AERIAL PHOTOGRAPHY AND NATIONAL FOREST 



.AIAPPING 



By Rolph ThelEn 

 Forest Products Laboratory, Madison, JVis. 



The development of aerial photography as a highly important and 

 indispensable phase of modern warfare has been one of the many won- 

 ders of the great war. Photographic reconnaissance was practically 

 unthought of during the early stages of the conflict, and may be said to 

 have been an outcome of trench warfare. At the time the United States 

 entered the war this art had already become of tremendous importance, 

 and in the final stages a complete detailed photographic map of each 

 sector had to be made daily. General Squier states that the British army 

 made 17,000 photographs before the operations at St. Quentin in order 

 that a relief map of the whole sector might be prepared before under- 

 taking the drive. 



Military maps of this character are commonly called mosaics, and are 

 made as follows: An airplane (other forms of aircraft could be used 

 under certain conditions) equipped with a magazine camera flies over 

 the area to be mapped, maintaining as uniform an altitude as possible, 

 and exposures are taken at the proper intervals to insure a sufficient over- 

 lapping of the resulting negatives. If the area is too wide to be mapped 

 in one flight, a number of parallel flights must be made, and the nega- 

 tives of each succeeding flight must overlap those of the previous flight. 

 After the negatives have been developed, prints are made from them. 

 If the accuracy of the map warrants it, the prints are all made to the 

 same scale ; this is done by making them in an enlarging camera instead 

 of by contact. Distortion, caused by obliquity of the plate at the instant 

 of exposure, can also be corrected in the enlarging camera if proper base 

 points are available. The cameras are usually rigidly attached to the 

 planes, and since it is impossible to fly continuously on an absolutely 

 even keel, a certain amount of distortion is bound to occur. After the 

 prints are made, they are matched up, trimmed, and assembled into the 

 finished mosaic. It is obvious that in the case of flat terrain it is possible 

 by this means to produce an accurate scale map. However, in the case of 

 mountainous country, this is not possible, since the scale wnll vary un- 

 evenly throughout the negative with variations in elevation. Thus a 



515 



