NR 
  
150 
  
  
  
  
Qoo 
ERRORS IN 
Fig. 3 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Second trial was made in connection with about 340 km lono 
bridging test. Flying altitude and camera arrangement Were 
lar to those in the previous case. However, the oblique camera 
was not properly focused and as a result the photographs Were 
of poor quality. The test area, from Renfrew, Ontario, to Roxton 
Falls, Quebec, is a flat country throughout and was accepted only 
because no other test area of similar length was available, The 
line on the diapositives was constructed using only a steel 
straight edge, knife, magnifier and mirror stereoscope because no 
special equipment was available. Adjusted coordinates Were 
checked against 47 ground control points. Mean square was found 
to be + 8.0 metres. Maximum error was —20.6 metres. 
Despite longer distance and worse quality of photographs the 
mean error is smaller than in the first case. This is probably due 
to the fact that the flight was very straight and the control line 
could therefore be kept close to the centre line of the strip. 
These results may be regarded as very satisfactory, particu 
larly if the very crude equipment used for the construction of the 
control line is considered. The use of the Straight Line Plotter, 
now under construction, will inerease the accuracy the economy 
and speed up the whole operation. 
It is considered that the method described above represents a 
definite advantage in solving one of the inherently difficult pro- 
blems of aerial triangulation. In long distance bridging the use of 
infrared oblique photographs is an essential part of the procedure 
developed by this laboratory. However, the control of a direction 
on the earth surface, as described above, can also be successfully 
applied to many other photogrammetric operations.