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(663) 
neers and the Department of Commerce Weather Bureau gives an excellent ac- 
count of the problems and possible solutions in this connection. 
A second practical use of air photos has been investigated by the State of 
California Department of Public Works in obtaining snow depths on selected 
courses. Markers were established with cross arms at 2” spacing at measured 
heights above the ground. Oblique photos were taken of these from aircraft 
flying 50' above ground and it was found that actual snow depths could be 
obtained to within 3 inches by direct observation of the photos. Past Snow 
Survey records in this area have indicated that the percentage of water content 
of the snow blanket does not vary too greatly over wide areas of the Sierra at 
any corresponding time of measurement. Snow content and percentage density 
found at an easily accessible snow course at approximately the same elevation 
as the snow depth markers can therefore be applied to the depths determined 
from the photographs. 
It would appear that much is yet to be done in standardizing the tech- 
niques and in developing a common approach to the problem. Perhaps at this 
stage it is well to have individuals trying and testing and out of the pot the 
solution will rise. Or perhaps there will be several methods evolved each appli- 
cable to a certain set of conditions and catering to special requirements. 
As in all fields of endeavour in this commercial world the approach select- 
ed for any particular area will be governed to a considerable extent by economic 
factors. 
The following cost data compiled by the Water Rights Branch, Department 
of Lands and Forests. Province of B.C. is of interest in this connection. 
During the snow season of 1952 a total of 111 samplings in 36 snow courses 
were made. Of these 76 were made by the B.C. Government at a cost of 
$ 1375.00 which works out to an average cost of $ 18.00 a sample. Some courses 
were sampled as many as 6 times and others only once. A sample, incidentally, 
covers 15 stations extended over about 1,000 lineal feet which makes up one 
snow course. 
The 36 courses are strategically located to sample an area of 39,500 sq. mil, 
and total run off predictions based on these samplings can be made to within 
1096 accuracy when correlated with actual snow measurements and run off 
records collected over the past years. 
It would require about 1,000 line miles of flying to cover this same area 
by a photographic aircraft to obtain one set of photos of each snow course. 
Operating cost figures for this photographic mission would vary consider - 
ably, but it can be seen that careful consideration would have to be given to 
the value of the photos, in order to justify this expenditure. 
In some areas where the supply of run off water is critical and a closer 
accuracy of forecast is required it may well be that the air photo approach 
would be well justified. 
(1) Cooperative Snow Investigations Technical Report No. 12 Department of 
the Army Corps of Engineers Department of Commerce Weather Bureau 
March — 1948 
(2) A Photographic Snow-Survey Method of Forecasting Run Off, Harry L. 
Potts, Board of Water Commissioners, Denver, Colorado 
1944