851 
Fig. 9 
Slope measurement for a topographic slope scanned 
from opposite directions 
Substituting h from equation (7) gives: 
_ Ap Ap 
^ a _ L2tg Q 1 + L2tg Q 2 - APtg 0 ^ Lltg 0 1 + Lltg0 2 + APtg0 x (J) 
The parallax difference AP and the slope length LI or L2 can be measured. 
Depression angles can be calculated from the flying height and ground range 
towards the end of the shadow. A small error will be introduced when the shadow 
does not lie in the reference plane. 
McCOY (1967) has worked this out for slant range representation. From the fore 
going it can be seen that from stereo imagery, relative altitude differences can be ob 
tained by approximation. Moreover, slope measurements can be made in lines 
perpendicular to the flight direction. For conversion of these apparent slopes to 
true slopes not perpendicular to the scan direction, a conversion nomogram can 
be used (fig. 10). Between apparent slope and true slope the following relation 
exists: 
tg /$ = sin y tg a 
in which /5 is the apparent slope, a the true slope, and y the angle between 
strike and scan direction.