106 PHOTO INTERPRETATION PICTURE, COLWELL 
understanding the geometry of such photography become apparent, however, when he is 
asked to explain with the aid of a diagram exactly why the images appear as they do. 
Yet, until he understands this geometry, he is likely to misinterpret many features on 
such photographs, including even those terrain features that happen to be much higher 
or lower than the datum plane for which image motion compensation has been set. 
  
Fig. 12. Stereogram showing a bridge under construction. Many photo 
interpreters answer incorrectly the question, “What is the main reason why 
the top surface of this bridge does not appear to be rectangular in shape?" 
For discussion of correct answer, see text. 
Figure 12 is still another illustration of the fact that many photo interpreters have 
an inadequate comprehension of the geometry of aerial photographs. This stereogram 
shows a feature that I often use to question photo interpretation students after I have 
discussed with them the nature of displacements due to tilt, relief, and improper image 
motion compensation. The question I ask is simply “why doesn't the surface of the bridge 
which is under construction here appear rectangular in shape?" Some reply that it is 
because of relief displacement on the photographs; but there is virtually no difference 
in relief on the surface of the bridge to cause such displacement. Others say it is because 
of tilt displacement; but it is fairly apparent on this small stereogram, (and very ap- 
parent when the two entire aerial photos are studied), that both photos are so nearly 
vertical that very little tilt displacement exists. Still others ascribe what they see to 
improper image motion compensation at the instant of photography; but none of the 
other features has suffered and in fact the camera used in taking this photography was 
not equipped with an image motion compensating device from this difficulty. The simple 
fact is that the main reason why the surface of the bridge does not appear rectangular 
in shape on these photographs is because it actually is not rectangular in shape; for in 
these two nearly distortion-free photos the bridge is shown essentially as it really is. 
This is not to say that no displacements are to be found on the photographs. (For 
example it may be quite revealing to ask the photo interpreter to indicate by means of 
a diagram why a light spot is seen between the bridge and its shadow on one of these 
photos but not on the other.) But examples such as this have convinced me that once the 
photo interpreter becomes aware of the fact that there can be displacements and dis- 
tortions on photos he may still be unable to determine whether they appreciably affect 
any particular photo image he is trying to interpret. 
To summarize this section of my report, many photo interpreters need better to 
understand the nature of displacements and distortions commonly encountered on aerial 
  
  
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