Ridgelines, divides, and saddles form the natural separation between drainage units and 

 are logical choices for boundaries. They have the advantage that they can be easily identified 

 without costly surveying on maps and photos as well as in the field. The boundaries of drainage 

 units should be marked on a map of adequate scale; one -fourth inch to the mile is usually suf- 

 ficient. The same boundaries should be marked on a series of maps of larger scale, such as 

 1 to 2 inches per mile. These are used to show the position of lakes and streams as the 

 inventory progresses. 



On both the large and small scale maps, the drainage basin units should be systematically 

 numbered for identification and reference of inventory data. 



PURCHASE AND PREPARATION OF AERIAL PHOTOS 



When the boundaries of the area to be studied have been determined, the photos nay be 

 ordered. Median scale (1:20,000) aerial photographs are readily available at moderate cost 

 and cover most of the United States . Such photos are usually taken with panchromatic film and 

 a filter to remove blue light, permitting moderate penetration of haze. They can be purchased 

 from the U.S. Agricultural Stabilization and Conservation Service or from the U.S. Forest 

 Service. Index sheets show the photographic coverage available over any specific area, and 

 the required prints should be ordered from these sheets by individual photo number. 



In aerial photography, images of all terrain features visible on one photo are duplicated on 

 one or more adjacent photos; that is, there is a large amount of overlap between adjacent photos. 

 Therefore, it is particularly important that the effective area of each photo be marked before 

 the inventory is begun, so that no water areas are either omitted or inventoried more than once. 

 The effective area is an approximately rectangular area in the central portion of each photo. 

 Procedures for delineating effective area are given in appendix A. 



Because the scale on an aerial photo varies with the height of the camera above ground 

 level, it not only changes from one photo to another, but from one portion of a photo to another, 

 even within the effective area . Changes in scale are particularly great in mountainous regions 

 where ground elevation changes abruptly and where conditions require frequent changes in flying 

 height. Unless the photo interpreter recognizes scale variations, he will get erroneous impres- 

 sions or measurements. For this reason, care must be taken to determine the scale of each 

 aerial photo to be used in the inventory. This is not necessarily a difficult or time-consuming 

 procedure, as convenient tables may be worked out to simplify the calculation. Detailed dis- 

 cussion of procedures for scale determination may be found in "Manual of Photographic Inter- 

 pretation ." In general, scale determination requires the use of accurate contour maps from 

 which the elevation above sea level can be read for specific points on a given photo. 



American Society of Photogrammetry. Manual of photographic interpretation. 868 pp., 

 Washington, D.C., 1960. See chapter 3, Fundamentals of photo interpretation. 



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