom of the label being toward the edge. Never paste the label as a seal to keep the roll from unwinding, as it must not be broken to open the record. The label should be filled out as completely as possible. The first marigram or tide roll of the series should be numbered 1 and the others numbered consecutively throughout the series. The gauge number will generally be found on the face of the clock upon the tide gauge. On this label the beginning and ending of the record

TIDES.

-155

refers to the particular tide roll and not to the entire series of observations at the station, as on Form 138.

458. Reading tide staff. — Every time the gauge is visited the tide staff should be read to the nearest 0.05 foot, and recorded on the marigram. If the water is too rough to obtain directly a reliable mean reading, record both the highest and lowest oscilla-

PIG. 22.

tions of the waves. The tabulator will take the mean of these two readings for comparison of staff and scale on Form 455. The exact position of the recording pencil of the gauge at the time each staff reading is made must be indicated by an arrowhead with a line connecting it with the note of the staff reading. The manner of connecting the note and tide curve is shown in figure 22.

156. GENERAL INSTRUCTIONS FOR FIELD WORK.

The note may be placed either above or below the curve, but it must be connected by a line with the exact position of the record- ing pencil at the time the comparisons are made.

At the time the above note was made the recording pencil was at the point in the curve indicated by the arrowhead, and only that part of the curve to the left of this point was completed. The por- tion of the curve to the right of the arrowhead was made after the note had been entered.

459. Time comparison and clocks. — The observer should have some means of obtaining correct standard time. There are two clocks on the automatic gauge now in general use ; one, designated the motor clock, which merely controls the movement of the main cylinder and paper; the other, which is on the left hand whei; facing the clocks, is designated the time clock, and makes the hour marks on the curve. The time indicated by the motor clock is of no consequence ; it is sufficient if this clock moves with ap- proximately uniform speed. The time indicated by the time clock and also the correct time and date should be recorded on the inarigram near the staff reading, on each visit to the gauge.

460. A rubber stamp of the proper form for these notes is fur- nished to each regular observer. After this note has been made on the marigram, the time clock, if wrong, should be corrected. The fact of this correction is indicated in the note on the mari- gram by the word " corrected." If no correction is necessary, the word " correct " may be used instead. Figure 22 shows the proper form for these notes.

461. The clocks on the gauge may be corrected as ordinary clocks, by turning the hands forward or backward ; but the hands of the time clock must not be turned backward when the minute hand is between 10 minutes before the hour and 5 minutes after the hour, as it would probably injure the hour-marking device. The regulating device is similar to that on ordinary clocks. Both clocks should be wound regularly twice a week, care being taken not to wind them too tightly.

462. Tension weight. — This weight, which winds the paper on the receiving roll, should be wound up every day that the gauge- is visited. It is connected with the receiving roll by pawl and ratchet, and while winding it up with one hand the receiving roll must be held steady with the other hand.

463. Pencils. — These should be examined frequently. The datum pencil points furnished for the gauge are usually too long and should be broken to the proper length. The point should be care- fully adjusted so as to make a distinct line and still not tear

TIDES. 157

the paper under unfavorable conditions. It should project about one-sixth inch from the holder, and this adjustment can be kept by screwing down the cap a little from time to time as may be necessary. When tho holder is lifted to remove or put in paper, care must be taken not to lose the lead from the brass holder. For the recording pencil that makes the tide curve, a good quality of No. 2 pencil is best. This should be kept carefully sharpened and adjusted in its holder. Upon this adjustment depends the efficiency of the hour-making device. It should be such that the hook attached to the pencil holder will just clear the tripping rod.

464. Lost record. — If any portion of the record is lost from any cause, move the paper forward a few inches before starting the gauge again. To do this the main cylinder must be disconnected from the motor clock in the manner described in paragraph 454, but the cylinder must be held steady with one hand when dis- connected, as otherwise the paper would be jerked forward by tho tension weight. This may be done by placing the hand on the paper over the main cylinder.

465. Changes in adjxistment. — After an automatic tide gauge has been properly set up, changes in the adjustment of the float pulley or in the position of the datum line should be avoided unless actually necessary ; and when any change is made, a note of the fact and the reason for the change should be immediately entered on the ma ri gram.

466. Cleaning the pencil screw. — The pencil screw must be cleaned from time to time, otherwise the pencil arm may be raised from the paper on a falling tide, or pressed so firmly against the main cylinder on a rising tide as to tear the paper, either accident causing a loss of record. A small rag moistened with gasoline passed around the pencil screw and drawn back and forth several times will keep it clean. Never put oil on the pencil screw, as it soon catches dirt and makes it worse than it was before.

467. Operating troubles. — Some of the difficulties likely to be met in the operation of an automatic gauge are:

Failure to keep the float box in a vertical position, causing tin1 float to scrape on side of pipe.

Breaking the float wire, which is nearly always due to sharp bends or kinks in the wire when it is slack ; a new wire should be put in, rather than to splice the old one.

Vibration of pencil on record ; this is due to too large openings in the tide box and if serious may require repair of box.

158 GENERAL INSTRUCTIONS FOR FIELD WORK.

Failure of gauge to record full range of tide, as shown by com- parison with staff readings; this is due to clogging of openings in float box, and is likely to render the record worthless, and calls for immediate examination of box and clearing of openings.

Jamming of paper rolls, due to paper not running true; rolls should be examined to see that they are in their right position, and paper rewound if necessary. Often the paper can be slipped away from the flange by hand.

Failure of hour-marking device. This is usually due to lack of proper adjustment of the recording pencil, causing too much space between the tripping rod and the hook attached to the pencil holder, or causing the hook to be jammed too tight against the rod. The adjustment should be such that the hook just clears the rod. If the trouble is due to the time clock itself it may be necessary to remove the clock and have it repaired by a clock maker.

468. Tabulation of tide records. — In order to establish uniformity in the records to be filed in the office, the tabulations should be neatly made in black ink and in accordance with the forms as outlined in the following paragraphs. For interpolated values to fill in gaps caused by lost record, red ink is generally used. In all forms in which both sides are to be used and in which there is a difference in the width of the left-hand margin on the two sides, the side having the wider left-hand margin should be taken as the front or first page of the sheet and be used in beginning the tabulations on that sheet. In all the forms the heading on the front side of each sheet should be filled out as completely as possible. On the back of the sheet the name of the station and the year and month of the observation should be repeated. The words " Party of " or " Chief of party " may be taken as equivalent to " Ob- server " when the latter has charge of the tide station. On the label of a tide roll, the dates of the beginning and end of observa- tions refer to the particular tide roll only, but in the headings of the forms for the tabulation of these rolls these dates refer to the entire series of the observations at the station. If the observa- tions are still being continued at the time the tabulations are made, the space after " Observations end " should be left blank. In the tabulations, the hours of the day are to be designated con- secutively from Oh (midnight) to 23h (11:00 p. m.), thus avoiding the terms a. m. and p. m. Before tabulating a marigram or tide roll the following preliminary work is usually necessary:

469. Marking the hours. — The time notes should be examined and if it is found that the time clock never varies more than

TIDES. 159

three minutes from the correct time, the small horizontal hour marks automatically made by this clock may be accepted as cor- rect and marked accordingly. The hour itself begins at the in- stant the mark leaves the curve, and no account need be taken of the length of the stroke. These marks should be numbered consecutively from 0 (midnight) to 23 (11:00 p. in.), and the numbering should be checked at each time note on the marigram. 470. In cases where the hour-marking device has failed to work, or when the hour marks are unreliable on account of the time clock being more than three minutes in error, the following method may be used : First : From the time notes ascertain the position on the curve of the nearest exact hour. This may usually be best accomplished by laying off 1 inch on a piece of paper and subdividing it in 12 equal parts. The inch, measured parallel to the datum line, will represent one hour on the tide curve, and each of the subdivisions will represent five minutes. This is known as a time scale. The correct time of the point on the curve indi- cated by the time note being known, the nearest exact hour may be readily laid off by this little scale. Second : Draw lines through the points indicating the exact hours perpendicular to the datum line and extending across the paper. Third: Prepare a strip OL paper somewhat longer than the greatest distance between the time notes on the marigram, pasting successive pieces together if one is not long enough. On one edge lay off equal divisions a little greater than 1 inch long, and if made about Is^ inches long they will generally be about right. These divisions should be numbered consecutively from 6h to 23h, and then from O11 to 23", repeated as often as may be necessary, the last division ending with any hour. In general it will be convenient to start the num- bering with 6h at the left-hand end of the scale, as the time notes will seldom be made at an earlier hour in the morning. This strip is known as a dividing scale. This scale is then adjusted obliquely between two consecutive" cross lines passing through the correct hour points, so that the numbers on the scale will agree with the hours represented by the cross lines. With the scale in this position, held fast from slipping by paper weights, each division may be marked on the marigram or tide roll by a dot. Fourth : By means of a square and a straightedge placed near the bottom of the paper and parallel to the datum line, these hour dots may be readily referred to the tide curve and indicated by vertical lines crossing the curve. These hour lines should be num- bered in the same manner as the hour marks that are made auto- matically by the machine.

160 GENERAL INSTRUCTIONS FOR FIELD WORK.

471. Comparative readings (Form 455, tig. 23). — This form is used to obtain the relation between the scale of an automatic tide- gauge record and the fixed tide staff. It is fundamental and of the greatest importance, for upon it chiefly depends the accuracy of the reference of all tide planes to fixed bench marks, so that they may be recovered at any future time. The work must be thoroughly checked by the observer so as to remove all acci- dental errors, for any mistake made here will affect all tabula- tions for the month.

472. The scale reading of the datum line for the comparison may be taken at any arbitrary number, but for convenience and to avoid negative differences it is desirable to have it such that the scale readings of the curve will be from \ to 1£ feet less than the corresponding staff readings. The reading scales are usually made of glass or celluloid and are graduated to represent feec and tenths. The foot divisions may be numbered with ink as desired. To choose an original scale setting for the datum line, place the scale with one of the foot divisions on the datum line at a point where a staff comparison is noted, and then number the foot divisions of the scale so that the point on the curve will read by the scale about a foot less than the staff reading. It would be well to test this same setting with several notes. The number of the foot division on the datum line will be adopted as the original scale setting and entered in the heading of the form. In using a glass or celluloid scale the side on which the division lines are cut should be kept down next to the paper. The num- bering of the divisions should be written with india ink on the upper surface. On the underside of scale draw an india-ink line across the scale through the foot division that is to be used a* the datum line.

473. Prepare table of comparative staff and scale readings on Form 455 as in the accompanying sample. Staff and scale read- ings should both be given to two decimals of a foot, but it will be sufficient to give merely the nearest 0.05 foot. The scale read- ing is subtracted from the staff reading for the difference. If the scale reading is larger than the staff reading, the difference will be negative.

474. If the gauge has run without any accident, the differences "A-B " for the marigram ought to be approximately equal ; but if there has beeu any change in the machine, such as moving the datum pencil, breakage of float wire, or increase of kerosene in float tube, etc., the differences will form distinct groups, one set of differences for each adjustment of the gauge.

TIDES.

161

TIDES: COMPARATIVE READINGS.

Sttlut •

. Party c} ... John .1. 5atkl3»

ott t*!:-! Jan»23« 1906 <*•.»•*.. T.d, c,*t t ,v». .76. ... 5<-oif iiia

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July 1

7 32

jn

4.90

•.„ 3.90

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2

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0 95

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5

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6

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4.70

3.90

0.90

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7

9 54

5 25

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5

8

7 33

9.46

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9

7 49

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ll

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FIG. 23.

-11

162 GENERAL INSTRUCTIONS FOR FIELD WORK.

475. In case there has been no change of adjustment during the month, examine the differences carefully to see that they agree fairly well with one another. Small variations in the differences are to be expected, but if any one of them varies by half a foot (0.50) or more from the apparent average, revise the difference and also the scale reading. If both are found to be correct, an error must have been made in the staff reading, and as this can not then be corrected, the difference must be rejected, which i.s done by making a pencil line around it, and that difference i.s then omitted from the computation of the mean difference.

476. In case there has been one or more changes in the adjust- ment of the gauge during the month, consider each set of dif- ferences by itself, and proceed as in the preceding paragraph, omitting accidental discrepancies.

477. Sum all those differences which are approximately similar and obtain their mean by dividing the sum by the number of differences used, carrying the quotient to two decimal places. In case a change was made in the adjustment of the gauge there may be two or more separate means required for the month. In either case, the mean difference plus the scale reading of the datum line for this comparison is the true setting of the scale on the datum line for tabulating high and low waters and hourly readings for the period covered by the uniform differences. Erase the temporary mark on the scale which was used for Form 455, and mark the true scale setting with india ink on the lower side of the scale, if transparent; or on its upper side, if opaque. Be- fore using this mark it should be tested by seeing that when this mark is set on the datum line the scale reading of the curve will be nearly the same as any staff reading.

478. Tabulation of high and low waters (Form 138, tigs. 24 and 25). — This form is used for the tabulation of high and low waters, which may be either read from the tide curve made by an axito- matic gauge or taken from plain staff readings as recorded in a tide book. The times are to be expressed in hours and tenths instead of hours and minutes. This is the general practice in the office work, and has been found to save considerable time in the computation of the lunitidal intervals. One-tenth of an hour is equivalent to six minutes, which is as close as an ob- served high or low water can usually be determined. The fo(-

TIDES.

163

Sp«eic*n of front of Jot* 138 . TIDES. HIGH AND LOW WATERS

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FIG. 24.

164

GENERAL INSTRUCTIONS FOR FIELD WORK.

3caela»n of back of Tom ^8. TIDES: HIGH AND LOW WATERS

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FIG. 25.

TIDES.

165

lowing tables give the equivalents of the minutes in tenths of an hour:

Minutes.

Tenths of hour.

Minutes.

Tenths of hour.

Minutes.

Tenths of hour.

Examples.

h. m. TITS.

Oto 3

0.0

21 to 27

0.4

45 to 51

0.8

4 02=4.0

4 to 8

.1

28 to 32

.5

52 to 56

.9

4 31=4.5

9 to 15

.2

33 to 39

.6

57 to 59

1.0

4 50=4.8

16 to 20

.3

40 to 44

.7

4 58=5.0

479. The heights should usually be referred to the zero of the tide staff, and should be given in feet and tenths of a foot. If the position of the tide staff has been changed during the observa- tions, the heights should all be referred to the zero of one of the staffs, and a full explanation given in the column of " Remarks." Any point of an automatic tide-gauge curve is readily referred to the zero of staff by using the true or corrected scale setting, as calculated from the comparative readings (par. 477) on the datum line.

480. When the series of observations is less than 6 months the high and low waters should be tabulated in groups of 29 days t?ach, beginning each group on the first line of the front side of a sheet. Allow two lines for each day, which will enable 17 days of record to be tabulated on the front page, and the remaining 12 days of the group will be tabulated on the back of the form. If any part of the record is lost, leave vacant lines for missing tides. If the series is longer than 6 months, the high and low waters should be tabulated by calendar months. Begin oar-h sheet with the first of the month, and after 29 days have been tabulated, place the remaining days of the month below the long black hori- zontal line near the bottom of the back of the form. For February of common years, insert March 1 after February 28 in order to complete the 29-day group. The high and low waters for March 1 should be repeated at the beginning of the sheet for March.

481. The method of representing the year, month, and days is shown in the specimen forms that follow. The repetition of the name of the month in the date column on the same side of the sheet is not desired ; and although two lines are allowed for each day, the day of the month is written only on the first of these lines. Generality, the morning tides are entered on the first line

166 GENERAL INSTRUCTIONS FOR FIELD WORK.

and the afternoon tides on the second line for each day. A tide occurring at midnight (Oh) is taken as belonging to the morning of the day just beginning. While tabulating the times and heights of the high and low waters, the columns of " Moon's transits " and " Lunitidal intervals " are left blank. These are to be filled in afterwards in case the reductions described in paragraph 493 are made.

482. After the times and heights have been tabulated, the high- est and lowest tide occurring during the entire month, or during the period represented by the sheet if the tabulations are not made by calendar months, should be selected and entered in the heading on the back of the sheet. If during this time the observer was unable to obtain a complete record because of some abnormal weather conditions, an estimation of the height of an extreme high water or extreme low water referred to the tide staff may be made from the evidence at hand and an explanation entered in the column of " Remarks."

483. Tabulation of hourly readings (Form 362, fig. 26). — The heights in this form should generally be referred to the zero of the tide staff, and should be expressed in feet and tenths of a foot. When heights are taken from a tide roll they are readily referred to staff by using the corrected scale setting on datum line (par. 477). The month and day of month are to be indi- cated in the spaces provided. The name or abbreviation for the month should be written only once on a page, except that the month should always be given for its first day. The series niay begin at any time and the days are to be entered consecutively without regard to calendar months or to time of changing tide rolls, seven days to the page, and using both sides of the form. After a year of observations has been completed a new series should be started. If any part of the record is lost, unless it is great enough to break the series into separate parts, blank spaces should be left for the missing tides. In the space after the " Day of series," the days are to be numbered consecutively, 1, 2, 3, etc., throughout the series, without regard to the month or day of month.

484. As stencils are to be used in connection with this form, it is important that the heights be written in their proper spaces in the columns headed " Feet " and in the lines opposite the number of the hour. In the form, these columns have the decimal points already printed. The columns without the decimal points are to be left vacant.

TIDES.

167

1

! o Form Ml

3p«olai«Ti of Fora 363

TIDES: HOURLY HEIGHTS

ura). Ckltfornl& v..r- l«ia

V f. COASI Mit cioenic lumii 1

Si. twin San n'*»*» (LR Fl

Chief of Pai Time Meric

tv ^lohn A. *»tkln« T... 38«43' H. T^.^ 117»14« *

i»n 12ata • Ti

de Gauge Ko.7.6 Scale 1: 18 Reduced to Staf

1

n-».

Ifooib

tnd Day

July 1

4

3

d 3

4 4

d. S

d.

6

4

7

Hori- { tonul

- fac..

1 Day of, 'S*n« |j 183

184

185

186

187

188

189

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r<tt.

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r«r.

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0 < 7. ft

8.2

1,9

9 4

8 I

7 S

6 •

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1

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7.6

7 g

7,9

7«6

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fi.l

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53

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7 3

7.S

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34.9

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4.9

8.5

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5,5

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4,4

4 1

3 9

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4.2

4,7

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5.3

4.9

4.5

4.3

4.3

4.7

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8

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4.9

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7.1

6>9

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6 4

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7.1

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7,5

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6 3

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7,3

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9 5

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i ^

a. a

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7 3

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it t

fi,l

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8.7

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153.4

153.3

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: Sum (or 29 days, 1 to 29 of <* Divi»or=6«i; me*n for 29 d»ys—

Tib-jilted ky

6.1912 Summit hyJohn S3ith

FIG. 26.

168

GENERAL INSTRUCTIONS FOR FIELD WORK.

485. When the record is to be continued for many years it is generally divided into series of 369 days each, commencing on the 1st day of January of each year, the last three or four days of one series being repeated as the first days of the following series. As a check on the arrangement of the days in the form, the fol- lowing table gives the page, column of page, and day of series, for the first of each calendar month, when the series commences with January 1. This table is not applicable to a series beginning on any other date than the 1st of January.

Common year.

Leap year.

Month.

Page.

Col- umn.

Day of series.

Month.

Page.

Col- umn.

Day of series.

Jan. 1

1

1

1

Jan. 1

1

1

1

Feb. 1

5

4

32

Feb. 1

5

4

32

Mar. 1

9

4

60

Mar. 1

9

6

61

Apr. 1 May 1

13

18

7 2

91 121

Apr. 1 May 1

14 18

1 3

92 122

June 1

22

5

152

June 1

22

6

153

July 1

26

7

182

July 1

27

1

1S3

Aug. 1

31

3

213

Aug. 1

31

4

214

Sept. 1

35

6

244

Sept. 1

35

7

245

Oct. 1

40

1

274

Oct. 1

40

2

275

Nov. 1

44

4

305

Nov. 1

44

5

306

Dec. 1

48

6

335

Dec. 1

48

7

336

Dec. 31

53

1

365

Dec. 31

53

2

366

Jan. 4

53

5

369

Jan. 3

53

5

369

486. If the series of observations is to be less than four years long it is usually best to begin on the first complete day of rec- ord as the first day of series. After 369 days have been tabu- lated, a new series should be commenced on the same month and day of month as the first series, the last three or four days of one series being repeated as the first day of the following series.

487. At permanent tidal stations the tabulated hourly readings when complete are to be summed both vertically and horizontally and the results written in the spaces provided on the form. The total of the horizontal sums on any page must equal the total of the vertical sums, and this total page sum should be written in the proper space near the lower right-hand corner of the form. If any of the columns are incomplete because of lost record, the missing readings may be estimated by interpolation, and entered in the form with red ink to distinguish them from the actual observed readings. Generally if the tide observer does not tinder- stand the method of interpolation, he may leave this part of the

TIDES. 169

work to be done in the office. In this case he should enter the sums of all incomplete lines, columns, or pages, with lead pencil, in order that they may be modified after the interpolations have been made. The final sums are to include both observed and interpolated readings. A tide observer who tabulates his own ivcords should send to the office only those pages of Form 362 which have been entirely completed so far as his gauge record will permit, retaining any partially filled form to complete when the record for the following month is removed from the gauge.

488. Interpolations. — Before beginning the reductions, if any portion of the record is lost, it is desirable that the missing tides he supplied by interpolation. Interpolated tides should be written in red ink or else inclosed in parenthesis to distinguish them from observations. If only a few tides are lost, plot time and height upon profile paper for a number of days before and after the break, leaving space for the missing tides; connect the alternate tides with curved line, which must be extended across the gap on the plotting as nearly as may be in accordance with the ap- parent law of change in time and height. There will usually be eight distinct curves — four for height and four for time, including both high and low waters. In order to prevent the curves from becoming confused by intersecting too much, they may be plotted upon scales which are dropped one below the other.

489. If there is a simultaneous series in the vicinity the missing tides may be supplied by comparison with it.

490. If the series is long enough take the means of the times and heights occurring 29 days before and 29 days after each missing tide, smoothing out the results by plotting.

491. In some cases a direct linear interpolation will suffice, espe- cially when only a few tides are missing. In this mode of inter- polation divide the difference between the times of alternate tides and also that of the corresponding heights by one more than the number of missing tides and add the quotient to the preceding time and heights. If it happens that one of the tides occurs near noon or midnight, it may chance that only one tide of that phase occurs in the civil day, and care must be taken to allow for this in counting up the number of missing tides^

492. Reduction of tide records. — To obtain tidal constants and tidal datums for any station, the tabulated tides, together with intervals and ranges depending upon them, must be reduced to their mean values. The mean of a series of items is obtained by dividing the sum by the number of items included in that sum. In order to secure uniformity, the spaces for the sums are

170 GENERAL INSTRUCTIONS FOR FIELD WORK.

generally indicated in the forms, and the number or items in- cluded in each sum should usually be written in small figures just above the sum, as indicated in the specimens of Form 138 (pp. 163-164). In the tabulations, the individual times and heights are given to one decimal place, but in obtaining the means the results should be carried to two decimal places. The last decimal figure should be taken to the nearest hundredth, but if the remainder should be exactly one-half of the divisor, the second decimal should be made even, if not already even, by adding one. The means directly obtained require certain cor- rections, which are explained in the following paragraphs.

493. Lunitidal intervals (Form 138). — This computation is made directly on the form on which the high and low waters have been tabulated.

First: The moon's transits for the meridian of Greenwich are copied in the column provided for the purpose. If the transits are obtained from an almauac, care must be taken to refer them to civil time and to convert the minutes into tenths of an hour. Photostat copies of a table of transits expressed in hours and tenths, Greenwich mean civil time, may be obtained from the office. The times inclosed by parentheses are for the lower transits of the moon, the unmarked ones being for the upper transits. They should be similarly indicated in the copy.

Second: Subtract from the time of each high and low water the time of the first preceding moon's transit, and write the dif- ference in the appropriate column on the same line as the tide from which it was obtained. In case the time of high or low water is nearly the same as that of the moon's transit, take the transit which precedes the tide by about 12 hours, but in no must the same transit be used for two consecutive high waters or for two consecutive low waters. The lower transit of the moon applies to both high and low waters, just the same as the upper transit does. When the time of the moon's transit is on one day and the following high or low water is on the next day. the time of this tide must be increased by adding 24 hours before attempting to subtract the time of the transit. The high-water intervals will usually be approximately six hours greater or less than the low-water intervals, but the intervals for each phase of tide will rarely vary among themselves more than several hours. Intervals from the lower transits of the moon are to be indicated by parentheses.

Third : Sum both columns of the intervals for 29 days, placing the results In the spaces provided on the bark of the form.

TIDES. 171

Fourth : Obtain the means by dividing each sum by the number of intervals combined to obtain it, carrying the results to two decimal places, and enter the results .just below the sum.

Fifth: Apply the correction to intervals, as obtained from the table on pages 173 to 175, and enter the results in the spaces pro- vided below the second horizontal black line near the bottom of the form. The corrected high-water interval thus obtained is known also as the corrected establishment of the port.

494. Corrections for lunitidal intervals. — The true lunitidal in- terval is the difference between the mean local time of the tide and the mean local time of the moon's transit over the local meridian. But on account of the tree of standard time instead of local time and the Inconvenience of changing the moon's transits to the local meridian, it is customary to compute fictitious lunitidal intervals, which are the difference between the standard time of the tide and the Greenwich time of the moon's transit over the meridian of Greenwich, and then correct the mean once for all, thus saving considerable work. Let L =west longitude of station in degrees and decimals. S =west longitude of time meridian used for tides. S'=west longitude of time meridian used for transits. X =correction to lunitidal intervals in hours and decimals. Then the correction for lag of the moon is —

(1) X*=

360

The correction for reduction of standard time to local time is —

24

( 2 ) A',=_ ( S - L ) =0.06666667 ( S— L )

360

Combining (1) and (2) gives —

(3) X=0.06666667 (8- L) + 0.00233667 (S{-L)

When Greenwich transits are used 8'=O°, and —

(4) X=0.06666667 8 -0.06900334 L

When Greenwich transits and mean local time are

used L=S, and (4) becomes —

(5) x= -0.00233667 L.

495. The following table has been computed from formula (4) for west longitude. For east longitude reverse the signs in this table.

172 GENERAL INSTRUCTIONS FOR FIELD WORK.

496. It is directly applicable when standard time has been used for the tides. Take the correction for the degrees of local longi- tude from the column headed by the time meridian used and add to this the correction for the minutes of local longitude. The latter part of the correction is independent of the time meridian.

497. For any other time meridian Si the table may be adapted by using the nearest standard time meridian (S) of the table, pro- ceeding as before, and then apply the following correction to the result from the table :

X= ±0.06667 (Si-iS), + for west longitude, — for east longitude.

498. When transits for the meridian of L' are used, the table may be adapted by proceeding as before and applying the follow- ing correction to the result :

X= ±0.00234 L', + for west longitude, — for east longitude.

499. When mean local civil time and Greenwich transits are used, it is probably more convenient to use formula (5) inde- pendently of the table.

TIDES.

173

Correction for lunitidal intervals (in hours and decimals). [For west longitude use sign given; for east longitude reverse sign.]

Time meridian,

Time meridian,

Time meridian.

Time meridian,

0°.

15°.

30°.

45°.

Longi- tude.

Correc- tion.

Longi- tude.

Correc- tion.

Longi- tude

Correc- tion.

Longi- tude.

Correc- tion.

0

Hour.

0

Hour.

0

Hour.

0

Hour.

5

+0. 655

20

+0.620

35

+0.585

0

+ .586

21

+ .551

36

+ .516

7

+ .517

22

+ .482

37

+ .447

8

+ .448

23

+ .413

38

+ .378

9

+ .379

24

+ .344

39

+ .309

10

+ .310

25

+ .275

40

+ .240

11

+ .241

26

+ .206

41

+ .171

12

+ .172

27

+ .137

42

+ .102

13

+ .103

28

+ .068

43

+ .033

14

+ .034

29

- .001

44

- .030.

0

0.000

15

- .035

30

- .070

45

- .105

1

- .009

16

- .104

31

- .139

46

- .174

2

- .138

17

- .173

32

- .208

47

- .243

3

- .207

18

- .242

33

- .277

48

- .312

4

- .276

19

- .311

34

- .346

49

- .381

5

- .345

20

- .380

35

- .415

50

- .450

6

- .414

21

- .449

36

- .484

51

- .519

7

- .483

22

- .518

37

- .553

52

- .588

8

- .552

23

- .587

38

- .622

53

- .657-

9

- .621

24

- .656

39

- .691

54

- .726

10

- .690

25

- .725

40

- .760

55

- .795

Time meridian,

Time meridian.

Time meridian,

Time meridian.

60°.

75°.

90°.

105°.

Longi-

Correc-

Longi-

Correc-

Longi-

Correc-

Longi-

Correc-

tude.

tion.

tude.

tion.

tude.

tion.

tude.

tion.

.

Hour.

.

Hour.

0

Hour.

,

Hovr.

50

+0.550

65

+0.515

80

+0.480

95

+0. 445

51

+ .481

66

+ .446

81

+ .411

96

+ .376

52

+ .412

67

+ .377

82

+ .342

97

+ .307

53

+ .343

68

+ .308

83

+ .273

98

+ .238

54

+ .274

69

+ .239

84

+ .204

99

+ .169

55

-f .205

70

+ .170

85

+ .135

100

+ .100

56

+ .136

71

+ .101

86

+ .066

101

+ .031

57

+ .067

72

+ .032

87

- .003

102

- .038

58

- .002

73

- .037

88

- .072

103

- .107

59

- .071

74

- .106

89

- .141

104

- .176

60

- .140

75

- .175

90

- .210

105

- .245

61

- .209

76

- .244

91

- .279

106

- .314

62

- .278

77

- .313

92

- .348

107

- .383

63

- .317

78

- .382

93

- .417

108

- .452

64

- .416

79

- .451

94

- .486

109

- .521

65

- .485

80

- .520

95

- .555

110

-- .590

66

- .554

81

- .589

96

- .624

111

- .659

67

- .623

82

- .658

97

- .693

112

- .728

68

- .692

83

- .727

98

- .762

113

- .797

69

- .761

84

- .796

99

- .831

114

- .866

70

- .830

85

- .865

100

- .900

115

- .935

174 GENERAL INSTUITCTIOXS FOR FIELD WORK.

Correction for lunitidal interval* — Continued.

Time meridian, 120°.

Time meridian, 135°. Time meridian, 150°.

Longitude.

Correction.

Longitude.

Correction.

Longitude.

Correction.

4

Hour.

.

Hour.

.

Hour.

110

+0. 410

125

+0. 375

140

+0.340

111

+ .341

126

+ .306

141

+ .271

112

+ .272

127

+ .237

142

+ .202

113

+ .203

128

+ .168

143

+ .133

114

+ .134

129

+ .099

144

+ .064

115

+ .W5

130

+ .030

145

- .005

116

- .004

131

- .039

14«

- .074

117

- .073

132

- .108

147

- .143

118

- .142

133

- .177

148

- .212

119

- .211

134

- .246

149

- .281

120

- .280

135

- .315

150

- .351

121 - .349

136

- .384

151

- .420

122 - .418

137

- .453

152

- .489

123

- .487

138

- .522

153

- .558

124

— .556

139

- .591

154

- .627

125

- .625

140

- .660

155

- .696

126

- .694

141

- .729

156

- .765

127

- .763

142

- .798

157

- .834

128

- .832

143

- .867

158

- .903

129

- .901

144

- .936

159

- .972

MO

- .970

145

-1.005

160

-1.041

Time meridian, 157° 30'.

Time meridian, 165°.

Time meridian, 180°.

Longitude.

Correction.

Longitude.

Correction.

Longitude.

Correction.

o

Hour.

Hour.

.

Hour.

148

+0.288

155

+0.304

170

+0.269 j

149

+ .219

156

+ .235

171

+ .200

150

+ .149

157

+ .166

172

+ .131

151

+ .080

158

+ .097

173

+ .062

152

4- .011

159

+ .028

174

- .007

153

- .058

160

- .041

175

- .076

154

- .127

161

- .110

176

- .145

IK

- .196

162

- .179

177

- .214

156

— .265

163

- .248

178

- .283

157

- .334

164

- .317

179

- .352

158

- .403

165

- .386

180

- .421

159

- .472

166

- .455

181

- .490

160

- .541

167

- .524

182

- .559

161

- .610

168

- 593

183

- .628

162

- .679

169

- .662

184

- .697

163

- .748

170

- .731

185

- .766

164

- .817

171

- .800

186

- .835

165

- .886

172

- .869

187

- .904

166

- .955

173

- .938

188

- .973

167

-1.024

174

-1.007

189

-1.042

168

-1.093

175

-1.076

190

-1.111

TIDES.

Correction for minute* of longitude.

175

Longi- tude.

Correc- tion.

Longi- Correc- tude. lion.

Longi- tude.

Correc- tion.

,

Hour.

' Hour.

/

Hour.

1

-a ooi

21 I -0.024

41

-0. 047

2

- .002

22 - .025

42

- .048

3

- .003

23 - . 026

43

- .049

4

- .005

24 - .028

44

- .051

5

- .006

25 • - . 029

45 - . 052

6

- .007

26 : - . 030

46 ! - . 053

7

- .008

27 I - .031

47 • - . 054

8

- . 009 28 - . 032

48 - . 055

0 - .010

29 - .033

49 - .056

10 - .012

30 - .034

50 - .058

11 - .013

31 ; - .036

51 - .059

12 - .014

32 - .037

52

- .060

13 - .015 33 - .038

53

- .061

14 - .016

34 - .039

54 - .062

15

- .017

35 1 - . 040

55 - .063

16

- .018

36' -.041

5« : - . 064

17

- .020

37 - . 013

57 - .066

18 - . 021 38 ' - . 044

58 - . 067

19 - . 022 39 1 - . 045

59 - . 068

20

- .023

40 - . 046

j

60

- .069

500. Mean high water (HW). mean low water (LW), mean range (Mn), and mean tide level (MTL), (Form 138).— First: Add the high and low water heights for 29 days. Second: Obtain the means by dividing each sum by the number of high or low waters Included. Third: Obtain the mean range (Mn) by subtracting the mean of the low waters from the mean of the high waters, and enter the result in the space before the symbol "Mn." Fourth: Obtain the mean tide level (MTL) by taking one half of rhe sum of the mean high water and the mean low water, and enter in the space before the symbol " MTL." Fifth : The mean range should be corrected for longitude of moon's node in ac- cordance with paragraph 503.

501. Mean higher high water (HHW), mean lower low water (LIW), and diurnal inequalities (DHQ and DIQ) (Form 138). — First : Check off the higher of the two high waters and the lower of the two low waters of each day for 27 days, omitting the first and last days of the 29-day group. When only one high or one low water occurs on a calendar day, by reason of one of the tides having occurred after midnight and therefore on the next calendar day, the single tide should be checked if the tide just above it is unchecked, otherwise it should not be checked. If, however, the tide has become diurnal and only one high and one low water occur during the tidal day, these should both be checked. Second :

176 GENERAL INSTRUCTIONS FOR FIELD WORK.

The higher high waters and lower low waters thus checked should be summed and the results entered in the spaces provided in the column of •' Remarks " on the back of the form. Third : Obtain the means and enter the results in the line below the sums. Fourth : Subtract the mean of all the high waters from the mean of the higher high waters and enter the difference after the symbol DHQ on the back of the form. Fifth : Subtract the mean of the lower low waters from the mean of the low waters and enter the difference after the symbol DLQ. Sixth: Correct the DHQ and DLQ in accordance with paragraph 506.

502. Correction for the longitude of the moon's node. — The moon's node is the place where the lunar orbit intersects the ecliptic or earth's orbit, and the position of the node is continually changing. The effect of this change in the longitude of the node is to vary the maximum declination of the moon by more than 10 degrees, there being periods of years during which this maximum is less than that of the ecliptic, and other periods of years when the maximum declination of the moon is greater than that of the ecliptic. A change in the moon's declination affects both the mean range and the diurnal inequalities of the tide, so that these quantities as found from observations for any year must be corrected to obtain a true mean value.

503. To correct the mean range for the longitude of the moon's node, apply the factor "F(Mn)(" or "Factor for mean range," which may be obtained from the following table for the years 191.r> to 1934. The factors have been computed for the middle of each year, but as they change very slowly, the same value may be taken for any month of the year. The line in the table from which the value is to be taken is determined by the argument

2 (DHQ+DLQ) , . ,

— --ff » which may be computed to one decimal place from

the uncorrected values of DHQ, DLQ, and Mn as obtained from Form 138.

TIDES.

177

504. Factor F(Mn). — For mlttring the obsci-ved range of tide to its mean value.

XDHQ+DLQ)

1Q17

1Q1&

1O1O

1O9f>

1Q91

1O99

Mn

0.0 to 0.2...

1.02

1.02

1.01

1.00

0.99

0.98

0.97

0.97

0.97

0.98

0.3 to 0.4.

1.02

1.02

1.01

1.00

.99

.98

.97

.97

.97

.98

0. 5 to 0. 6

1.02

1.02

1.01

1.00

.99

.98

.98

.97

.97

.98

0.7to0.8

1.02

1. 01

1.00

1.00

.99

.98

.98

.98

.98

.98

0. 9tol. 0 . . .

1.02

1.01

1. 00

1.00

.99

.99

.98

.98

.98

.98

1. ltol.2

1.01

1.01

1.00

1. 00

.99

.99

.99

.98

.98

.99

1.3 to 1.4

1.01

1.01

1.00

1.00

1.00

.99

.99

.99

.99

1.5 to 1.6

1.01

1.00

1.00

1.00

1.00

.99

.99

.99

.99

.99

1.7101.8

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

2(DHQ+DLQ)

Mn

0. 0 to 0. 2

0.98

0.9.5

1.00

1.01

.02

1.03

1.03

1.03

1.03

1.02

0.3 to 0.4

.98

.99

1.00

1.01

.02

1.03

1.03

1.03

1.03

1.02

0.5to0.6

.98

.99

1.00

1.01

.02

1.02

1.03

1.03

1.02

1.02

0. 7to0.8

.98

.99

1.00

1.01

.02

1. 02

1.02

1. 02

1. 02

1.02

0.9 to 1.0

.99

.99

1.00

1.01

.01

1.02

1.02

1. 02

1. 02

L01

1. ltol.2

.99

1.00

1.00

1.01

.01

1.02

1.02

1. 02

1.02

1.01

1.3 to 1.4

.99

1.00

1.00

1.01

1.01

1.01

1.01

1.01

1. 01

1.01

1.5tol.6

1.00

1.00

1.00

1.00

1.00

1.01

1.01

1.01

1.01

1.00

1.7 to 1.8

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

505. For stations on the Atlantic coast of the United States

from Maine to Florida, 2-(ggg+-PLQ> is usually small, and if

M n

the values of DHQ and DLQ have not heen obtained, the top line of the tahle may generally he taken without material error. For sta- tions on the coast of the Gulf of Mexico from Key West to the Rio Grande, the mean range of the tide is very small and the facto»- F(Mn) need not be applied. For other localities, where DHQ and DLQ are usually computed, as on our Pacific coast, Alaska, etc., the

value of 2(DHQ+DLW should be obtained and entered in the Mn

space provided in the heading on the back of Form 138. If it is larger than 1.8, no correction need be applied to the mean range.

506. The diurnal inequalities DHQ, DLQ, should be corrected by the factor 1.02 F,, which may be obtained from the following table for the years 1915 to 1934. These factors have been computed for calendar months and may be used without modification for series of 29 days beginning on the 1st day of the month. If a 29-day

13027°— 21-

-12

178

GENERAL INSTRUCTIONS FOR FIELD WORK.

series begins on any other day of the month, the factor may be obtained from the table by interpolation.

507. Factor 1.02 Fi. — For correcting DHQ and DLQ.

Month.

1915

1916

1917

1918

1919

1920

1921

1922

1923

1924

Jan. . .

0.81

0.83

0. 86

0.89

0.92

0.97

1.01

1.03

1 03

1.02

Feb

94

.97

1 01

1 05

1 11

1 16

1 22

1 26

1 26

1 °3

Mar

1.08

1.11

1 16

1.21

1.29

1.37

1.45

1 49

1 49

1 4C

Apr

1.01

1.03

1.08

1.13

1.21

1.27

1.33

1.37

1 3G

1.33

May

.86

.88

.91

.96

.99

1.04

1.08

1.10

1.10

1.07

June

78

.81

83

.$6

91

94

.97

98

98

%

July. .

.82

.84

.86

.90

.95

.98

1.01

1 03

1 02

1.00

AUP. . .

.95

.98

1.01

1.06

1.13

1.18

1.22

1.24

1 23

1.19

Sept Oct

1.10 1.02

1.13 1.05

1.19 1.11

1.26 1.17

1.34 1.23

1.42 1.31

1.48 1 35

1.51

1.37

1.4S 1 35

1.43 1.30

Nov

.86

.88

.92

.96

1.01

1.06

1.09

1.09

l.OS

1.04

Dec

.81

.82

.85

89

.92

.96

.98

98

97

94

Month.

1925

1926

1927

1928

1929

1930

1931

1932

1933

1934

Jan

0.98

0.94

0.90

0.86

0.83

0.82

0.80

0.80

0. SO

0.82

Feb

1 17

1.12

1.06

1.01

98

.95

93

93

93

95

Mar

1.37

1.29

1.22

1.15

1.11

1.08

1.06

1.05

1.06

l.Ofi

Apr

1.26

1.19

1.12

1.07

1.03

1.00

.99

.99

1.00

1.02

May

1.02

.98

.93

.90

.87

.85

.84

.84

.85

.86

June

.92

.88

.85

.81

.79

.78

.77

.77

.78

.79

July. .

.96

.91

.86

.84

.82

.81

.80

.80

.81

.82

Ang

Sept

1.13 1.43

1.07 1.26

1.02 1 19

.98 1.13

.95 1.09

.93 1.07

.92 1.06

.92 1.06

.93 1.07

.95 1.10

Oct

1.22

1.15

1 10

1.04

1.01

1.00

.99

.99

1.00

1.03

Nov

1.00

.95

.91

.88

.85

.84

.83

.83

.84

.87

Dec

.90

.86

.83

.60

.79

.7t>

.77

.77

.78

.80

508. Annual inequality in mean sea level. — At most stations there is a variation in the reading of mean sea level upon the staff at different times of the year, depending in general upon the seasonal changes in the direction and strength of the wind, and in river stations also upon the rainfall or melting snow. There is a rough periodicity in these variations of mean sea level, but they can not accurately be foretold. The inequality may to some extent be eliminated from a short series of observations by a comparison with simultaneous observation at a near-by station where the tidal planes have already been determined from a long series of observations.

509. Comparison of simultaneous observations (Form 248, fig. 27). — If tidal data have been well determined for one station, satisfactory data for another station near by may generally be obtained by observing the tides simultaneously for several days

TIDES.

179

Specimen of Form E4S ronaM*

TIDES : Comparjso^ of Simultaneous Observations CA) suixjdtmt» .UUOT irf.»ta. Jhi^+siEibfliAt..^, r,»i \ r. ut ...4C?..si.'...iii LOIS, ..ua*?_Q7j_K....

(B) Stu«)a.-d(uuion.il.j«tty.-ii»adln&.....-?..! !__ L»t. ...4Q?..45!..S... ' """ """ ~

Chief at party

Time MeridUn. (A)

...«^..| .9..Q

za.7 lia.a_l_32^a

.CL8. -0.5-

.*.&.. l.B"

jua

l.Q-

_5.i_.l..iQ^Q aa.a

.7^.1 4.fe

ta_L

.e.fla..|.,n.5&

;^:

B in time of high ftod low wiier r«opectiv«ly UCerann in longitude (T>Uo on Ivk ol (arm.) =(l)-M2)-"M«ui diE«nnce la high ud low w>Ur inlorvnb, r«rp«Kivel;-

'.4,-iO. 4Q...-Mo«n HHW height >t (A). It - 9TOQ ->f~. LHW height M ( A>.

-•...9.7a.---U(4H.(«;l-U<*n HW bright U (A).

-..S.3a...-!10)-fll)-Mn » (A).

(() -. 6.00.-«Mcio HLW ).-i?hl X (A). (7) -..2.65_-Kaai LLW hcigSt M (A). {») -..J.35 -(S>-

»n LW height U (A).

-..1.B5 _»Mc»n LUW _0. 13 ..-04) -(Id)- 2OHQ diffdvnn.

(SO)— .. 1.92...- H(H)+(16))-«»n (»)-...0»£7 ..-;K»-;:t)-.Mn differen

H(i imtio. -(»)]-DL<l ratio.

TU»ult* from c*o.i*"lsor. o

An4>4 KUta. fr-i 4. JDOB

B Oi an, on, as>, c«)

f/. -.-I. 11.80...S..4a.

|l2»66..

--7.AI.

£U1A

Mau LWon ettfl >t mbordinto d IIMO LLW «o Miff U nbordiaaui <

FIG. 27.

ISO GENERAL INSTRUCTIONS FOR FIELD WORK.

at the two stations and comparing the results. For this purpose there should be at least two days of both day and night tides observed, and if the stations are very far apart a longer series should be observed. It is generally most satisfactory, especially when there is a large diurnal inequality, to have the tides coin- pared consist of a whole number of tidal days so that the num- ber of higher high waters, lower high waters, higher low waters, and lower low waters will be the same.

510. Explanation of Form 248. (See page 179). — This form is designed for the comparison of tides at a subordinate station for which tidal results are sought, with the tides observed simulta- neously at a standard station for which tidal constants are known. For short series of observations the high and low waters observed at the subordinate station may be tabulated immediately in this form, in which case it will be unnecessary to tabulate them also in Form 138. The time and height differences are to be obtained by subtracting the values at the standard station from the values at the subordinate station and the results entered with proper signs in the columns indicated.

Find the sums and means of columns of time difference, height of tide at both stations, and height difference. For stations on the Pacific coast, where the plane of reference is mean lower low water, the heights of the higher high, lower high, higher low, and lower low waters are to be summed separately, the higher highs and lower lows being indicated by pencil check marks. For sta- tions on the Atlantic coast, where the plane of reference is mean low water, the heights of the high waters may be all combined into a single sum, and similarly the low water heights ; the head- ings of their sums being made to read HW and LW, respectively, by striking out the extra letters. All mean results should be given to two decimals of its unit, whether hour or foot If any indi- vidual difference varies greatly from the apparent average, and an examination of the original record fails to show an error, that difference should not be included in the sum ; and such a value should be encircled to show that it has been rejected.

For stations on the Atlantic coast omit (4) to (9), (14) to (19), (25), (26), and the computation of DHQ and DLQ at the bottom of the form. Take (10)= mean high water height at the subordi- nate station, (11)= mean low water height at subordinate sta- tion, (20)= mean high water difference, and (21)= mean low water difference. For stations on the Pacific coast the lower part of the form should be filled out completely as indicated.

TIDES.

181

The correction for difference in longitude (2) may he obtained from the following table. Find the difference in longitude by sub- tracting the longitude of the subordinate station from the longi- tude of the standard station, considering west as positive and east as negative. The correction has the same sign as the resulting difference of longitudes. If the kind of time used at the two sta- tions is different, apply this difference, expressed in hours, to the difference in the time of tide as directly obtained, adding if the time meridian of the subordinate station is west of the time meridian of the standard station, and subtracting if the time meridian of the subordinate station is east of that of the standard station.

511. Correction for difference in longitude. —

Dif- fer- ence.

Corre"- tion.

Dif- fer- ence.

Correc- tion.

Dif- fer- ence.

Correc- tion, j

Dif- fer- ence.

COTTPC-

tion.

Dif- fer- ence.

Correc- tion.

Dif- dif- ence.

Correc- tion.

.

Hour.

.

Hnnr.

0

Hour. \

0

Hour.

0

Hour.

.

Hour.

1

0.069

31

2.139

61

4.209 !

91

6.279

121

8-349

151

10. 420

2

0.138

32

2.208

62

4.278 !

92

6. 348

122

8.418

152

10. 4S<>

3

0.207

33

2.277

63

4.347 |

93

6.417

123

8.487

153

10.558

4

o 27;

3!

2,3-16

64

4.416 !

94

6.486

124

8.556

154

10.627

5

0.345

35

2.415

65

4.485

95

6.555

1 125

8.625

155

10.606

6

0.41!

36

2.4S1

66

4.554

96

6.624

126

8.694

156

10.765

7

0.483

37

2.553

67

4.623

97

6.693

127

8.763

157

10.834

8

0.5.32

38

2.622

68

4. 692 i

98

6.762

128

8.832

158

10.903

9

0.621

39

2.691

69

4. 761

99

6.831

129

8.901

159

10.972

10

O.b90

40

2.760

70 , 4. 830

100

6.900

130

8.970

160

11.041

11

0.759

41

2.829

71 4. 899

101

6.969 131

9.039

161

ll.llff

12

0.82S

42

2. sys

72

4.968

102

7.038

m

9.108

162

11. 179

13

0.897

43

2.967

73

5. 037

103

7.107

133

9.177

163

11.25S

14

0.966

44

3.036

74

5.106

104

7.176

134

9.246

164

11.317

15

1.035

45

3. 105

75 5. 175

105

7.245

135

9.315

165

11.386

16

1.104

46

3.174

76 ! 5. 244

106

7.314

136

9.384

166

11. 455

17

1. 173

47

3.243

77 5.313

107

7.383

137

9.453

167

11.524

18

1.242

48

3.312

78 5. 3S2

108

7. 452

P»

9.522

168

' 11.593

19

1.311

49

3.381

/9 : 5.451

109

7. 521

139

9.591

169

11.662

20

1.3SO

50

3.450

80

5.520

110

7.590

140

9.660

170

11.731

21

1.449

51

3.519

81

5.539

111

7.659

141

9.729

171

11,800

22 1.51S

52

3. 5<i.S

82

5.658

112

7.728

142

9.798

172

11.869

23 1. 587

68

3.657

83

5.727

113

7.797

143

9.867

173

11.938

24 1. 656

54

3.726

84

5.796

114

7. 866

144

9.936

174

12.007

25 1. 725

55

3.795

85

5.865

115

7.935 | 145 10.005

175

12.078

20 1. 794

56

3.864

86

5.931

llo

8.004

146 10.074 i 176

12.145

27 ' 1.863

57

3. 933

87

6 003

117

8. 073

147 '10.143 177

12.214

28 1.932

58

4.002

88

6.072

118

8.142

148 |10. 212 j| 178

12.283

29 2.001

59

4.071

89

6.141

119

8.211

149 10. 281 ! 179

12.352

30 2. 070

60

4.140

90

6.210

120

8.280

1.50 10.351 180

12. 421

i

i 11

182

GENERAL ISTSTRUCTIONS FOR FIELD WORK.

Dif- fer- ence.

Correc- tion.

Dif- fer- ence.

Correc- tion.

Dif- fer- ence.

Correc- tion.

Dif- fer- ence.

Correc- tion.

.Dif- fer- ence.

Correc-j tion. !

S? Correc- & tion

!

,

Hour.

,

Hour.

,

Hour.

,

Hour.

i

Hour. '

JTowr.

1

0.001

11

0.013

21

0.024

31

0.036

41

0.047 i

51

O.Q3U

2

0.002

12

0.014

22

0.025

32

0.037

53

0.048

52

0.060

3

0.003

| 13

0.015

23

0.026 1

33

0.038

43

0.049 !

53

0.061

4

0.005

14

0.016

24

0.028

34

0.039

44

0.051 i

54

0.062

5

0.006

15

0.017

25

0.029

35

0.040

45

0.052 '

55

0.063

6

0.007

16

0.018

26

0.030

36

0.041

4G

0.053 1

56

0.064

7

O.OOS

17

0.020

27

0.031

37

0.043

47

0.054

57

0.066

8

0.009

18

0.021

28

0.032

38

0.044

48

0. 055

58

0.087

9

0.010

19

0.022

29

0.033

39

0.045

49

0.000

59

0.068

10

0.012

20

1

0.023

30

0.035

40

0.046

50

0. 058 '

60

0.069

512. Planes of reference. — Upon the Atlantic and Gulf coasts of the United States, including Porto Rico and the Atlantic coast of the Panama Canal Zone, all soundings are reduced to mean low- water. Corrected mean low water is obtained by subtracting one-half of the corrected mean range from the corrected mean tide level. In localities where the mean rise and fall of the tide is less than 1 foot, as in the greater part of Albemaiie and Pamlico Sounds, a plane one-half foot below the mean water level should be taken as the equivalent of the datum of mean low water.

Upon the Pacific coast of the United States, Alaska, Hawaii, and the Philippines, all soundings are reduced to the plane of mean lower low water, except that for Wrangeli Strait the datum is 3 feet below mean lower low water. Corrected mean lower low water is obtained by subtracting the corrected diurnal low-water inequality (DLQ) from the corrected mean low water.

Upon the Pacific coast of the Panama Canal Zone the sound- ings are reduced to the plane of mean low water springs. For this datum the spring range of tide is first obtained from tat- high and low waters observed at the time of new and full moon. One-half of the spring range is then subtracted from the mean tide level to obtain mean low water springs.

513. Difference in time of tide. — When there is much difference in the time or height of the tide at the place of sounding and at the tide gauge, allowance should be made in the reduction of the soundings. The difference may generally be estimated from ob- servations made at several stations in the vicinity of the work, but when it has been impossible to establish more than one tide station in the locality, the following formula may be useful in

TIDES.

183

estimating the velocity or a progressive tidal wave, and enable one to obtain the approximate difference in the time of the tide:

per second,

when y — 32.17 feet per second and d — depth of water for the average cross section between stations, in feet.

In order to convert feet per second into nautical miles per hour,

OCAA

multiply by gQgQ=0.592, and we have

v—3.3G\d nautical miles per hour. The time required for the tide wave is

_ 6080 *~~60'

17.87 . . . ..

— -r= minutes per nautical mile.

5280

15.51 = minutes per statute mile.

For convenience the following brief table is given : Time required for the tide wave to travel.

Depths.

1 nautical mile.

1 statute mile.

Depths.

1 nautical mile.

1 statute mile.

Fathoms.

Minutet.

Minutes.

Fathoms.

Minutes.

Minutes.

1

Z.3

6.3

9

2.4

2.1

2

5.2

4.5

10

2.3

2.0

3

4.2

3.7

15

1.9

1.6

4

3.6

3.2.

20

1.6

1.4

6

3.3

2.8

30

1.3

1.2

6

3.0

2.6

40

1.2

1.0

7

2.8

2.4

-50

1.0

0.9

8

2.6

2.2

60

0.9

0.8

" ••'•••': •

514. Bench marks. — A. bench mark is a definite point on a per- manent object used as a reference for elevations. No matter how temporary the occupation of a tide station is, if any plane of ref- erence is computed or assumed, the tide staff should be referred to at least three permanent bench marks. This is of great im- portance to make the results of the tidal observations available for future use. These marks should be sufficiently scattered so that they are not likely to be all destroyed by a common cause.

515. Qualities of a good bench mark. — The principal qualities of a good bench mark are that it is not likely to be destroyed or it* elevation changed, and that it may be easily found and identified.

184 GENERAL INSTRUCTIONS FOR FIELD WORK.

It should be so placed that a leveling rod can be held vertically over it. A mark set horizontally is generally more convenient to use than one set in a vertical wall. In a settled community perma- nent and substantial buildings afford the best location. In a rocky country, a ledge of rocks will serve. If the ground Is sandy, a mass of concrete containing not less than a cubic yard of material and buried so that its top projects a few inches above the surface of the ground, will generally constitute a suitable foundation for the bench. For a concrete bench mark, great care should be taken to obtain a proper mixture; one part sand, two parts cement, and three parts broken stone constitute the usual proportions. Bench marks should not be placed on buildings which are known to rest upon filled-in ground, as such structures are liable to settle. Water hydrants, curbstones, and growing trees are very unsatisfactory as bench marks except for tempo- rary use.

516. An identification mark is required, for no matter how well the bench mark may b'e described there is nearly always danger of mistaking the point used, unless it is clear y marked. The best identification mark, which should be used when it is practicable to do so, is the standard disk or cap bench mark of this Survey. These are made of brass about 3$ inches in diameter and have the following inscription " U. S. COAST & GEODETIC SURVEY B. M. $250 FINE OR IMPRISONMENT FOR DISTURBING THIS MARK." The disk bench mark has a shank about 3 inches long for insertion in a building or other substantial support. It should usually be set with its face flush with the wall and secured with cement. On the face is a short line, which should be p'.aced hori- zontal and which serves as the reference point. The disk bench mark may with advantage be set with its stem vertical in rock or cement, in which position it is more convenient to place a rod on than when set in a wall. The cap bench mark is threaded in- side and is designed to screw on top of an iron pipe which is sunk in the ground and secured to a mass of concrete, the top of the pipe projecting a few inches above the ground. This cap bench mark may be set directly in the top of a concrete base, when no suitable pipe is available. In the center of the top of cap is a space inclosed by a circle which is the .point of reference. When a standard disk or cap bench mark is used, the year of establish- ment and the number of the mark should be stamped into the metal. Duplication of numbers should be avoided, and a number that has previously been assigned to another bench mark in the

TIDES. 185

same locality, whether destroyed or extant, should not be used again for a new bench mark. Sets of dies for stamping letters and figures may be secured from the office.

517. If the standard bench marks are not available, a small cross •+- cut on a rock, building, or other structure, or in some cases a drill hole, will serve to indicate the point on which the leveling rod was held. The addition of lettering, especially in rock, will make the identification more certain. In a rocky ledge subject to weathering, a copper bolt is desirable, as a cross or inscription may become too indistinct to identify with certainty.

518. Permission from the Treasury Department to place stand- ard disk bench marks on any Federal building has been obtained, as indicated by the following letter :

TREASURY DEPARTMENT, Washington, November 11, 191.}. The honorable the SECRETARY OF COMMERCE,

Washington, D. C.

SIR: By direction of the Secretary I have the honor to acknowledge the receipt of your communication of the 7th instant, requesting that permission be granted to the officers of the Coast and Geodetic Survey to place on the Federal buildings under the control of this department small inscribed metal tablets, which are to bo used as bench marks in connection with the system of leveling, the custodians of the buildings to designate where the tablets are to be placed.

In reply, you are advised that no objection will be interposed by this department to the placing of the tablets on the various public buildings, as desired, and this letter, or a copy thereof, upon its presentation to the custodian of a Federal building, is to be considered by him as his author- ity for permitting the placing of one of the tablets on the building In his custody.

Respectfully,

B. R. XEWTON,

Assistant Secretary.

*

519. Leveling. — The bench marks and tide staff should be care- fully connected with each other by forward and backward lines of spirit levels. Great care should be taken to keep the instrument in proper adjustment. When the forward and backward measures between two bench marks differ in feet by more than 0.05 K (in which K is the distance leveled between the two bench marks in statute miles), or for distances under 500 feet by more than 0.015 foot, both the forward and backward measures are to be repeated until the difference between two such measures falls within the limit. No one of the questioned measures is to be used with a new measure to get this agreement. If work has been previously done in the vicinity, all the old bench marks that can be recovered should be connected with the tide staff by spirit levels. It is

186 GENERAL INSTRUCTION 8 TOR FIELD WORK.

desirable that beiich mark* established by other organizations, and also city and railroad datums, should be connected~with the Survey bench marks.

520. Record of bench marks and leveling. — Before leaving the general locality where bench marks have been established, a de- scription of each one must be written and sent to the office, together with the leveling record, and must also be written in the Tide Book, Form 277, when this is used for recording staff read- ings at the station, or in the Leveling Record, Form 258. Care should be taken to make the descriptions of the bench marks as clear and distinct as possible, and sufficiently complete to enable another person to readily iind and identify the marks. When a bench mark is made on a building in a city or town, the street and number should be given when possible. When not on a prominent structure, the distance and direction to several land- marks that are more or less permanent in the neighborhood should be given. Sketches or photographs which would aid in locating or identifying the bench marks are desirable.

521. Inspection of tide stations. — The tide observer should be interviewed to ascertain whether he thoroughly understands his work. The care taken to keep the gauge clocks correct, and the manner of making the staff comparison should be noted, special attention being given to the observer's habit in regard to the scale on the gauge to see whether he is inclined to enter the scale read- ing rather than the actual staff reading on the marigram. The value of many of our tidal records has been impaired because of the uncertainties arising from the habit of some of our tide ob- servers of entering modified scale readings instead of the actual staff readings on the tide roll. In general, the metal scales serve no useful pui»pose in securing the tidal records and have been already removed from some of our gauges. Unless there is some special reason for leaving the scale on the gauge, it may be re- moved and returned to the office.

522. The gauge should be adjusted so that, 'at approximately mean tide level, the recording pencil will be near the center of the paper, and the float pulley and the counterpoise pulley about half filled with wire or cord. The datum-line pencil should be set near the center of the paper. The counterpoise weight should be so arranged that it may move freely between the limits of extreme high and extreme low water. In order to secure a sufficient range of motion for the counterpoise weight and also for the tension weight, it is generally desirable to carry each supporting cord over a pulley in the ceiling of the tide house. After these adjustments

TIDES. 187

have been made, there will be no occasion for the tide observer to change them unless the float wire breaks or other accidents oc- cur. When this happens the observer should enter a clear explana- tion of the fact on the tide roll. Such changes are very undesir- able except when actually necessary, and the observer should be so informed.

523. The position of the float in the well should be examined to ascertain if it is free to move within the limits of the tide without scraping on the sides of the float box and, if necessary, the gauge should be moved to bring the float into a more nearly central po- sition. If there is any evidence of the float box being clogged with mud, barnacles, or other matter, the opening should be cleared.

524. The tide staff should be examined to see if it is in good condition. If there is more than one staff at the station, make a note of which one the tide observer is accustomed to using. If the staff is a portable one, it should be placed in position for use and a note made of the reading on the staff that is level with the support on the fixed guide. It is important that the staff be actually placed in position, as it may happen that some obstruc- tion in the guide will prevent the staff from being lowered to the position indicated by the shoulder or angles attached to the staff. If there appears to have been a change in the position of the staff, ascertain, if possible, from the tide observer or other source, the time when such change took place, as this information is very important in the interpretation of our records. The fixed guide for the portable tide staff should be examined to see that it. is in good condition.

525. As many bench marks as practicable should be connected with the tide staff by spirit levels. If there is more than one staff at the station, all should be connected with the bench marks by levels. If a portable staff is used, it should be placed in posi- tion to determine its true relation to its support. At permanent tide stations not less than five permanent bench marks should be maintained. One of these should be as near the tide staff as practicable to afford a ready means of checking the elevation of the staff. It is also desirable that a temporary mark be placed directly alongside of the staff, which would enable the staff to be replaced without the use of a level should it be removed from any cause. The pi'esent condition of the bench marks should be noted, deficiencies in the descriptions supplied, and new marks established when necessary.

188 GENERAL INSTRUCTIONS FOR FIELD WORK.

526. A separate report should be made for each tide station in- spected. This report should include i-ecoiumendations concerning repairs to the station, with an estimate of the cost whenever it is possible to do so.

527. Estimates for the inspection of tide stations should be sub- mitted as soon as possible after the receipt of instructions. A small amount should be included to cover small incidental repairs which may be required, and these repairs should be made if pos- sible while the inspecting officer is at the station. At remote stations, when repairs are urgently required which exceed in amount the sum allotted for repairs, and when it is desirable that these repairs be made while the inspecting officer is at the station, to avoid the expense incidental to sending some one else to super- intend the work, telegraphic approval of the estimates for this work should be requested from this office in case circumstances will not permit the delay of obtaining authority by mail.

CURRENTS.

528. General remarks. — Currents are of two types, tidal and non- tidal. Tidal currents are due to the tides and nontidal currents are due to winds, land-water discharge, differences of density, and other such causes. What chiefly distinguishes these two kinds of currents from each other is the fact that tidal currents are periodic while- nontidal currents are not periodic.

In passages from one body of water to another, in narrow en- trances to bays and in constricted parts of rivers, currents fre- quently attain considerable velocity. In such places, as well as off capes and wherever strong currents or countercurrents are be- lieved to exist, current observations are desired.

529. location of stations. — The location of each current station should be determined by angles between three or more objects plotted on charts or hydrographic sheets, and angles should be taken during both flood and ebb. When observing currents off- shore out of sight of land the position of the station should be given by latitude and longitude with as much precision as the means at hand will permit. In all cases soundings should be fre- quently made, as -these aid in identifying the station.

530. Length of observations. — Continuous observations covering a period of at least 2 tidal days or 50 hours are desirable. In every case the aim should be to secure at least 25 hours of con- tinuous observations. In places where daylight observations only

CURRENTS. 189

are feasible two sets of observations should be made, each covering a period of 2 days and separated by an interval of 1 or 2 weeks.

531. Frequency of observations. — Observations should be made as frequently as practicable and preferably at definite intervals of time, as quarter-hourly, half-hourly, or hourly. Near the time of strength of current it is of advantage to make observations? every 10 minutes or even oftener.

532. Measuring current velocity. — For measuring the velocity of the current either a log line and current pole or a current meter Is generally used. Where dangerously swift currents prevent the anchoring of a bout the velocity of the current may be determined by noting the time taken by a free float to pass a measured dis- tance between two ranges. Observations made by following up a free float with a boat are, as a rule, of little use, since the station is continually changing.

533. Log line. — The log line should have a sufficient length of stray line to permit the current pole to attain a position beyond the effect of the disturbed waters in the wake of the vessel. One hundred feet of stray line is generally satisfactory. Part of this stray line should be much heavier and stronger than the log line, because it is used to lift the heavy-current pole out of water; A one-half inch rope is often used for the first 25 feet of stray line, to which is attached a log line three-sixteenths of an inch in diameter.

When the log line has been thoroughly wet, it is marked by- tags or otherwise into principal divisions representing knots, or nautical miles per hour, and secondary divisions for the tenths of a knot. The length of these divisions will vary according to the period of time that the float is allowed to run, and may be conveniently found from the following equation:

Where JT=the length in feet of the graduation representing one

knot, r=number of seconds during which the pole runs out.

The log line should preferably be marked for an observation interval of 60 seconds for use with a stop watch into principal divisions representing knots, each 100 feet 4 inches in length. Tenths of knots will then be shown by lengths of 10.13 feet, which should be marked in a suitable manner.

Occasionally, as with very strong or very weak currents, it is more convenient to use a shorter or a longer time interval. In

190 GENERAL INSTRUCTIONS FOR FIELD WORK.

that case the following formula will give the velocity. If L= length of line in feet run out during an interval of T seconds, the velocity in knots is

V_L 3600 L

The log line should be measured occasionally, when wet, and any error in length noted in the record book. If the line breaks it should be carefully repaired and a note to that effect entered into the record book.

Velocities expressed in feet per .second may be converted into knots, or nautical miles per hour, by the following equation :

Where F=the velocity in feet per second.

534. Current pole. — This is generally a pole or other cylindrical body 2 or more inches in diameter and from 6 to 20 or more feet in length, according to the depth of current to be measured. The pole may be made of wood or of sheet metal, and in either case is to be weighted at one end so as to float vertically with about. 1 foot out of water. The weight of lead or other material required may be estimated from the following equatiou :

W=64 I*>K(L — 1) — 10, in pounds. Where 64=pounds in 1 cu. ft. of sea water. r= radius of pole, in feet. »=3.1416. L=length of pole, in feet, which is diminished by 1

because 1 foot is out of water. w?=weight of pole, in pounds.

When a wooden pole is used, while most of the lead may be a solid casting, some of it should be in the form of sheet lead wrapped around the lower part of the pole and secured by n nail ; for the pole will absorb water after it has been in use some time, as will be indicated by a gradual decrease !n the length of pole above water, and then by unwrapping a portion of the sheet lead, and cutting It off, the former line of flotation may be restored.

535. Observing with log line and pole. — The reel carrying the log line should be mounted upon a stand or other support. About five minutes before the time for observations lower the pole into the water and allow It to run out the length of the stray line. On the exact time for observing, release the line and press the

CURRENTS. 191

stem of the stop watch. If necessary assist the line by hand, but do not pay out faster than the pole cau take it away. When the stop watch has completed one minute, stop the reel and at the same instant press down the stem of the stop watch. Then count the number of knots and tenths that have run off the log line.

536. Direction of current. — For determining the direction of the current one of the two following methods may be used, either alone or preferably together: (a) Measuring with a sextant the angles between the pole and fixed objects on the shore; (6) noting the angle the current line makes with a graduated circle called a pelorus.

537. Use of the sextant. — When measuring the angle between the float and some fixed object on shore, the angle should be marked " R " if the float is to the right of the object of reference and " L " if it is to the left. This rule should be invariably fol- lowed, ft using fixed objects on shore for azimuth, it is of ad- vantage to choose objects rather remote and few in number.

538. Use of the pelorus. — A simple form of the pelorus consists of a circular brass disk about 8 inches in diameter graduated every 10 degrees, from 0° to 360°, clockwise. It is fastened to the boat so that the 0° and 180° marks fix a fore-and-after line parallel to the keel of the ship with the 0° forward.

After the current pole has run out the observation interval for determining the velocity of the current, the log line is stretched across the center of the pelorus and the angle made by the line to the nearest 5 degrees is recorded. The heading of the ship by the ship's compass at the same time is noted, which, with the pelorus reading, gives the direction of the current

When the log line is stretched across the pelorus, two angles, differing from each other by 180°, are determined. In order that no confusion may arise, that arc is read which is farthest away from the pole. This will be the forward semicircle of the pelorus, unless the pole happens to be drifting forward of the beam, when the after semicircle is read.

539. Current meters. — All meters whose measurements depend upon the impact of water against a rotating wheel or propeller should be rated at frequent intervals. This may be done at the Bureau of Standards or may be accomplished by attaching the meter, well submerged, to the bow of the boat and drawing it at uniform rates through still water. The boat is driven at vari- ous speeds over a measured course and the number of revolutions of the meter per second is noted. A rating curve may then be

192 GENERAL INSTRUCTIONS FOR FIELD WORK.

drawn by plotting on cross-section paper the speed in feet per second as abscissas and the revolutions per second as ordinates. With all meters sent out from the office rating tables will be fur- nished.

540. TTse of current meters. — In smooth water measurements with a meter may be successfully made within a foot of the sur- face. If waves exist, the measurements should be taken at such ;i depth that the flow appears to be practically steady.

If the velocity is to be determined at a depth of only a few feet below the surface, the meter may be attached to a pole; if at a considerable depth, it must be suspended by a strong slender cord or cable, and to the lower extremity of the meter sufficient weight should be attached to keep the meter in a nearly horizontal position.

When using a meter which measures velocity only, a current pole should be used for determining the direction of fte current. Care should be taken to make the average depth of the resisting surface of the pole about equal to the depth at which the meter is suspended. Another method for ascertaining the direction of the current at the depth of the meter is to suspend a heavy body of suitable specific gravity to that depth, the position taken by the suspending wire indicating the direction of the stream.

Photostat copies of instructions covering the care of current meters may be secured from the office.

541. Kind of time used. — Since it is desired to determine not only the velocity and direction of the current, but also the time of maximum and minimum velocities, correct time is essential. The record should clearly state the kind of time used, whether standard time, mean local time, or apparent local time. If stand- ard time is used, as is generally the case in coastal waters, the standard meridian should be specified.

542. Checking the time used. — The time used should be checked frequently; of the observer can readily consult a reliable clock, a time comparison can be made each day. A note in the column of remarks of the record book should state the observer's time when the comparison was made, the correct time, and whether or not the observer's time was corrected.

If the time is obtained from a noon mark or from sextant ob- servations upon the sun it can be reduced to local mean time by applying the equation of time. Local mean time is reduced to standard time by adding a number of minutes equal to four times the number of degrees which the standard meridian is to the east- ward of the local meridian.

CURRENTS. 193

In localities having a water horizon, the time of sunrise or sunset (using the upper limb of the sun) should occasionally be noted in the column of remarks.

543. Record of observations. — The form of record book now available for current observations is numbered 270, and all items called for should be filled in as completely as possible. Particular attention is directed to the items at the beginning of the record calling for the deviation table and information regarding the kind of time used, location of stations, and description of current apparatus used.

544. Reduction of observations. — The method used in reducing any given series of current observation, other than by the har- monic analysis, depends on the type of current under discussion. In the inland waters and close inshore along the coast, both on the Atlantic and the Pacific, the currents are of the rectilinear or reversing type. Offshore, on both coasts, the currents are of the rotary type. But whereas on the Atlantic coast the currents show practically no diurnal inequality, the Pacific coast currents, both of the rectilinear and rotary types, show considerable diurnal inequality. The method of reduction of each of these types of currents is outlined below.

545. Atlantic coast rectilinear currents. — Where the current has a distinct period of flood followed by a slack and then by a period of ebb current, the observations should be plotted on cross- section paper, taking the hours of the day ;is abscissas. Two curves will then result for each day, one the curve of velocities and the other the curve of directions (azimuths). From the smoothed curves there is then entered on Form 451 the times of slack and strength of flood and ebb, together with the velocities and directions pertaining to them. On the same form, under the heading " Moon's Transit or H. W., L. W.," there is entered the times of tide, predicted or observed, at some near-by station. If no satisfactory tides be available, the times of the moon's upper iind lower transit may be used. The tidal-current or luni-current interval for each element of the current is then entered in the proper column, and a mean value from the whole series of obser- vations derived.

546. Atlantic coast rotary currents. — Offshore the currents do not flow in one general direction during the flood and in an oppo- site direction during the ebb. Instead, the direction of the cur- rent changes continually at an average rate of about 30° per hour. In reducing a series of observations of this type of current, the

13027°— 21 13

194 GENERAL INSTRUCTIONS FOR FIELD WORK.

observations are plotted on cross-section paper, as indicated for the rectilinear currents. In addition, the times of high and low water, predicted or observed, at some near-by place, are indicated on the cross-section paper. The hourly velocities and directions of the current from three hours before to three hours after high and low water, are then entered into Form 507, and the mean for all the hourly values found. Where no suitable tide is avail- able, the hourly values of the velocity and direction of the current with reference to the moon's upper and lower transit from six hours before transit to six hours after transit should be used.

547. Pacific coast rectilinear currents. — Because of the consider- able diurnal inequality found in the Pacific coast currents, some modification of the procedure outlined for the reduction of the corresponding current on the Atlantic coast is necessary. The ob- servations are plotted on cross-section paper and the times of slack and ebb referred to some suitable tide. But it will be necessary to distinguish the two high waters and the two low waters. One set of the current elements will be referred to higher high water another to lower low water, the third to lower high water, and the fourth to higher low water. The tidal-current intervals and the velocities of the various currents with reference to the various tides must be kept distinct, and the means of each group found separately.

548. Pacific coast rotary currents. — Offshore the currents on the Pacific coast become rotary. In the reduction of tins type of cur- rent the method followed is that outlined for the corresponding current on the Atlantic coast, except that the hourly values of the velocity and direction of the current are referred to both higher high and lower high waters and to lower low and higher low waters from three hours before time of tide to three hours after.

549. Weak rotary currents. — Where the velocities of rotary cur- rents do not exceed half a knot, the effect of wind or river dis- charge will frequently completely mask the tidal currents. In such cases, the tabulated hourly values of the current, with refer- ence to the time of tide, must be resolved into north-and-south and east-and-west directions, before summing for the mean. This may easily be done by means of a traverse table, but is best left for computation at the office.

550. Winds. — In the open sea the effect of a continued wind is to cause a nontidal current, setting somewhat to the right of the wind in the Northern Hemisphere and to the left in the Southern Hemisphere. Near the coast, local conditions modify the above rule. It is therefore essential that the direction and velocity of

MAGNETIC OBSERVATIONS. 195

the wiud be recorded every hour in the record of current obser- vations, so that the local effect, of the wind on the curent may b« ascertained.

MAGNETIC OBSERVATIONS.

551. General remarks. — For detailed information in regard to in- struments and methods of observing reference should be made to " Directions for Magnetic Measurements " published in 1911. New edition in preparation (1920).

To secure the best results, particular attention should be ps:id to the following points :

Be sure that all articles of iron and steel are removed, to a safe distance before beginning magnetic, observations. This applies parlicularly to articles about the person of the observer, such as knives, keys, belt and suspender buckles, eyeglasses, watch, stool in briin of stiff hat, etc.

Be sure that the instrument is level and the levels in adjustm-cii ! before beginning observations, especially in latitude and azimuth observations.

Be careful to keep the magnets and dip needles dry and clean, especially the pivots of tlie dip needles.

Handle the chronometer irith care at oil times.

552. Equipment. — Observers engaged exclusively in magnetic work are supplied with a complete magnetic outfit, consisting of theodolite-magnetometer, dip circle, half-second pocket chronome- ter, and nonmagnetic observing tent. AVhen magnetic observa- tions are to be made only as opportunity offers in connection with other branches of the field work of the Stirvey. the equipment is often less complete, either a dip circle with special needles for total intensity observations and a compass attachment for deter- mination of the magnetic declination,- or simply a compass dec- linometer for declination alone. In such cases the true meridian is usually known from triangulation, or else the instrumental equipment includes a theodolite and timepiece with which the necessary astronomic observations can be made.

553. General survey parties working in remote regions, such as Alaska or the Philippines, will in general be furnished with a compass declinometer for measuring the magnetic declination. In connection with triangulation where the true azimuths are known, the magnetic declination can readily be obtained, and this should be done at intervals of about 20 miles along the progress of the triangulation, or at shorter intervals where there is indication of local disturbance. In the regions mentioned this should be con-

196 GENERAL, INSTRUCTIONS FOR FIELD WORK.

sidered a regular part of the work of general survey parties. The making of complete magnetic observations, including dip and in- tensity, will be required only when there are special instructions. The declinometer may be set up directly at the triangulation sta- tion ; or if this is impracticable because of the presence of iron, height of tripod, or other cause, a magnetic station may be estab- lished by alignment between the triangulation station and the mark.

554. Selection of stations. — The conditions to be satisfied in choosing a magnetic station are freedom from present and prob- able future local disturbance, combined with convenience of access. A station on suitably situated public property, or property be- longing to an educational institution, is to be preferred, as it is less likely to be disturbed. Proximity of electric railways, masses of iron or steel, gas or water pipes, buildings of stone or brick, should be avoided. A quarter of a mile from the first. 500 feet from the second, 200 feet from the third and fourth may be con- sidered safe distances. The station should be at least 50 feet from any kind of building. If any doubt arises in the selection of a station on account of the possible existence of local dis- turbances, two intervisible points a hundred yards or more apart should be selected and the magnetic bearing of the line joining them observed at both. A lack of agreement between the two results is evidence of local disturbance.

555. Description of station. — Each point occupied should be de- scribed with' sufficient detail to render possible its recovery. The description should begin with the general location — enough to indicate the park or field in which the station is situated — this to be followed by measured distances to fences or other near-by fixed objects, and the manner in which the station is marked. It should include the approximate distance and direction from the center of town or from some point which can be definitely located oil a map. so that a rough check on the latitude and longitude may be made. In case a new station is established in a locality where observa- tions have been made before, the distance and direction from the old station should be given if possible. It is desirable to give a rough sketch showing the relation of the station to surrounding objects, indicating on it the direction of north (which should always be toward the top of the sketch) and the direction l>f the marks of which the true bearings are determined.

556. Azimuth marks. — These marks should be well-defined ob- jects as nearly in the horizon as practicable and likely to be

MAGNETIC OBSERVATIONS. 197

available for future use. Where an observing tent is used, it is preferable to have the mark to be used in azimuth and declination observations -in a southerly direction, so that it may be sighted upon through the opening in the south side of the observing tent. Jt should be one-quarter of a mile or more from the station if possible, so that a small error in recovering the station or a small change in the position of the marking stone would not materially affect the azimuth of the mark. As an angle of 1' subtends an arc of approximately 1 inch at a distance of 300 feet, the effect at any given distance may be readily computed.

557. Marking of stations. — Every .station intended for future use should be marked in as permanent a manner as conditions will warrant, to assist in its subsequent recovery, using the bronze magnetic station marking disk whenever possible. To avoid being disturbed the station mark should project little, if any, above the surface of the ground and should extend 2 feet or more into the ground. :

558. Meridian lines. — When a meridian line is to be established the magnetic station should be selected so as to form one end of the line and the distance; to and location of the other (Mid should be given in the description. The line should be not less tlmn 300 feet long, and extra precaution should be taken to secure the marking stones against future disturbance. The azimuth obser- vations must be made with special care and the computations revised before the second stone is set.

559. Repeat stations. — Where observations are to be made at an old station for the purpose of determining the secular change, especial effort should be made to occupy the precise point at which the earlier observations were made. Any change in the immediate surroundings should be noted in the description of station. If local conditions have changed to such an extent that a reoccnpation of the old station is clearly undesirable, then a new station must be established. There may be cases, however, in which it will be best to reoccupy the old station and also estab- lish a new one, as for example when the old station, while not satisfying the requirements of future availability, may stiii suffice to determine the secular change since the former observations. When, owing to change in the immediate surroundings or defect of the original description, it is impossible to locate the exact spot from the measured distances, the desired result may some- limes be accomplished with the aid of the bearings of prominent objects. Having three well-defined objects which were connected

19S GENEKAI, iNSTIMTCTIOXS KOK FIKIJ) WOUK.

by angular measures at the time of the former occupation, suc- cessive trials with the theodolite will serve to locate the spot at which those angular measures are reproduced.

560. Care of instruments.— Care should be taken to keep the in strument in good adjustment and free from dust. The magnets should be touched with the hands as little as possible and should always be wiped with clean chamois or soft tissue paper at tlie close of observations. They should not be allowed to touch each other nor come in contact with iron or steel objects and should in the Northern Hemisphere be kept in the box with north end down. The dipping needles should be wiped with tissue paper both before and after observations and the pivots and agnte edges cleaned with pith. In reversing polarity the bar magnets should be drawn smoothly from center to ends of needle, as nearly parallel to the axis of the needle as possible. The bar magnets should be wiped after using to prevent rusting and should not be allowed to touch except at ends of opposite polarity.

561. Order of observations. — When a complete instrumental outfit is supplied the observations at a station comprise morning and afternoon azimuth, latitude "at noon, one set of dip with each of two needles, two sets of declination, deflections, and oscillations, and angles between prominent objects. It is desirable that the azimuth observations should be made at nearly equal times not less than two hours before and after apparent noon. Latitude observations should begin about 10 minutes before maximum alti- tude of the sun (apparent noon) and continue until about 10 minutes after. They need not be made when a reliable latitude is available. As the declination and horizontal intensity are usually changing more rapidly in the morning than in the after- noon, it is preferable to make the magnetometer observations in the afternoon. They should be made in the following order : Declination, oscillations, deflections, deflections, oscillations, dec- lination. At stations far removed from a magnetic observatory, particularly where the diurnal variation is large, as in western Alaska, it is desirable to make additional declination observations at other times of the day, preferably at about the times of maxi- mum and minimum, as a control on the correction of the results for diurnal variation. The mean of the maximum and minimum values of declination is usually a close approximation of the mean value for the day.

562. Thermometer. — The same thermometer must be used through- out a set of intensity observations and placed as near the lonri

MAGNETIC OBSERVATIONS. 199

magnet as possible. Before beginning observations the thermom- eter should be examinee! to see that the mercury column is not broken and that none of the mercury is in the upper recess. A broken column can usually be joined by holding the thermometer in the hand and striking the wrist sharply against the knee or by Attaching it securely to a string and swinging it rapidly in a circle.

563. Discrepancy limits. — Before leaving the station the compu- tation should be carried f.-ir enough to show that there is nothing radically wrong with the observations. Thus, in good work, the two consecutive sets of azimuth should agree within one minute, and the morning and afternoon sets within two minutes. A greater difference is usually due to lack of adjustment or level of the theodolite or to a mistake in pointing on a wrong limb of the sun. The effect of changes in level of theodolite should he elimi- nated by the method of observing described under " Elevations by vertical angles" (p. 00). In case the difference between morning and afternoon azimuth amounts to more than five minutes, the observations should be repeated. The two sets of declination should not differ more than two or three minutes when allowance is made for diurnal variation. The average time of 70 oscillations, or whatever number is used, should not differ more than a half second in the two sefs, and in the deflections the two values of log sin 11 should not differ more than 0.00100 for either distance, when allowance is made for the difference of temperature of the two sets. When the dip results for the two needles differ by more than five minutes fn excess of tJtc normal difference, the observa- tions should be repeated. Thus, if previous observations show rliMt needle No. 1 gives on the average a dip three minutes greater than needle No. 2, the observations should be repeated when No. 1 gives a result more than eight minutes greater or two minutes less than No. 2.

564. The record should be kept with a hard pencil (or fountain pen) and entered at once on the proper form (not recorded on blank paper and afterwards copied on the form). All computa- tions should be made in ink. The different sheets should be punched and fastened together in the covers provided (Form 367), arranged in the following order: (1) Description of station, (2) angles connecting the azimuth mark with other prominent objects and chronometer correction on standard time (Form 441), (3) latitude (Form 267), (4) azimuth observations (Form 266), (o) azimuth computations -(Form 269), (6) declination (Form 37), (7) dip (Form 42), (8) oscillations (Form 41), (9) deflections (Form 39).

200 GENERAJL INSTRUCTIONS FOR FIELD WORK.

565. Abstract. — Before the record is sent to the office the compu- tations should be completed and a copy made (on Form 442) -of the results and also such quantities as would be needed to replace the computations in case the record is lost. No duplicate of the records is to be made. All records must be turned in promptly, especially at the end of the calendar year, in order that the results may be included in the annual publication of results, which covers the calendar year.

566. Observations with compass declinometer or with the com- pass attachment of a dip circle are recorded on Form 38a. Stand- ardization observations should be made at the beginning and end of the season at some place where the declination is known from magnetometer observations.

567. Total intensity. — The total intensity may be determined with a dip circle by Lloyd's method (Form 389) when suitable standardization observations have been made at a station where the dip and intensity are known. As the determination of total intensity by this method is relative, it is necessary to guard, as far as ijossible, against any change in the magnetism of the two needles and to use the same weight in the field as during the stand- ardization observations. Their polarities m-ust never be reversed, therefore, and they must not be allowed in close proximity to the bar magnets when these are being used to reverse the polarity of the regular dip needles. Standardization observations should be made at the beginning and end of the season's work to determine the intensity constant.

568. Observations on board ship. — On shipboard declination is determined with the standard compass, dip and intensity with a Lloyd-Creak dip circle mounted on a suitable gimbal stand. The successful determination of declination, dip, and intensity at sea requires, first, that observations should be made with the Lloyd- Creak dip circle at a base station on shore at the beginning and end of the cruise to determine the intensity constant for the par- ticular weight used at sea Knd the correction to the dip as de- rived from the deflection observations; and, second, that the ship be swung at the beginning and end of the cruise (and if possible in the highest and lowest latitude reached) at a place near shore where the declination, dip, and intensity are known from shore observations, in order to determine the deviations of the standard compass and the deviations of dip and intensity at the dip-circle position.

569. The accuracy of the results depends principally upon the successful determination and elimination of the effect of the ship's

DESCRIPTIVE REPORTS. 201

magnetism. For this reason observations are usually made on 8 or 24 (preferably 24) equidistant headings, steaming in a circle forward and baok (with port and starboard helms), holding the ship long enougli on each heading to secure good results, and tak- ing usually not over two hours for both swings. Since a complete determination of dip and total intensity on each of 24 headings of the forward and back swings would consume too much time, the practice has been adopted of observing deflections alone while swinging ship in one direction and loaded dip alone while swing- ing in the opposite direction. Besides the total intensity derived from the combination of these observations, a value of dip on each heading results from the deflection observations, since the sus- pended needle is deflected by approximately equal amounts in opposite directions from its normal position. On each heading, observations with dip circle are made in only one position of circle and needle, as follows :

0° to 75°, Circle East, Needle Face East; 90° to 165°, Circle West, Needle Face West ; 180° to 255°, Circle West, Needle Face East ; 270° to 345°, Circle East, Needle Face West. In this way the observations with the dip circle can be made in about the same time as required for the compass observations, which are being carried on at the same time.

570. When instructed to make magnetic observations at sea the ship should be swung at least once a day if possible. When cir- cumstances would not permit a complete swing, results have some- times been obtained from observations on and near the course ; e. g., on course one or two points to starboard, one or two points to port, and back on course. This requires a knowledge of the deviations on those particular headings, which may be derived from the complete swings preceding and following. (See Appen- dix 3, Report for 1904, pp. 192 to 197, and Forms 354, 355, 356, compass, and 390, 391, 392, dip circle.)

DESCRIPTIVE REPORTS.

571. Descriptive reports must be submitted to cover all hydro- graphic and topographic surveys. It is preferable to have a sepa- rate report for each sheet, but in some cases it may be more con- venient to have a single report cover the consecutive sheets of a season's work in one locality where much of the information is common to the different sheets.

(a) The descriptive report should not be in the form of a letter, it should not be a journal of the work, and it need not contain any-

202 GEXERAL, LNSTKtXTIOXS FOR FIKU) WORK.

thing about tlie movements of the party ; it should be entirely distinct from the season's report and should give the date of the instructions under which the work was done.

(6) It should be headed " Descriptive report to accompany sheet (insert number and title of sheet or sheets)." Writing must not be nearer than 1 inch to left edge of paper.

(c) The descriptive report is for the purpose of supplementing original sheets, either hydrographic or topographic, by informa- tion not readily shown thereon, and which will be useful in the interpretation of the sheets, in the compilation of sailing direc- tions, and in chart construction. Preference should, however, be given to showing information on original sheets themselves when practicable to do so.

(d) The descriptive report should be written concisely, omitting all unimportant detail, and should be arranged in a systematic manner with each class of information in separate paragraphs under suitable underscored headings.

(e) Bearings given in connection with sailing directions and hydrographic information should in general be expressed as from seaward and in degrees, and it must be clearly stated whether the bearings sire true or magnetic.

572. Subject heads. — No general rules can be laid down, but the following points will be suggestive in preparing descriptive reports .so far as applicable to any particular region and according to the character of the survey, made. The amount of detail to be given requires much judgment; overminute details tend to obscure the most useful facts. Obviously certain classes of informaton may be useful as to a new country previously unsurveyed \vhich may not be necessary to give in connection with the resurvey or" a well-known coast.

(a) General description of the coast, following the geographic- sequence of the published Coast Pilots or Sailing Directions, and includiug the aspect or appearance of the coast on making the land ; describing prominent objects, as, on a bold coast, the head- lands, peaks, etc., with their form, color, and height; or, on a flat coast, the spires, beacons, etc. Especially describe' the first landfall and objects useful as guides to navigation. (See pa.-s. 194 to 19G.)

(b) Outlying dangers and islands, the limits of tide rips and breakers, and their relation to wind and tide.

(c) Currents, tidal or not tidal. — General conclusions from observation or other information. How long does flood run after

DESCRIPTIVE REPORTS. 203

high water and ebb after low water? Does current set fair with channel?

(d) Landmarks. — Description of all prominent landmarks likely to be useful to navigation or to future surveying operations should be submitted as directed in paragraphs 194 and 212. If mountains, state whether summits are often clouded. Give measured or esti- mated heights of mountains, hills, cliffs, islets, or rocks referred to. Describe ranges in use by pilots and means of identifying them.

(e) Inshore dangers. — Extent and nature, least depth over them ; whether visible ; if breaking, at what stage of tide ; how much, if any, is bare at low water : marks or ranges for clearing them by day or night.

(f) Bars and channel*. — Least depth, best time or place for crossing or entering, permanency of bars aud of channels ; breakers on bars and their extent and with what winds or tides they occur.

(g) Anchorages, with descriptions relative to their capacity, holding ground, amount of protection, and circumstances of weather under which tested.

(h) Change of coast line or deaths. — Mention any reliable evi- dence as to recession or growth of shore line or change of depths. If a resurvey, note any important facts regarding changes ob- served. Give evidence, if any, of subsidence or emergence of shores.

(i) Dangers reported or shown on previous charts or surveys; if not found, or if more water found, give in each case detailed statement of effort made to find former shoal water, and any important evidence as to the reliability of the previous report.

(/) Survey methods. — Explain any unusual features of survey methods used ; mention if any part of the Avork is incomplete or requires further examination, and the reason ; also if any portion is less reliable; state the system of control of the work; mention any discrepancies and adjustments made.

(fc) New place names. — When an original sheet contains new place names, i. e., place names which have not hitherto appeared on the charts, chiefs of parties will list them in the descriptive reports of the sheets affected under two heads: (1) Well-estab- lished local names ; (2) names assigned by field officers. In other respects the instructions under the heading " Geographic names " will be followed. Reports should be supplemented with photographs which will illustrate the apparatus used or add to the knowledge of the locality. (See par. 592.)

204 GENERAL. INSTRUCTIONS FOR FIELD WORK.

PROGRESS SKETCHES.

573. A progress sketch faithfully representing the extent of the entire season's work should be prepared and forwarded at the end of each season. Each progress sketch must have a projection.

lu order that the office progress charts may be kept closely corrected, a progress sketch on tracing vellum showing the hydrog- raphy and topography accomplished, shall be forwarded to the office at the end of each month. The information thereon will be transferred to the progress chart and the .sketch returned to the chief of party for each succeeding month's work. If not other- wise designated, the scale of the progress sketch will correspond to that of the published chart showing the entire area outlined for the season's work.

(a) Progress sketches should be made on tracing vellum, u black ink only. They must uot be of excessive dimensions, usu- ally not over 18 by 24 inches. Scales of -nnjWoi sjo'ooo. or ^Wro, are recommended according to the extent and detail. The scale <»f the sketch must be stated hi the title. They should be drawn sufficiently strong to be suitable for blue printing.

(l>) In the Philippines progress sketches of general coast work should, if practicable, be on a scale of ioo1«6o (the scale of the Philippine coast charts) ; for harbor surveys a larger scale may tie used if necessary to show the triangulation clearly. The stamped title form is to be used on such sketches, giving the fol- lowing information: Class of work, island, locality, scale, dates, chief of party, vessel.

(c) The progress sketch .should give the approximate limits of the topography by parallel ruled lines, not closely spaced, the approximate limits of the hydrography by widely spaced dots, and the triangulation as indicated below, including the various oper- ations of a single party for one season on one sketch.

(d) Principal triangulation schemes should be in heavy i.- and ba?3 lines should be of double width. A line observed at both ends should be full throughout. A line observed at one end should be full at the observed end and broken at the other. Kecon- noissance lines should be dotted if shown on the sketch with triangnlation. When the sketch contains reeonnois.sance only, the lines .should be full if they are to be observed at both ends, A line should be broken at the end from which it is not to be ob- served. Old Stations recovered, including spires, stacks, etc.. should appear thus : @ New stations should appear thus :

205

(e) All important points determined, including mountain peaks, should be shown as far as practicable. Lines to intersection sta- tions should he drawn lighter than those of the main scheme. -: fusion of lines may often be avoided by indicating with short lines radiating from intersection points, the stations from which they were observed. All lines, letters, figures, etc., shown on the sketch should be sufficiently bold to make a good blue print.

GEOGRAPHIC NAMES.

574. Distinct names of points, islands, shoals, rocks, town>. mountains, etc., are necessary to the intelligent use of charts and sailing directions, and the surveyor should ascertain the accepted or native names, and use such nanu-s in all possible cases. Atten- tii-n should be called to all new names of geographic features; that is, names not previously used in the publications of the Survey, with a statement whether the name is in local use, and if not, what name is in use, with the reasons which prevented its adoption.

(or) The orifiin of each now name should be stated. Geographic features must not he given the names of living persons as the rules of the United States Geographic Board only permit the retention of such names in rare cases.

(6) All ncic names are submitted to the Geographic Board by the office before publication and the decisions and rules of the board in regard to names are to be followed in all cases. In the Philippines the decisions of the Philippines Committee on Geo- graphic Names govern in the same manner.

i r) y<niic.< already in u*e on charts and maps and in the Coast Pilots should be verified ; if well established and appropriate they should be adhered to, even though found to differ from the native or original name, especially if the feature is of more importance to navigation than it is to the inhabitants, and if the native name is an awkward or difficult one,

(d) Dual names for the same object lead to confusion and nvenience, and special care should be taken to avoid giving

:i new name to an object already named, or changing a name

already established. Where two names are in use it should be

"tained which is the more appropriate and the more ac-

,.hle ro the people of the locality, and report should be made

giving the authorities.

(e) For such objects as require them, and for which ac- knowledged names can not be found, names should be recom- mended, selecting as far as practicable designations that convey

200 GENERAL INSTRUCTIONS FOR FIELD WORK.

some idea of the form, character, productions, or traditions of the place, or some characteristic of its inhabitants; convenience of length of word and pronunciation should also be considered. Report should be made of names so recommended.

(/) In new applications of the terms "shoal," "bank," and " reef" to forms of secondary size and limited extent, but clearly separated from the surrounding bottom by a steeper slope, the following distinctions should be made, but these terms already in use should not be changed :

Shoal should be applied only to areas on which then; is a depth of 6 fathoms or less.

Bank should be employed for areas of greater depth.

A reef is always rocky, and the term should not be used where there is more than 6 fathoms at low water.

(g) Where the native names ascertained have not an estab- lished written form, they should be spelled according to the system of the Geographic Board, a? follows :

(ft) The true sound of the word ns locally pronounced is taken as the basis of the spelling.

(i) An approximation only to the sound is aimed at. An attempt to represent delicate inflections of sound and accent would often result in forms of words too complicated for use.

(;') The vowels are to be pronounced as in Italian and on the continent of Europe generally, and the consonants as in English.

a has the sound of a in father. Examples : Java, Banana. Somali, Bari.

c has the sound of e in men. Examples : Tel el Kebir, Medina, Peru.

< has the sound of i in ravine, or the sound of ee in beet. Ex- amples : Fiji, Hindi.

o has the sound of o in mote.

u has the sound of oo in boot. Examples: : Umnuk. Ung.

ai has the sound of i in ice. Example : Shanghai.

au has the sound of ow in how. Example : Fuchau.

ao is slightly different from above. Example: Nanao.

ei has the sound of the two Italian vowels, but is frequently slurred over, when it is scarcely distinguishable from from ey in the English they. Examples : Beirut, Beilul.

c is always soft and has nearly the sound of *; hard c is given by fc. Example: Celebes.

ch is always soft, as in church. Example : Chingchin.

f as in English; ph should not be used for this sound. Thus, not Haiphong, but Haifong.

ADDITIONAL INSTRUCTIONS. 207

g is always hard (soft g is given by ./). Example: Galapagos.

h is always pronounced when inserted.

j as in English ; <1j should never be used for this sound. Ex- amples: Japan, Jinchuen.

k as in English. It should always; bo used for the hard c. Thus, not Corea, but Korea.

kh has the sound of the oriental guttural. Example: Khan.

gh is another guttural, as in the Turkish : Dagh, Ghazi.

ng has two slightly different sounds, as in finger, singer.

q should never be employed; qu is given by Jar. Example: Kwangtung.

b, d, I, m, n, p, r, s, t, v, w, x, and z as in English.

y is always a consonant, as in yard and should not be used for the vowel i. Thus, riot Mikindany, but Mikindani.

All vowels are shortened in sound by doubling the following consonant. Examples : Yarra. Tanna, Jidda, Bonni.

Doubling a vowel is only necessary where there is a distinct repetition of the single sound. Example : Nuulua.

Accents should not, generally, be used ; but where there is a very decided emphatic syllable or stress which affects the sound of the word it should be marked by an acute accent. Examples : Tonga tabu, Galapagos, Palawan, Sarawak.

(fc) In the Philippine Islands, in translating from Spanish into English nouns which are combined with geographic names, the following system should be followed, except in specific instances where a different usage has already been established:

River, island, bay, point, and gulf are to follow the proper name.

Mount, port, and cape are to precede the proper name.

Rio Grande is to be translated simply rircr. unless these words form the specific name of a stream.

ADDITIONAL INSTRUCTIONS.

575. Completion of field results. — It should be the aim of a chief of party to turn in field records, computations, and sheets in a completed condition, as far as circumstances may permit. All records and results must be transmitted as early as practicable, and in any event before the commencement of another season's work.

576. Records in general. — All records should be kept in a sys- tematic manner on the standard forms as far as provided. They must be sufficiently distinct and clear to avoid all chance of mis- understanding, particularly numbers must be written plainly. Ex-

208 * GENERAL INSTRUCTIONS FOR FIELD WORK.

planation must be given wherever necessary so that the record may be intelligible to one not familiar with the field work.

577. Original records should not be made on loose sheets of paper to be copied afterwards into the regular form of record book, but should in all cases be made at once in the book which is to be transmitted to the office, and must be consecutive and continuous in the order of time in which the observations are made.

578. Erasures should not be made iu original records. Where an error is discovered, draw a line through it and write the cor- rected figures above or to one side.

579. Original records in pencil must not be inked. Pencils softer than No. 3 should not be used in making records. It is prefer- able, but not essential, to make original records in ink.

580. The duplication of records is usually to be avoided, except in cases where called for in the general or specific instructions. The requirements are specified under each head. The function of duplication is the insurance against loss in transmission, and this should be kept in view in deciding special cases.

581. Records or computations sent by mail are to be svell wrapped and registered. When there is duplicate information (in whatever form) it should not be forwarded by the same mail as the original, and in general should not be kept in the possession of the observer any longer than necessary after the completion of the work.

582. Computations in general. — Computations should be kept up during the field work as far as practicable, and at least far enough to show that the observations are sufficient and the record com- plete.

583. Computation? should be transmitted to the office promptly, as soon as reasonably complete. In no case should computations be held with the idea of making them perfect in the field, as the final revision of the computations is the function of the office.

584. All computations must be in a neat and orderly form, and complete, so as to be readily intelligible to others. Every impor- tant operation must be shown.

585. Standard forms for computations should be followed wher- ever practicable.

586. Every computation must show by whom m;ide and by whom checked.

587. Proper titles should be written or pasted on each csihier <>i computations, giving all essential information, as kind of work, locality, date, observer's name, computer's name, etc. Printed labels are available to cover ordinary requirements.

ADDITIONAL INSTRUCTIONS. 209

588. No writing should be placed within 1 inch of the binding margin of the sheets.

589. Information affecting navigation, reports of dangers, and changes in aids to navigation. — (See pars. 374-402.)

590. Suggestions and recommendations of a definite character are invited as to survey methods or instruments, need of surveys or charts in any particular locality, economies in work, improvement or correction of charts or other publications, and concerning aids to navigation.

591. Maps, charts, and sketches (or copies of them) containing information as to geography, topography, or hydrography likely to be of value to the Survey should be obtained when practicable and forwarded to the office.

592. Photographs. — Photographs illustrative of the geographic features of new regions visited are desirable — more especially views from seaward of important features of the coast, harbor en- trances, and prominent landmarks. Views illustrative of survey- ing operations, or of the people of the region, may also be of value whon unusual. The following information should accompany every photograph : Subject, locality, position from which taken (an exact location for views of important coast features is desir- able), date, and by whom taken.

593. All negatives worth preservation taken with supplies and outfits furnished by the Survey are to be transmitted to the office.

594. In the Tropics, owing to climatic conditions, plates and films should be especially cared for, used as fresh as practicable, and developed soon after exposure. If necessary, they should be forwarded for development.

595. Special effort should be made to protect plates and films from being fogged or light struck.. Orthochromatic plates are rec- ommended.

596. Care of instruments. — Proper care of instruments is impor- tant in all classes of surveying work. The officer using the instru- ment should personally see that it is kept in good order and not leave this to anyone else. Instruments in good condition and ad- justment are essential to good work.

597. The arc of a sextant may be cleaned by wiping lightly with chamois skin or a soft rag dipped in weak ammonia. Never polish the arc with paper or cloth, as this is liable to deface the gradu- ation.

598. Sounding wire, even when galvanized, is subject to rust if not well cared for. The reel should be wrapped around with oiled

13027°— 21 14

210 GENERAL INSTRUCTIONS FOR FIELD WORK.

cloths and well covered from rain. When the sounding machine is idle for a short period the wire should be dried by running through cloths, and oiled, and this should be repeated once a month when the machine is not in use.

599. All surveying instruments should be cleaned from time to time. Surfaces that are liable to stick together when left in place for a long time should be moistened slightly with oil or tallow after cleansing and before assembling; this applies to the cells holding object glasses.

600. Particular care should be taken of invar and steel tapes, steel parts of drawing instruments, etc., as all steel instruments are subject to rapid deterioration, particularly on board ship or in a tropical climate. Invar and steel tapes should be cleaned and oiled after use, and the chief of party should make sure that they are carefully handled at all times ; special care is required in reel- ing tapes.

601. A lens may be dusted with a camel's-hair brush, and when necessary may be cleaned by rubbing gently with soft tissue paper, first moistening the glass slightly by breathing on it. A lens should be examined occasionally to see that it is tight in its cell..

INDEX.

Paragraph.

Additional instructions 575-601

Aero-mercurial tide gauge 420

Aids to navigation:

Report on 75

To be located 273,371,372

Anchorages, development of 230, 243

Angles:

Secondary triangulation-

Accuracy required 17

Record of observations 62

Tertiary triangulation—

Accuracy required 59

Areas offshore, development of. 237

Astronomic positions 1

Automatic tide gauge 421-439

Clocks 422

Counterpoise pulley 431

Counterpoise weight 432

Datum pencil 436

Float 429

Float box 441

Float pulley 430

Freezing, to prevent 443

Hour-marking device 439

House for gauge 444

Installation 440

Operation 453

Paper 426

Pencilarm 435

Pencil screw 434

Roll, tide 426

Rollers 424

Scale 437

Setting up gauge 445

Sliding grooved pulley 433

Tension spring 428

Tension weight 427

Azimuth 2, 144

Bars, development of 235, 242

Baselines, secondary triangulation:

Frequency of 13,44

Inclination-correction tables 55

Measurement 15, 16, 46-55

Nets 14,45

Record, form of. 51

Sites for 14,45

Paragraph. Base measurement, primary, general

instructions for 4

Bench marks 514

Box gauge reference 417

Identification mark 516

Leveling record 520

Number of bench marks 514

Permission to use Federal build- ings 518

Qualities of a good bench mark 515-516

Standard bench mark of Survey. 516 Temporary bench mark for tide

staff 408

Boundary marks to be located 97, 98

Box gauge 413

Care of instruments 596-601

Care of property 373

Channels, development of 230

235,242,243 Character of figures:

Secondary triangulation 10

Tertiary triangulation 23

Charts to be obtained 591

Coast Pilot 374-404

Aids to navigation 388

Anchorages 395

Approaches 381

Bars 382

Canals 394

Changes 400

Currents 389

Dangers- Inshore 385

Outlying 376

Description of shore 384

General description of coast 375

Ice 392

Information affecting navigation. 401

Information required 374

Inside route pilot 402

Landmarks 377

Landing places 396

Obstructions to navigation 391

Pilots 380

Ports 386

Refuge 379

211

212

INDEX.

Coast Pilot— Continued. Paragraph.

Rivers 393

Sailing directions 378,387

Tides 390

Watering places for vessels 397

Weather 398

Wrecks 399

Compass deviation ranges 369

Computations, field 100-104

Computations 582-588

Current observations 528-550

Current direction 536-538

Current meters 539

Use of 540

Current pole 534

Forms for record 543

Location of stations 529

Logline 533,535

Observations required 530, 531

Observations with log line and

pole 535

Pelorus 538

Record 543

Rectilinear currents 545,547

Reduction of observations 544

Rotary currents 546, 548, 549

Sextant, use of. 537

Time to be used 541,542

Velocity 532

Wind 550

Curvature correction 143

Dangers to navigation 366,367,370

Dangers, reported, to be investi- gated 366, 367

Depth curves, hydrography 321

Depth units, hydrography 336-338

Descriptions of stations 85-96

Examples 96

Standard marks 89,91

Notes. . . 89,92-95

Reference marks 94

Warning for disturbing 90

Use of notes 87

Descriptive reports 571,572

Difference in time of tide 513

Direction instrument, secondary tri- angulation:

Record of observations. 62

Useof 19

Direction instrument, tertiary tri-

angulation, use of 61

Drag work (see Hydrography).

Paragraph.

Duplication of records 109

Eccentric signals 69,70

Eccentric stations 69,70,105

Elevations:

By vertical angles 112-138

Computations 123-138

Factors, tables of 132-138

Nonreciprocal observa-

tions 130-131

Radii of curvature, loga- rithms 138

Reciprocal observations. . . 128, 129

Zenith distances 123-126

Directions for observations 117, 118

Hours of observation 115

Instrumental adjustment 116

Method of observation 114-116

Plane of reference 113

Record of observations 1 19-122

Engineer stations, United States,

marking 84

Field computations 100-104

Field results, completion of 581

Float box 441

Freezing in float box, to prevent 443

Gauges:

Tide 407

Aero-mercurial 420

Automatic 421

Box 413

Manometer 420

Pressure 419

Staff 408

Geographic names 574

Gravity 139

Harbors, soundings in 238

Horizontal angles, examples of rec- ord 65,68

Horizontal directions, example of

record 62

Hydrography 198-373

Data to start survey 198-199

Drag work —

Dangers 243

Drag depths 245-246

Improvised drag 243

Long wire drag 243-244

Pipe drag 243

Plotting work 243-251

Records 252,247-250

Plan of development 218-220

INDEX.

213

Hydrography— Continued. Paragraph.

Position angles 253-276

Beginning and end of lines. . 269

Buoys to be located 273

Changes of course 268

Numbering 271

Offshore 276

Plotting 266, 276, 317-318

Range, how noted 272

• Record 270,288-312

Selection of objects for 257-265

Signals for observing 275

Theodolites on shore 274

Time interval 267

Scale 201

Shore line 200

Signals:

Character of 213-217

List of locations 212

Location of 202-210

Names 211

Offshore 237

Soundings:

Anchorages 230

Bars 235,242

Channels 230,235,242

Compass lines 232

Danger indications, develop- ment of. 227,228

Deflection scale, use of 281, 283

Depth curves 229

Harbors 238

Interval —

Distance 224

Time 225

Lead and line, use of. 278, 354, 360

Lines 221,223

Machines, use of 285

Offshore 237

Parallel lines 233

Plane of reference for 313-315

Plotting 317-319

Pressure tubes, use of 284

Range lines 231

Records 286-309

Bearings, how recorded 294

Character of bottom 300, 301

Corrections 304

Correctness and clearness . . . 306

Courses, how recorded 294

Directions, how recc.aeu... 294

Soundings— Continued.

Records — Continued. Paragraph.

Duplication of 297

Identification. 287, 307, 309

Index of signals 2%

Information notes 288

Information required 293

Instruments, verification.of.. 289

Location of tide guage 295

Marinegrowth 300

Miscellaneous information. . 292

Separate for each sheet 286

Soundings, how recorded . . . 298 Time of soundings and posi- tions 303

Time to be used 291

Reducers or tie corrections. . . 311-312

Reduction of 310

Reefs 235,236

Locating 240

Sailing lines 230

Shoals- Development 235,239

Exposed 234

With vessel under way 279,280

Hydrographic sheets 317-353

All notes to be used 323

Boat sheet to be sent to office... 347

Celluloid, use of. 346

Comparison with previous work. 322

Dangers to be indicated 332

Depth curves 321

Depth units 336-338

Distances and scales 348

Drafting, character of 324

E rrors and omissions hi records. . 340

Errors to be avoided 353

Features to be distinct 330

Locating lines 352

Numbers of, to be limited 344

Orientation 341

Overlap of 333

Paper for boat sheets 345

Plotting 317-319

Necessary details 319

Positions to be marked 325-328

Protractor, use of 349, 350

Reefs, definition of. 339

Scale 201,332

Size 342,343

Soundings, election of. 331

214

INDEX.

Hydrographic sheets — Contd. Paragraph..

Spacing soundings 351

Table of statistics 320

Tide rips to be indicated 335

Inclination correction tables 55

Indefinite objects 72

Description of 73

Observations on 72

Initial positions 1

Installation of automatic tide gauge. 440

Adjustment of pencil arm 449

Attaching counterpoise weight. . 446

Attaching float 447

Attaching t ension weight 450

Floatbox 441

Setting up gauge 445

Starting gauge 451

Tidehouse 444

Instrument for secondary triangula- tion, selection of 18

Instruments, care of 596-601

Instruments for tertiary triangula- tion, selection of 00

Interpolation of tide records 488

Intersection stations 21, 66, 74

Invar tapes, care of 600

Invar tapes, use of 47-58

Lamps, signal 52

Land survey marks to be located ... 97, 92

Latitude determination 2

Leadlines 354-367

Comparison of, to be recorded. . . 357

Corrections 358,359

Marks to be used 360,361

Material to be used 354

Verification of length 355, 356

Length of lines:

Secondary triangulation 12

Tertiary triangulation 43

Lightkeeper's instruction 58

Location of tide gauge 404

Longitude determination 2

Magnetic observations 551-570

Abstract of results 565

At sea 568-570

Care of instruments 560

Compass declinometer.' 566

Equipment 552

Instructions 551

Meridian lines 558

Observation- Allowable discrepancy 563

Required 561

Magnetic observations— Con. Paragraph. Observations by survey parties.. 553

Records required 564

Stations —

Description of 555

Selection of 554

Total intensity 567

Manometer 420

Maps to be obtained 591

Marking stations : 76-84

Navigation, information affecting. . . 370

Objects, large, observations on 71

Offshore hydrography, positions for. 276 Old stations, supplementary descrip- tions 85

Operation of automatic tide gauge . . 453

Adjustment, changes in 465

Cleaning pencil screw 466

Clocks, comparison of 459

Duties of tide observer 45cv

Label for tide roll 457

Lostrecord 464

Operating troubles 467

Pencils 463

Placing paper on gauge 454

Reading tide staff 458

Removing paper from gauge 455

Time comparison 459

Tension weight 462

Winding tide roll 456

Photographs to be made 592-595

Plane of reference, hydrography . . 313-316

Planes of reference 512

Plotting hydrographic sheets 317-353

Position angles. (See Hydrography.)

Position computation 103

Precise leve'ing 141

Pressure gauge 419

Pressure tubes, use of 284

Primary triangulation :

Accuracy of 4,7

General instructions for 4

Reconnoissance for 4

Progress sketches 573

Recommendations 590

Keconnoissance instructions 4, 142, 143

Records:

Duplication of 87,109,297,580,581

Of hydrographic work 285-312

Preparation for filing 576-588

Transmission of 581, 587

Triangulation 62, 65, 68, 109-1 1 1

Recovered stations, marking 8$

INDEX.

215

Paragraph.

Reduction of tide records 492

Annual inequality in mean sea

level 508

Comparison of simultaneous ob- servations 509

Correction for diurnal inequali- ties 506

Correction for intervals 494

Correction for longitude of

moon's node 502

Correction for mean range 503

Correction for mean sea level — 508

Diurnal inequalities 501

Highwater 500

Higher high water 501

Low water 500

Lower low water 501

Lunitidal intervals 493

Range, mean 500

Tide level, mean 500

Redaction to center 105

Reefs, development of 235, 236, 240

Reference marks 80, 81

Remarking stations:

Recovered stations. 83

United States Engineer stations . 84 Repeating instrument:

Record of observations 65,68

Use of, secondary triangulation. 20 Reports required, aids to naviga- tion. 75

Results of secondary triangulation,

accuracy required 17

Results of tertiary triangulation, ac- curacy required 59

Sailing lines, development of. 230

Secondary triangulation:

Accuracy in general 7

Angles, accuracy required 17

Base measurement 15,16,47-55

Base nets 14,45

Base sites 14, 45

Character of figures 10

Frequency of bases 13

Instructions for 10-21,56-58,69-111

Length oflines 12

Record of observations 62

Results of, accuracy required ... 17

Selection o f instruments 18

Strength of figures 11,25-12

Seeing, poor 99

Sextant glasses 363-365

Paragraph.

Sextants, care of 59

Sheets:

Hydrographic (see Hydrographic

sheets).

Topographic (see Topographic survey).

Shoals, development of 235, 239, 241

Signals:

Characterof 213-217

Tertiary triangulation 56

Sketches to be obtained 591

Sounding (see Hydrography).

Sounding en route to field 368

Sounding machines, use of 285

Sounding poles, use of 362

Sounding wire, care of 598

Spherical excess 106, 107

Staff, tide 408

Glasstube 409

Multiple 412

Portable 410,411

Standard marks, triangulation sta- tions 89-91

Standard notes, description of sta- tions 88-96

Station marks 77, 78

Statistics, hydrography 320

Steel tapes:

CareoL 600

Use of 47-55

Strength of figures:

Secondary triangulation 11, 25-42

Tertiary triangulation 25-42

Suggestions 590

Survey marks to be located 97,98

Tabulation of tide record 468

Comparative readings 471

Dividing scale 470

High and low waters. 478

Hourly readings 483

Marking hours on tide roll 469

Timescale 470

CareoL 600

Invar, use of. 47-55

Steel, use of. 47-55

Temporary objects, observations on. 72

Tertiary triangulation:

Accuracy in general 7

Angles, accuracy required 59

Base measurement 46-55

Base nets 45

216

TNDEX.

Paragraph. Tertiary triangulation — Continued.

Base sites 45

Character of figures 23

Frequency of bases 44

Instructions for 6, 23-1 1 1

Length of lines 43

Results, accuracy required 59

Selection of instruments 60

Strength of figures 25-42

Three-point problem 108

Tide gauge (see Gauges, tide).

Tide station, inspection 521-527

Tide station, report 452

Topographic survey:

Approximate locations 171

Contour interval 151

Control 146

Descriptive report 168

Elevations:

Beyond limit of sheet 153

How obtained 155

How shown 152

Plane of reference for 154

Features to be included 158-166

Inking of sheets 183-191

List of plane-table positions. . . 194-196 Magnetic meridians required.... 148 Marking old triangulation sta- tions 76-96

Photographs and tracings of

sheets 192,193

Plane table, use of 145

Plans and local maps 174

Revision 172,173

Rivers, mapping of 167

Scale 150

Sextant, use of. 167

Sheets:

Dimensions 157

Howlaidout 156

Inking 183-191

List of positions 194

Objects to be located 194-196

Photographs of. 192,193

Stadiaerrors 169

Stadia rods... 170

Paragraph. Topographic survey— Continued.

Symbols and lettering 175-182

Theodolite, use of. 167

Traverse lines, allowable closing

error 147

Traverse 140

Triangulation:

Accuracy of. 4,5,6,7

Boundary marks to be located. . 97, 98

Classification 3

Computation 100-104

Corrections to directions 19

Descript ions of stations 84-96

Direction instrument, use of — 19, 61

Duplication of records 87, 109, 580

Eccentric signals 69, 70

Eccentric stations 69,70

Horizontal angles, example of

record 65,68

Horizontal directions, example

ofrecord 62

Indefinite objects, observations

on 72

Land survey marks to be lo- cated 97,98

Large objects, observations on . . 71

Lightkeepers 58

Marking stations 76-96

Primary 4

Records 62,65,68,109-111

Reduction to center 105

Reference marks 80, 81

Repeating instrument, use of. .. 20,63

Secondary ' 5

Seeing, poor 99

Signallamps 57

Signals 55

Spherical excess 106

Stationmarks 77,78

Survey marks to be located 97, 98

Tertiary 6,23

United States Engineer stations

marking 84

Witness marks 82

Wire drag. (See Hydrography.)

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