2221 HYDROGRAPHIC MANUAL PaGE 74 



2221. Use of Air Photographs in Reconnaissance 



Reconnaissance for the selection of sites for control stations may be greatly facili- 

 tated by the use of suitable air photographs. Unless the air photographs are of the 

 nine-lens type on a scale of 1:20,000 or smaller, or a carefully made mosaic is available, 

 it will be advisable to make a preliminary compilation of the photographs and reduce 

 the compilation to a small enough scale so that the control scheme may be plotted on 

 a single sheet. 



Generally a simple inspection of this plot, supplemented by examination of the 

 photographs under a stereoscope, will be sufficient to select the sites for the triangula- 

 tion stations along the shore. If the adjoining terrain has much relief or obviously 

 advantageous means of access, it may also be practicable to select the sites for most of 

 the inshore stations by simple inspection. 



Approximate heights of tentative station sites may be determined from the photo- 

 graphs if a point of known elevation and the proposed station sites appear in the zone 

 of overlap of two of the photographs. Heights may be bridged from one overlap to 

 another, but large errors are likely to accumulate from the use of instruments and 

 methods ordinarily available to the hydrographer in the field. Generally this will be 

 unnecessary, since most of the air photographs taken in connection with hydrographic 

 surveys will show portions of the coastline, thus affording ready reference to sea level. 



To determine an approximate difference in elevation, the two overlapping photographs should 

 be oriented very carefully under a stereoscope. Then the distance between the two photographic 

 images of the point of known elevation and the distance between the two photographic images of the 

 point whose elevation is desired should be measured to the nearest tenth of a millimeter. The 

 difference between these two distances is called the difference in stereoscopic parallax dp. The 

 distance between the centers of the two photographs on the preliminary plot, or base B, should also 

 be scaled in meters. The symbol used for the known elevation is h. The altitude of the airplane H 

 is obtained from the ratio — focal length of camera / divided by scale of plot S. The focal length of 

 the camera and scale of the preliminary plot wiU be furnished with the photographs and other advance 

 data. Thus for the nine-lens camera and a 1: 20,000 scale plot: 



„ 209.75 mm (/) . ,or ^ 

 ^^ 1/20,000 iSr ^'^^^ ^^^^'"^ 



The difference in elevation dh is given by the equation: 



,j_ dp (in mm)X (H—hy (in meters) 

 ~~ / (in mm) XB (in meters) 



Thus for dp equal to 2.0 mm, H equal to 4,185 meters, reference point at sea level, B equal to 

 2,000 meters, and / equal to 209.75^mm: 



„ , , , 2.0X (4,185-0)2 . 



dh (meters) = 209 75X2 000 ^ ^^-^ meters 



If straight lines are drawn, preferably with white ink, between possible sites for 

 triangulation stations on each photograph, the lines will fuse to form lines in space when 

 the photographs are viewed under the stereoscope. It will then be immediately 

 apparent whether the lines are clear, blocked, or grazing. Reconnaissance on the 

 ground may then be reduced to visiting the sites from which the doubtful lines radiate. 



Study of the photographs under the stereoscope will also furnish valuable informa- 

 tion relative to the types of signals needed, the best landing places, and routes to the 

 stations. 



223. Requirements 



In table 3 are given the requirements for triangulation and traverse control, which 

 shall be strictly complied with, except as amended by the project instructions. 



