Dept±i classification without measuremem. - -In this method, the photos are examined 

 stereoscopically and a judgment is made on uianeasured estimates of bank slopes. Other clues 

 to the slope of the lake bottom are considered, including the general configuration of the lake, 

 the expected characteristics of lakes of known origin, images of underwater detail, such as 

 vegetation or rocks, and the lighter or darker tones of different parts of the lake. 



In the test study, two experienced interpreters worked independently to estimate the po- 

 sition of the 15 -foot depth line for each lake and to draw it on the photo. Their estimates were 

 inaccurate for the shallower lakes . None of the four lakes determined from field measurements 

 to be less than 15 feet deep was correctly classified by either interpreter. Only one lake was 

 judged to be less than 15 feet deep, and field measurements showed this lake to be actually 

 deeper than 15 feet. Although depth judgments were often wrong, lakes with steep banks were 

 seldom misclassified. Training of photo interpreters should include practice in comparing 

 estimates of bank slopes with parallax measurements of the same slopes. 



Depth classification using parallax measurement. --In this method, selected bank slopes 

 around each lake image are measured, using parallax wedges, as described by Moessner and 

 Choate.^ Accuracy may be improved if the banks most likely to characterize lake depth are 

 chosen according to indications such as the clues mentioned above. For example, if underwater 

 vegetation is evident near a steeply sloping bank, that bank is probably not a good choice for 

 parallax measurement. 



The underwater extension of each measured bank is calculated, and the distance from 

 shore at which a water depth of 15 feet would be expected is calculated. The resulting points 

 are then plotted on the photo. For example, a 20 -percent bank slope (20/100) would result in 

 a 15 -foot lake depth at 75 feet from shore if it continued under water without change: 



15 feet 20 



Distance from shore 100 



15(100) 



Distance from shore 



20 



= 75 feet 



In the test study, three trained interpreters measured bank slopes of 21 lakes on the 

 photos. Their measurements gave dependable depth information for steep -sided lakes. The 

 17 lakes that were deeper than 15 feet, as determined from depth measurements made from a 

 boat, were all correctly identified by parallax measurements. However, this procedure proved 

 undependable for lakes with the shallowest water and the gentlest bank slopes. Of the 12 depth 

 estimates made for the four lakes found to be less than 15 feet deep, only three were correct 

 (25 percent) . 



Lake size as an indicator of lake depth. - -To cut down the time required for depth classifi- 

 cation using parallax measurements, the relation of lake size to the steepness of general terrain 

 and lake banks may be used as an indicator of depth. The bank slope of a small lake must be 

 quite steep if the lake depth is to reach 15 feet or more. Conversely, large lakes are likely to 

 be 15 feet deep even when their bank slopes are relatively gentle. This general relation between 

 lake size and minimum average bank slopes may be tabulated for convenience, using general 

 estimates taken from the photos and based on field experience. For lakes on the North Slope, 

 the relationships between size and slope were as follows: 



^See footnote 3. 



9 



