Base Maps 43
show all objects related to the location of real-property boundaries, such as fences or driveways, at reasonably frequent intervals.
3.4 ACCURACY
Accuracy of the horizontal and vertical position information on the base map is fundamentally a function of the map scale and contour interval, respectively. National Map Accuracy Standards (Appendix B) have long been used as the primary standard to control the accuracy of plotted map information. For scales larger than 1:20,000, which include essentially all base maps that would be used to support a cadastral overlay, standards for horizontal accuracy specify that 90 percent of the points tested shall be plotted on the map within 1/30 inch of their true position. Standards for vertical accuracy specify that 90 percent of the points tested shall be shown in elevation within one half of the contour interval used on the map.
The Photogrammetry for Highways Committee of the American Society of Pho-togrammetry has prepared specifications for large-scale mapping for highways, with a horizontal accuracy requirement that 90 percent of all planimetric features be plotted within 1/40 inch of their true position (U.S. Department of Transportation, 1968). This is a more stringent requirement than the comparable 1/30 inch required by National Map Accuracy Standards and has also been suggested by the Task Committee for Photogrammetric Standards of the American Society of Photogrammetry in their recently proposed Accuracy Specifications for Large Scale Line Maps. Either the 1/30-inch or 1/40-inch requirements have been adopted by nearly all users in their base-mapping specifications for large-scale property-ownership maps.
The requirement that the base map of a local record system be compiled according to National Map Accuracy Standards (Appendix B) is primarily due to the need for the base map to satisfy the engineering needs of public works departments. When accurate information is necessary, specific boundary lengths would come from a recorded plat, boundary description, or other report of survey, not from scaling the cadastral overlay on the base map. A new Engineering Map Accuracy Standard has been proposed by the Committee on Cartographic Surveying of the Surveying and Mapping Division of the American Society of Civil Engineers (ASCE). These standards are intended to provide a clearer communication of accuracy requirements between those having the need for the map and those preparing the map. Also included are specific field-testing procedures to assess the compliance of the map with the standards.
Whatever standard is agreed on, the need exists for quality control within a base-mapping program to verify conforrnance of the mapping to standards. An accuracy-check ground survey is necessary to determine the ground positions of checkpoints for comparison with their corresponding mapped position. Evaluation of the checkpoint results, using the National Map Accuracy Standard, can be accomplished by$ 437