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HYDROGRAPHIC MANUAL 



Page 98 



Table 4. Corrections to sextometer distances 



[This table gives the corrections to sextometer distances for a standard Coast and Geodetic Survey navigating sextant, and the 

 corrected distances for a sextometer rod with 15-foot spaced targets. All distances are in meters.] 



239. Air Photographic Surveys 



Air photographs are replacing the planetable as the most efficient means of topo- 

 graphic mapping. The displacements and distortions, which at first limited the use 

 of air photographs to revision of former accurate ground surveys and to the addition 

 of inaccessible detail, have been gradually eliminated by improvements in materials, 

 instruments, and methods. In addition to yielding more complete information at 

 lower costs, modern air photographic surveys now give fully as accurate results with 

 no more control than that required for the planetable. In the Coast and Geodetic 

 Survey, a nine-lens air camera has been developed which gives the high accuracy 

 needed for the location of hydrographic signals with one-half to one-fourth the geodetic 

 control needed for the planetable. The nine-lens composite photographs are 35.4 

 inches square or 11.2 miles square on a 1:20,000 scale. Each photograph includes 

 an area of 125 square miles at this scale, from 4 to 20 times as much as a single-lens 



photograph. 



2391. Air Photographs in Mapping 



Photographs record directions from the position at which they are exposed. A 

 perfectly vertical photograph of perfectly flat land would give map positions directly, 

 but the two conditions are practically never attainable. In a perfectly vertical photo- 

 graph of land that is not level, only the points vertically below the camera and points 

 in the datum plane will appear in their true map positions (fig. 14). All other points 

 will be displaced radially from the point vertically beneath the lens by distances pro- 

 portional to their elevations and distances from the plumb point (fig. 15). Note that 

 the trace of the direction from the lens to the object on the plane of the photograph 

 is a line radiating from the principal point. The principal point is the foot of a per- 

 pendicular from the rear node of the lens to the plane of the photograph. It is usually 

 marked by a small cross near the center of the photograph or may be found at the inter- 

 section of straight lines connecting marks at the middle of the sides or near the corners 

 of the photograph. If the photograph is taken when the camera axis is not vertical, 

 the angles between the radial lines will not be exact; but if the axis is within a degree 

 of the vertical and the elevation differences do not exceed 5 percent of the altitude of 

 the camera, radial lines to control points may be used as a multiple-arm protractor for 

 plotting without graphically appreciable error, the center of the photograph on a 



