(14) 
work. The first is that several projectors may be treated as a unit if due con- 
sideration is given to the position and density of the control and the second is 
that extreme care must be used in the elimination of want of correspondence 
(*y" parallax) and that exact agreement must be obtained in the "pass" points 
used to work from model to model. 
An approximation of the methods used to compute extensions for 1/50000 
work has been adapted to 1/250000 compilation at the Army Survey Establish- 
ment and is in daily use. The tip curve is plotted from data arrived at by 
comparing the Multiplex projector heights with those given by altimeter readings 
taken at the instant of exposure of the vertical camera. This procedure has 
made possible a considerable reduction in the required field control. 
More recently the Wild Autograph has been tried. These methods, perhaps 
in combination, may permit the successful bridging of greater and greater 
distances, when used in conjunction with better centering of the air camera 
lenses and more experience. To aid research on this important subject, an 
extensive test area is being surveyed by ground triangulation, traverse and 
levelling in the vicinity of Ottawa, and part or the whole of this can be photo- 
graphed from various heights as needed. 
Interest in oblique photography was revived in about 1944, when demands 
arose for the aeronautic charts already mentioned. This work was carried on 
in the Surveyor General's office under the direction of F. H. Peters and B. W. 
Waugh. As the work developed plotting procedure was given more study, and 
improvements applied to methods and instruments. With the use of the 
improved trimetrogon mount having measured angular separation for the cameras, 
it is possible to employ McKay's computational system. This has resulted in 
a saving of at least 10 per cent in the time taken for plotting. An experienced 
technician plots about 1,800 sq. miles (4600 sq. km) per month. 
In general the control is astronomic fixes at intervals of 50 to 100 miles 
(80 to 160 km). The maximum positional error on the charts is estimated at 
0.25 mile (0.4 km). 
Of the total requirement of 2.5 million sq. miles (6,000,000 sq. km) over 
2.0 million sq. miles have been photographed and 1.5 million (3,900,000 sq. km) 
have been plotted. Control data for contouring is available from radial alti- 
metry over an area of 386,000 sq. miles (1,000,000 sq. km). In a country like 
Canada, where aviation is often the sole means of transport available, the value 
of these charts cannot be overestimated. 
FOREST PHOTOGRAMMETRY 
Immediately aircraft became available in Canada, they were applied to 
forestry (Ref. 2). Photographic mapping for forest purposes soon followed the 
initial operations of fire patrolling and inventory, and the Forestry Branch, of 
the Department of Resources and Development, Ottawa, has from the beginning 
assumed a leading role in its development. 
re re EEE 
   
    
  
   
  
  
  
  
   
  
  
   
  
   
  
  
   
  
   
   
   
   
  
  
  
  
  
  
  
    
   
  
  
  
  
  
  
    
   
  
  
    
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