Ai umm K(a/2,a/2) 
| 
Do | 
| 
| 
45° | 
0 A 
Figure 7. The region D, and the gradient line OK Figure 8. The geometrical form of Z 
Hr 
Dn 
  
EL 
(1) (2) 
Figure 9. The sufficient area condition on a slope: (1) the ground E. with the greatest ground slope € in the direction 
D, and its reference horizontal plane H, (R on H,); (2) the ellipse B, on E, corresponding to the circle with the 
radius R on H, ( e, - long axis and e, - short axis). 
  
41 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996