AERIAL PHOTOGRAPHY— BAISLEY 3g7 



THREE-DIMENSIONAL PHOTOGRAPHY 



Experiments indicate that depth perception or binocular vision 

 disappears at about 500 feet, and that when we look at more distant 

 objects we get no more information by observing with two eyes than 

 we do by observing with one. In observing the actions of people 

 blind in one eye we see illustrated another method of depth percep- 

 tion. Such a person will move his head from side to side and so get 

 slightly different views of what is in front of his eye, and he is thereby 

 able to estimate distances; thus he has, in fact, depth perception of 

 a kind. 



In flying an airplane we move rapidly along, so that even at high 

 altitude we get different views or a changing view of the same scene, 

 and we thereby get considerable depth perception, just as the man 

 blmd in one eye gets it by moving his head, but the third-dimension 

 sense of depth is not nearly so striking when obtained in this manner 

 as it is when brought out by observing stereoscopically two photo- 

 graphs of the same area taken a considerable distance apart. 



By taking aerial photographs in stereoscopic pairs, depth is added 

 to the picture, and it shows up as a visual model in its three dimen- 

 sions even more strikingly than the scene itself when observed from 

 an airplane. Photographically the two scenes, as observed by the 

 two eyes, can be and often are separated by a distance of several miles. 

 When observed in a stereoscope theso two pictures of the same terri- 

 tory taken from a fairly high altitude and a considerable distance 

 apart form a single visual model, which it is only possible for us to 

 approximate when viewing the terrain itself because of the close 

 spacing of our eyes and the inability to carry the image in the mind 

 for any appreciable time as we move along in flight. 



THE USE OF PHOTOGRAPHY IN MAPPING 



Single lens vertical pictures are useful individually as maps or col- 

 lectively for the preparation of maps, either photographic mosaics 

 or maps of the conventional kind. A single picture taken with an 

 8^-inch lens from an altitude of 21,780 feet will give a scale of 

 2 inches =1 mile, so that a picture of the size taken by our standard 

 camera, that is, 7 by 9 inches, will include 14% square miles and, 

 if used intelligently, can very well serve the purpose of a map of the 

 area shown. 



A great many photographs of this type are required by the Air 

 Corps and by the Corps of Engineers for use as maps. They have 

 the advantage of being very cheap in comparison with a map of 

 such an area made by ordinary means. They also contain much more 

 detail than does a conventional map and have the advantage of being 



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