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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
???*?.!_?•_
€ z«f.
t(fl/« reading of Datum .'.intfor tha etmpa
f—
1912
' kLAblNG
.
,
>->
TIDI-
.™
July 1
7 32
jn
4.90
•.„
3.90
)•
1.00
B
2
7' VI
A 55
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0 95
p
3
7 aa
4.^0
?,?0
o.°o
B
4
7 50
. 4.2S
3.30
0.95
S
5
8 11
4.50
3. SO
1,OQ
|
6
a is
4.70
3.90
0.90
t
7
9 54
5 25
A. an
0.95
5
8
7 33
9.46
4. TO
O.95
T
9
7 49
6.10
S.2O
0.90
T
10
6 27
6.90
5.80
1-10
H
ll
a QS
7, OS
6.10
Q,<«»
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12
7 40
7.20
6. 2O
i.m
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13
9 00
7-50
6-50
1,00
n
14
a 45
7.00
^00
1.00
T(
IB
8 37
3.9S
a. no
0.«5
«
1«
7 20
4. OS
3-10
0,95
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17
a as
4.90
4.00
0.90
H
1«
B 27
4.«S
3.<>0
^•M
*
19
6 OS
4.30
3.40
0.90
S
20
7 14
4.30
3-80
1.00
X,
21
6 4a
5.70
4-flO
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»
22
7 »
e.ao
5.30
0.90
T
a?
7 on
6.93
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"
a*
« 46
',«?
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T
2«
7 32
7. 23
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J
»?70 . 9m
M
7 BO
7.15
6.20
0,<«
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0.96 * Maan
27
7 22.
G.55
5.50
0.95
s
J,QO * Origin/ii Setting
28
7 40
6.55
s.so
0.95
8
6.96 » Corrcctad •
29
7 13
*,*fi
4.<»
Q,9S
S
30
7 32
5. BO
4.50
1,00
»
31
7 79
5.06
4.10
0.95
«
31
20,70
13027°— 21-
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
««*wttili« fefM
;/,. Jota.JU
. 1.906. ....... Otamal
j, 1913
(o«— »
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23.0
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( 8.3)
( 1.8)
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fi.l
2
a.i
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9.9
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7.3
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(14.5)
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( 9,0)
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12.5
8.0
9.7
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9.9
( 2.B)
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( 7.9)
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9.4
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( 8.3J
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( 9.0)
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(21,4)
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0.7
2.7 i 10.2
fi.O 1
12
9,9
8.5
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(11.1)
( 4.«) \ 7.5
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(29 4)
14 9
13,9
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a
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9.2
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( 4.6) S 7.* 2.21
(23. 6) i 20.0
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i 14
i. 10.0
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(10.4) i( 3.9) S 7.7
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21.0
14.5 j 8.9 2.4 II 11-Q S.7 '
IS
( o.«)
10.8
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1«
( 1.7)|
11.5
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14.2 23.2
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17
( 2.8) 12.3
B.S
( «-4)
( 2^9) |! 8.4 i fc£j
15.1 23.4
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i .
1
fi
i
FIG. 24.
164
GENERAL INSTRUCTIONS FOR FIELD WORK.
3caela»n of back of Tom ^8.
TIDES: HIGH AND LOW WATERS
MM .
ffa?M,Ut.9l io»r»l fufe DotrJulj.. 14,15. ... IMiU.Jk.Qfl. lmt
JtO)-<-*«= O.3.- F(.Mn)= 1J33- f,x/-0? = 0.80.. Tin* ntriJ;a».
DATE
1913
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Uv
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Low
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£
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27 daft- J«lT 2 - 2fl
July 1
Hffff UIT
15.9
1S.S
2.*
R,«
99 35
5«M 9»t, 9 ggj
19
( 4.3)
0.9
7.0 a.s
( 2.7)
8.7
4 a'
*«« 3.4S 3,B?
20 0 IV 9 51
20
1.5
7.8 jl 8.9
{ * *)
7,9
4.9
""'"LO* Oi9= 1.1?
17.4
14.5
21.2 |j( 9.4)
a.a
3=T —
ai
lfl,2
23. Q
4.9
<J l'
Cbrrert pj B//S =J 1 01 X 0. 80 = 0. 81
32
( 6.6)
8-0
9.5
ioTa
4.0
C»iT«irfCi«=i.ifl x 0.90 =0.98
19.0
lfi.0
( 9.4)
9.1'
33
( 7.4)
6.9
0.3
11.9
6.2
7.1
4 3^
19.8
17.5
1OTS
(10. 1 }
9.3*
S. 9
34
( 8.2)
7.8
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12.0
6.2
7.4
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20. 6
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(10.O)
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( 9.1)
8.8
1.8
11.0
4.9
7.9
21.5
28
(10.0)
8.5
2.2
11. O
4.7
7.4
1^
22.4
19. S
13.0
( 9.S)
( 3.0)
9.7'
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(10. a)
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30.0
13. S
( a-a)
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9.0
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(11.6)
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10.8
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99
( 8>9) [| *'*
(13.1)
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19.0
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( 9.6) 9.2
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68
184.4 -
472.4
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9 AA
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9.44
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9.55
A, 20
a.T*
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ft. 39
8.58
irri '
Una rat inttrml
&.3B
Jaljr 9C
O.B
10. a
4.2
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3.4
7.5
(IS 2)
91,8
J*i»
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( a a)
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T^tatUI,, J~» «,.<.».
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9.9
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7.7
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fl«'f »ir *. 19ia
Ms gj
99. a
Datt fag, 6. 1313
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
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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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