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Title
Transactions of the Symposium on Photo Interpretation

WORKING GROUP 4
DOVERSPIKE-HELLER
237
Accuracy of interpretation increased in going from the small scale to the two
larger scales (fig. 1). Accuracy in interpretation was consistently higher on the
color film but this advantages diminished as the scale increased. Very little
difference in accuracy was recorded between the two large scales on both types
of film.
Accuracy in interpreting color film was consistently higher for each tree
species than in interpreting panchromatic film (fig. 2). This was true even for
the best interpreter, although he did correctly identify all 19 Pirns strobus trees
on both films.
The effect of scale on the recognition of three particular species is shown in
fig. 3. Pinus strobus, which ranked first in accuracy of recognition, showed little
change between the two larger scales. Betula papyrifera, which ranked last,
showed little improvement in accuracy with increased scale. Recognition of
Thuja occidentalis was greatly improved with the increase in scale.
The morphological features and color characteristics of these species explain
why photo images of some species are harder to recognize than others.
1. Pinus strobus was accurately identified by all interpreters. They were con
sistent in recognizing the characteristic foliage arrangement, e.g., a pattern
of triangular branches that radiate like the spokes of a wheel.
2. Betula papyrifera was identified correctly when the tree was full crowned and
vigorous but when the tree was decadent and had thin foliage its image was
generally confused with Populus tremuloides.
3. Thuja occidentalis was recognized by crown margins and apices on the largest
scales. These characteristics were not recognized on the small scale. Image
color may facilitate making a correct appraisal of this species at large scales
since the interpreters consistently designated the tree as having the same
hue.
Other species where interpretation accuracy was high had distinguishing
characteristics, such as ragged foliage and occasional long branches on Pinus
banksiana.
Discussion and conclusions
Color film is superior to panchromatic film for use in identifying individual
tree species but its superiority reduces with an increase in scale. For the accu
rate identification of individual tree species a photo scale of 1 : 1584 or larger
is necessary.
A recent forestry graduate did an equally accurate job of interpreting at
these scales after three days of intensive training as did men having 20 years
experience in photo interpretation. Thus, it would seem possible to eliminate
interpreters’ performance as a variable in interpreting pictures of this kind if
proper training and training aids are used.
The cost of using color film at large scales should be little more than that
required for panchromatic film. While it is true that color film costs five times
as much as panchromatic film, the important point to remember is that film