QUALIFICATIONS AND TRAINING OF THE PHOTO DsTERPRETER 



The efficient use of aerial photography for a water resource inventory requires that per- 

 sons assigned the duty of interpretation have some technical knowledge of photogrammetry and 

 some training and experience in actual interpretation. In any practical situation, the success 

 of the inventory will depend largely on the skill of the interpreter. Every effort should be made 

 to secure a qualified person or to give adequate training to those who are assigned to the work. 

 Those without general aerial photogrammetric training may refer to the publications on this sub- 



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ject by Moessner and Moessner and Choate. Most colleges and universities offer courses in 

 photogrammetric techniques . 



A primary requisite for the photo interpreter is the ability to use the stereoscope. Normal 

 vision in both eyes is therefore required. Almost all the work with the photographs is done with 

 the three-dimensional image provided by stereoscopic viewing. In this, as in all skills, experi- 

 ence is valuable in that it allows the user to produce results faster and more accurately. Time 

 spent in practice with the stereoscope is a necessary part of training. 



In addition to specific training and experience with the photos themselves, the interpreter 

 should have field experience. He should have the opportunity to make a few comparative ground 

 measurements of features typical of those he will be expected to recognize or measure on photos. 

 He should travel through the inventory area with the aerial photos and make direct comparisons 

 between the photo image and the ground situation. In a water inventory, for example, he would 

 want to study the ground situation in terms of lake depth, stream width, slope of bank, vegetation 

 type, and similar features. 



LW'ENTORY OF LAKES 



The inventory of lakes has two aspects: (1) the determination of the total number of lakes 

 and their location, and (2) the physical description of each lake. Because this information is 

 valuable even for small lakes and streams, it is desirable that the inventory be as complete 

 as possible. 



RECOGNrriON OF LAKES ON PHOTOS 



For the most part, lakes are easily recognized on aerial photos. The smooth texture of 

 the lake surface is generally so different from the surrounding vegetation or ground surface 

 that the interpreter can instantly pick out the lake images. However, photo images of lakes 

 vary considerably in tone from deep black to light gray. When light is reflected from a lake 

 surface, suspended sediment, light -colored vegetation, or lake bottom material, the image on 

 panchromatic photos appears light gray. But when light is absorbed by deep water, dark materi- 

 als in the water, or the lake bottom, the resultant tone of the photo image is dark. 



^Moessner, KarlE. A simple test for stereoscopic perception. U.S . Forest Serv. , 

 Central States Forest Exp. Sta. Tech. Pap. 144, 14 pp., illus. 1954. 



"Moessner, Karl E., and Grover A. Choate. Estimating slope percent for land manage- 

 ment from aerial photos. U.S. Forest Serv. Res. Note INT -26, 8 pp., iLlus. 1964. 



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