258 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Fig. 2. Oblique sampling photograph. (Four-times enlargement to an average scale of 
approximately 1 : 600) 
measurements were then made and the results were compared with measure 
ments obtained in the field. The results were disappointing because of tilting 
of the camera axis due to air turbulence caused by the alternation of upward 
and downward air currents which developed over bare ground and snow- 
covered areas. Nevertheless, excellent winter photographs, which benefitted 
greatly from the use of a fast shutter speed, had been secured. 
In the spring of 1962, while the ground was still snow-covered, new sampling 
photographs were taken for the Department of Forestry (figs. 1, 2). In order to 
investigate further the problems of scale, these photographs were again taken 
with a Vintén camera using a 4-inch focal length lens and, in addition, lenses 
of 12- and 36-inch focal lengths. In most cases, a shutter speed of 1 /2000-second 
was employed. It was thought that by using lenses with a longer focal length, 
the effect of such minor fluctuations in flying height not controlled by the 
radial-line method of mapping, might be reduced. Also, if a short focal length 
is used, many corrections are necessary for small differences in ground elevation 
within one photograph, but by using a long focal length, the same differences 
in elevation can be ignored, with a consequent saving of time. Furthermore, 
the problem of tilt may be reduced by flying at higher altitudes where the air