GRAVITATIONAL METHODS 



393 



positions, and by recording the interior horizontal angles measured from the base line 

 to the point in question, the distance to the point could be determined by simple triangu- 

 lation. Knowing the distance, and by measuring the vertical angles directly from the 

 photographs, it was possible to compute the elevation dififerences between the instru- 

 ment and points under observation. This method has the advantage of obtaining eleva- 

 tions of inaccessible points, such as rugged coast lines, mountainous terrain, etc., but 

 involves considerable work for both field and office personnel. The method is also 

 subject to certain errors in visual readings made in the field. 



Fig. 236. — Topographic camera. A, 35 mm. camera; 

 B, telephoto lens; B', sunshade; C, reflex housing; D, 

 eyepiece; E, level bubble; F, compass; G, transit head; 

 H, tripod. (Courtesy of Republic Exploration Com- 

 pany.) 



With the recent American development of the topographic camera%, shown in Fig- 

 ure 236, all visual readings in the field, as well as the measured base line, are completely 

 eliminated. Both the horizontal and the vertical control data are combined on the same 

 film-frame with all the speed, accuracy, and dependability that can be achieved with 

 the photographic method of recording. Thus all of the data needed for the determina- 

 tion of distance, direction, and elevation difference between the instrument and rod 

 point are obtained on one picture with one shot (Figure 237). The resultant field data 

 are therefore free of the human element in reading and recording. 



Fundamental Theory and Mathematical Relationships. — Referring to Figure 

 238, the HI line is the level line to which the instrument has been adjusted. This is the 

 line numbered 50, as shown in the photograph. 



t World Oil, January, 1948; Oil and Gas Journal, Nov. 22, 1947; World Petroleum, November, 

 1947; Photogrammetric Engineering, March, 1949. Patent applied for. 



