(6) 
Concurrently, investigations were underway at the Geographical Section, 
Department of National Defence, under Major P. de L. D. Passy, who developed 
the “straight line” method for extending the plots (Ref. 18). It was based on 
the principle that the perspective image of a straight line is another straight 
line, so that, starting with the intersections of lines joining major control points, 
more straight lines could be drawn, the intersections of which could be then 
utilized, as minor control, and so on. 
In 1925 the first air map was issued {rom the Surveyor General’s office to 
a surveyor, compiled from oblique photographs of the area he had to traverse, 
thus greatly facilitating field work. 
About 1926 more attention commenced to be devoted to plotting from 
vertical photographs, as country less suitable for the oblique method had to be 
mapped, and the radial intersection method was applied. 
The following list gives an idea of the growth of air photography in Canada 
during these pioneer years. It contains the number of photographs taken each 
year by government-operated aircraft. 
Year Number Year Number 
1919... Nil 1925... 24,248 
1920... .... 1,233 1926... ..-. 42,169 
1921... 7,984 1927... 58,143 
1022: ....... 7,964 1928... 45. 89,071 
1923. ......- 4,421 1920....... 101,889 
1924.......: 13,894 1930. ...... 104,163 
Among the pioneers of this period, A. M. Narraway merits special attention. 
From his first journey in an aircraft he realized the value of the view of the 
country revealed to a surveyor from such a point of vantage. Henceforth, he 
devoted much energy to sponsoring the development of air mapping and to 
enlisting support for it from officials and the public generally. At the time, it 
must be remembered many had to be convinced of the potential value of air- 
mapping in the economic development of the country. 
Mention should also be given to D. H. Nelles, then of the Geodetic Survey 
of Canada, who proposed a system of photography using two low oblique cameras, 
one pointing forward in the aircraft and the other pointing aft. Nelles and his 
collaborator, Fortin, made a model of a projector for plotting the overlapping 
photographs from different air stations that would result from this method of 
photography. 
EARLY AIR CAMERAS AND PHOTOGRAPHY 
Deville had always been insistent on the retention of accuracy in the instru- 
mental equipment issued to Canadian surveyors, and in 1911 had established a 
small laboratory at Ottawa for the purpose of calibrating and testing instruments. 
In 1920 this laboratory made the first calibration of an air camera, to provide 
     
  
  
  
  
  
  
  
   
  
  
   
   
  
   
   
    
   
   
    
    
     
   
  
  
  
  
  
  
  
  
   
  
   
  
   
   
  
  
  
   
  
  
    
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