(10) 
with a military rangefinder on which a specially designed clinometer was mounted, 
the complete instrument being supported on a tripod equipped with a horizontal 
circle. In this way the position and elevation of prominent features could be 
obtained without the necessity of climbing to them. 
In unsettled country, P. E. Palmer and J. Carroll carried a sensitive alti- 
meter from lake to lake in a float-mounted aircraft. The altimeter was placed 
in a padded airtight case, to protect it against atmospheric and temperature 
fluctuations, and the case was opened to the air only at the observing stations. 
Statoscopes for yielding the variations in camera heights during photography 
were not found reliable, owing to the large thermal elasticity of air. 
The helicopter has been used for some years for extending barometric 
control, and the value of this method, in places where a landing can be made, 
is firmly established. For the ten per cent or so stations where a landing cannot 
be made, experiments are under way on methods for measuring the vertical 
height of the aircraft above the station while it is being passed slowly. This 
type of aircraft has also been found very useful in carrying observers and their 
equipment to stations for triangulation and ground camera observations for 
control purposes. It has solved many difficult transport problems, and generally 
increased the efficiency of field work in suitable areas. Important savings in cost 
have been achieved, and these may be still more marked as surveyors and 
helicopter pilots accumulate more experience. 
As mentioned before, in mountainous country horizontal ground photo- 
graphs are in use to yield horizontal and vertical control for vertical air photog- 
raphy. The adoption, about 1935, of infra-red sensitive emulsion for terrestrial 
photography expanded the scope of this method of obtaining ground control 
and it is now accepted as standard in suitable districts for mapping at scales 
of 1/50,000 and smaller. By using horizontal photographs from a network of 
stations controlled by triangulation, it is possible to intersect and locate any 
number of supplementary control points. These points are then identified on 
an air photograph and serve as vertical and horizontal mapping control. This 
method is being used extensively in mountainous terrain where the summits are 
above timberline. 
To permit a height record in the case of older cameras, which, unlike more 
modern types are not fitted with calibrated altimeters, an experimental instru- 
ment-recording camera was built in 1938. In this camera a photograph of a 
number of full-size instrument dials was automatically registered on 25 mm fiim 
each time the main camera shutter operated (Ref. 14). The model camera 
worked very well, but war intervened in 1939, and the experiments were dis- 
continued. Actually the camera was put to considerable use by the 
R.C.A.F. for obtaining instrument data during experimental flights of aircraft. 
In 1943, the expansion of flying activities in the uncharted areas of northern 
Canada accentuated the necessity, in the interests of aircraft safety, of adding 
contours and critical spot elevations to the aero charts then being prepared by 
the Surveyor General for all of Canada. Development was immediately com- 
menced on an airborne radar altimeter suitable for providing the information 
    
   
  
  
  
   
   
  
  
  
  
  
  
  
  
  
  
  
   
   
  
  
  
  
  
   
   
  
   
   
  
  
   
   
   
  
  
   
   
  
   
  
   
  
  
   
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