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Transactions of the Symposium on Photo Interpretation

The development of geographical photo-techniques in any area is guided by
the type and availability of air photographs. The size and relative inaccessibil
ity of Canada led to early adoption of photogrammetry as a mapping tech
nique [6]. As early as 1920, experimental air survey photographs were taken,
and by 1938 an aerial photography programme had covered 50% of the popu
lated area. The scale was primarily 4 inches : 1 mile (1 : 16,000 approxim
ately), chosen essentially for forest interpretation. Wartime requirements for
aeronautical charts saw the greater part of northern Canada covered with three
camera trimetrogon photography with the vertical scale of 1 : 40,000, and
systematic 1 : 40,000 and 1 : 63,000 vertical coverage from south to north was
later developed for mapping purposes. In 1948, 99% of the populated area
had been flown and by 1950, 99% of total area was covered. Probably the
most important factor is that of availability. Since the formation of the National
Air Photo Library in 1925, all Federal and Air Force photography - with few
exceptions - has been available to the general public.
Canadian geographers, faced with the same problems of size and inaccessib
ility that prompted the extensive air-photo coverage, have made considerable
use of the new tool. The following notes are an outline of the photo inter
pretation techniques that have been used - successfully — to gather geographical
For ease of reference, three broad groups of techniques may be recognized:
firstly, reconnaissance techniques which allow the first areal statements of
geographic data, particularly in remote areas; secondly, small scale inter
pretation techniques, where it has been possible to improve the overall recon
naissance classifications to a point where they can be used as an accurate
basis for regional and even systematic research; and thirdly, large scale tech
Reconnaissance techniques fall into two broad groups:
1. Those in which the scale of photography available and remoteness oi the
area make it the only possible way to obtain even elementary geographic
2. Those in which, while photo-scale and access are suitable for more detailed
work, the overall patterns are of prime interest.
These types of survey are essentially limited to the large and more static
elements of landscape, such as overall vegetation patterns in remote areas
[5, 14] and physiographic types emphasising the juxtaposition of bedrock and
drift, though they have been applied to the mapping of land use in the prairies
[29]. A typical classification is that found in the classic photo-reconnaissance
survey of Labrador-Ungava [17], where vegetation is divided into:
1. Forest - closed crown
2. Forest — open crown and woodland
3. Bog
4. Bare rock
5. Burnt areas.