GRAVITATIONAL METHODS 



395 



the vertical angle involved in the field work, without the need for secondary corrections 

 of any kind. 



Although the instrument is always level in operation, the lens used takes in a field 

 of view of ten degrees vertical angle range, thereby permitting operation over rela- 

 tively rough terrain. 



Instrumentation and General Constructional Details of Equipment. — The 



topographic camera, as shown in Figure 236, consists of a 35 mm. camera with tele- 

 photo lens and reflex housing, all rigidly mounted on a specially-constructed head 



Fig. 23S. — Method ot reading pbotogrammetric rec- 

 ord, showing elevation values H, fixed target distance i, 

 and direction angle. (Courtesy of Republic Exploration 

 Company.) 



similar in appearance to a standard transit head. The mount is constructed in such a 

 manner that no vertical angles can be turned with the objective lens. A sensitive level 

 bubble is mounted on the right side of the reflex housing, and the instrument line-of- 

 sight is adjusted parallel with the level position of this bubble. The adjustment is 

 easily accomplished by means of the eyepiece mounted on the back of the camera box 

 which permits viewing the image brought in by the objective lens and focused in the 

 plane of the mil scale lines which are located in the plane of the film. By sighting on 

 a Philadelphia rod, the instrument is adjusted by the standard two-peg method as used 

 in the adjustment of a transit or other conventional surveying instrument. After 

 adjustment is made, the eyepiece is covered and is not used again in regular field work, 

 except to check adjustment at periodic intervals. 



An auxiliary lens and prism system mounted on the left side of the reflex housing 



