848 
very large. Moreover the complement of the depression angle becomes smaller 
and many topographic slopes will be steeper so that equation 4 cannot be used 
any more for height determination, and heights will be somewhere between the 
amount obtained by equation 2 and 4, dependent on topographic slope. Fig. 5b 
gives similar curves now for a fixed height difference and variable shadow 
length and depression angle. 
Another draw back for height measurements is that we always measure the 
relative height difference between the highest point where the shadow starts, to 
a point, in most cases not on a basal plain, but somewhere on a topographic slope 
where the shadow ends. For reasons pointed out above, relative height measure 
ments from shadow length in single strip imagery is of restricted value for inter 
pretation purposes. 
LEWIS & WAITE (1973) have used radar shadow frequency for determining general 
terrain slope information of the backslopes. This is possible for medium to high 
relief areas where the landform must be homogeneous over the entire range of the 
image or where multiple coverage is obtained. 
Measurements from stereo SLAR imagery 
SLAR imagery may be viewed stereoscopically when strips are flown with a cer 
tain overlap. When using two overlapping strips with opposite scan directions it 
appears impossible to fuse the images and to obtain a true three dimensional 
picture. The opposite shadows make this impossible. On the other hand, when 
using one negative and one positive with opposite shadow working it is possible 
to obtain a stereo effect. For two consecutive overlapping strips flown under the 
same scan direction, stereo imagery may be obtained from two positive prints. 
For a vertical object we can calculate the height from the parallax difference, 
making an approximation by considering the wave front as straight instead of 
spherical (McCOY, 1967). For the far range this approximation is not very serious. 
From fig. 6 the following equation can be derived for stereo imagery with same 
scan direction and ground range representation: 
P 2 - P x = AP = h + tg0 2 - h tg0 ± 
U - Ap 
t S 0 2 -t S 0 1 ° 
in which AP is the parallax difference and Q and 0 2 are the smaller and the 
larger depression angle respectively towards the top of the object.