The photo interpreter can rapidly compute the lake area by the following procedures: 



Step 1. - -Determine the exact scale of the photo at the lake surface elevation. 



Step 2 . - -Determine the acreage per dot. For example, 1 square inch of photo surface 

 at a scale of 1:19,000 is: 



19,000 X 19,000 = 361,000,000 square inches 



= 361,000,000 144 = 2, 506, 944 square feet 

 = 2,506,944 -- 43,560 = 57.55 acres 



Consequently, for a dot grid with 256 dots per square inch, the acreage per 

 dot for a scale of 1:19,000 is 57.55 -=- 256 = 0.225 acre. 



Step 3. - -Count the dots superimposed on the photo image of the lake. This should be 

 done while the photos are being viewed stereoscopically. 



Step 4 . - -Multiply the number of dots counted times the acreage per dot. 



Lake Elevation 



The approximate elevation of a lake can be determined once the lake position is indicated 

 on the contour map. Elevation is critical in estimating the approximate nimiber of snow^- and 

 ice -free days that may be expected during the year, an important influence on general recrea- 

 tional use. The duration of the ice -free period also influences the amount of biological activity 

 (such as fish growth) in the lake . 



Depth 



In this study, determination of maximum lake depth from photos could not be done accu- 

 rately. Instead, photo interpretation was used to determine whether lakes were less or more 

 than 15 feet deep, and if more, to estimate the percentage of each lake area that was under 15 

 feet deep. The procediires used were essentially those reported by Moessner. 



Any appropriate depth contour may be used. The 15 -foot depth contour was judged to be 

 a significant lake measurement in the study area for three reasons: (1) It is about the minimum 

 depth that allows fish in high mountain lakes to escape suffocation and mortality during severe 

 winters; (2) it approximately defines the "littoral zone" in which light penetration and water 

 temperature permit the production of food for fish; and (3) it is the maximum depth at which light 

 penetration allows recognition of underwater detail on panchromatic photographs of clear lakes. 



The techniques used for estimating depth on aerial photos are based on the assumption 

 that the immediate bank slope surrounding a lake continues more or less unchanged for some 

 distance under water. Thus, lakes with steep banks on all sides are considered more likely to 

 be deep than are lakes with nearly level banks . 



Estimations of lake depth may be based mainly on the interpreter's judgment of steepness 

 of bank slope or may be calculated from bank slope measurements on the photo. Measurement 

 is, of course, more accurate, and it allows the interpreter to predict depth at specific distances 

 from shore. 



Moessner, Karl E. Estimating depth of small mountain lakes by photo measurement 

 techniques. Photogram. Eng. XX1X(4): 580-588, illus. 1963. 



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