226 



GRAVITATIONAL METHODS 



[Chap. 7 



PStvis Units 



Fig. 7-76. Terrain effects and contours at Caribou, Colorado (compare with 



Fig. 7-119). 



on gradients and curvatures from contour lines {cartographic correction). 

 The procedure is illustrated in Fig. 7-77. The terrain is divided into 

 elements bounded by successive contours (interval dp) and subtended by 

 an angle da, so that the coordinates of the center of the element are p and a. 

 Then the corresponding gradients and curvatures may be obtained from 

 (7-69) by neglecting f compared with h so that 



rT „, 5 ,2 dp cos a da 



Uxz = -3fc ^ h 



2 p^ 



jj ^, b ,2 dp sin a da 



LI vz = —6k - h 



[/a = 'ikbh 



2U^u = Sk8h 



2 p- 



dp cos 2a da 

 ? 



dp sin 2a da 



(7-81) 



(6) Graphical methods. The essence of graphical terrain correction 

 methods is that the influence of the terrain is evaluated not by measure- 

 ments in fixed distances and azimuths but by the use of contour lines and 



