th little or. 
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; hève pro- 
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ens stereo- 
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1ay not be 
17: 419-422 
> Interpreta- 
, Wood and 
erial Photo- 
No. 48. 
ia Fish and 
rial Photog- 
110-112. 
11:(3) 48-49 
rtical Aerial 
Note No. 1. 
13: 244—247. 
os," Forestry 
Petersham, 
dons," First 
man., 51:(4) 
  
  
  
   
   
  
   
   
   
   
  
   
   
  
   
  
   
  
  
   
  
   
  
   
   
  
   
  
   
  
   
   
  
  
  
  
  
   
  
  
  
  
  
  
  
  
  
  
     
(661) 
REPORT ON THE USE OF AIR PHOTOGRAPHS IN SNOW SURVEYS 
by 
William Hall. 
Measurement of water stored in the form of snow throughout the various 
drainage areas in the Pacific Northwest has been a standard procedure for a 
considerable number of years. Dr. J. E. Church is credited with establishing a 
snow observatory in Nevada in 1904 and since that time snow surveys have 
been organized by various agencies in every area where the increasing needs of 
industry, farming and cities have brought the annual use of water to a point 
where accurate information on the annual spring run off is necessary to plan 
storage requirements for flood control, hydroelectric supply and irrigation 
needs. In British Columbia snow surveys were instituted in 1937 to obtain run 
off data for that part of the Columbia River drainage area in Canada. 
This latter project is organized under the Water Rights Branch in coopera- 
tion with various agencies in the United States of America. 
The main objective of a snow survey is to obtain the total water content 
of the accumulated snow of the preceeding winter immediately prior to the an- 
nual spring melt. Ordinarily this is accomplished by sampling of snow depths 
and water contents on preselected snow courses distributed throughout the drain- 
age basin involved. The density of the snow courses will be dictated by the 
accuracy required for the run off forecast for any given area. In that part of 
the Columbia River drainage in British Columbia, which covers 39.500 sq. mil., 
36 snow courses have been established which give a representative sampling 
that can be applied to the entire area. Each snow course consists of 15 stations 
extending over about 1.000 lineal feet. 111 actual samplings are taken each win- 
ter, some courses being sampled only once, while others may be sampled 6 times. 
It is accepted that the spring and summer run off forecast can be computed 
to within about 1096 based on these samplings. 
The use of ground and air photos to supplement ground measurements in 
Western America has been the subject of investigations since about 1930, when 
Mr. H.C. Potts of the Board of Water Commissioners, Denver, Colorado initiated 
an investigation using ground photography. A sequence of ground photos taken 
at various stages of snow cover was obtained from a fixed point with the camera 
on a fixed orientation and the percent area covered by snow measured by the 
grid square method. The snow area as measured on photos was then correlated 
with observed run off as recorded by automatic gauges and curves drawn show- 
ing the percent snow cover against run off. A reasonably good correlation was 
obtained. The results were summarized in 1944 by Mr. Potts as follows; “Addi- 
tional years of record will undoubtably add to the accuracy and value of this 
study. Certain changes in methods and equipment may be madc from time to 
time as appear necessary, but it is our opinion that run off from accumulated 
snow in certain areas may be forecast from photographs with more accuracy 
than has been attained in the past. The advantages of observing a relatively 
large area will, we believe, more than compensate for the various minor errors 
51