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which are known to exist by this method.” — “A Photographic Snow Survey 
Method of Forecasting Run Off” — Harry L. Potts. 
In 1948 the Department of the Army Corps of Engineers and the Depart- 
ment of Commerce Weather Bureau published a comprehensive report summa- - 
rizing the results of an intensive investigation into the application of vertical 
air photos in the study of snow cover over large areas. 
The following factors were investigated: 
(1) Analysis of basic topography 
(2) Correlation of snow cover to topography 
(3) Sampling techniques for evaluating air photos 
(4) Run off as a function of snow covered area 
(5) Analysis of snow patterns and persistence of them 
(6) Evaluations of ground photographs of snow cover by comparison with 
air photos. 
The results of these studies brought to light the fact that topography and 
vegetation definitely affect the distribution of the snow fall and the melting of 
the snow cover and indicated that the photos will yield much data on these two 
factors that can be correlated with the routine measurement of actual snow con- 
ditions to give a much more accurate figure for actual water content per water 
shed than is available from present surveys. 
Use has been made by other agencies of high level obliques to obtain snow 
cover over large areas. 
Photogrammetric methods used in obtaining the required data from the 
photos, both vertical and oblique, are quite correctly confined to the simplest of 
equipment and techniques. 
To obtain the relative areas of snow vs. bareground, either grid square 
counts or photo electric cell scanning devices are used. 
One weakness encountered in obtaining accurate comparisons between pho- 
tos taken at different times is the difficulty in reocupying the same air station 
precisely so that a direct comparison of the photos taken at different times can 
be made with no error due to perspective. 
This can be overcome by having highly trained air crews who can duplicate 
the position of the plane for each photo sequence by visual observation of 
ground detail. This has been done in British Columbia in connection with pre- 
selected stations for photos to be used in Multiplex mapping. 
Inasmuch as it would be economically unsound and practically impossible 
to obtain complete air photo coverage of a large drainage area at specified times 
during the period for which snow measurements were required, and also im- 
practical to have the photos all examined even if they were obtainable, it would 
appear that sampling areas would have to be selected. 
The character and location of these would require careful investigation 
correlating the influencing factors of slope and vegetation. Also the photogenic 
qualities would play an important part. It would be essential on each photo to 
have a proportion of bare ground throughout the heaviest snow period other- 
wise no percentage snow quantity could be obtained. 
A good deal of initial survey work would be required to locate courses that 
will represent the best average conditions with the minimum number of courses. 
Technical Report No. 12 dated March 1948 of the Cooperative Snow In- 
vestigations published jointly by the Department of the Army Corps of Engi- 
  
   
  
    
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   
    
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