394 



EXPLORATION GEOPHYSICS 



The position of the datum or top target above or below this 50 line is shown as H 

 in the photograph. The interval between the top and bottom target is shown as i and 

 is the interval measured in millimeters directly on the film. Let R represent the number 

 of feet between the top and bottom targets ; then the elevation difference is given as 

 follows : 



A£ = H/i • R 



The quantities H and i are both measured directly on the film negative to the near- 

 est 0.01 mm. i? is a constant depending upon the rod construction. From this it can be 

 seen that the elevation difference is independent of the lens focal length, as well as of 

 the distance between instrument and rod. 



Fig. 237. — Typical photographic record showing dis- 

 tance, direction, and elevation difference between the 

 instrument and the rod, and general terrain conditions. 

 (Courtesy of Republic Exploration Company.) 



The distance from instrument to rod is given as a function of the lens focal length 

 (/), calibration constant {k), target spacing {R), and image size (j) as follows: 



D- 



__ j ' k ' R 



Since the instrument is leveled for all shots, the plane of the film is parallel to the 

 plane of the rod, resulting in true elevation differences and map distances, regardless of 



