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Photo-interpretors think in patterns, consequently they even 
emphasize the general tendency to see "in patterns". A geologist and a 
soil scientist are so keen on structures, that isolated trees and bushes 
line up in their eyes and minds to complete patterns, which are very 
difficult to get rid of, even by critical interpretation and careful 
analysis at a later stage (Vink, private comm. 1966). 
[Under this label of shape perception, object perception and judgment 
of size, length, area, and other absolute tasks can also be 
classified. Often the shape is better remembered if the object to 
which it belongs is named, or with which it is conventionally 
related, for instance because of drawing conventions of 3-D objects. 
Objects are often not seen as they are, but psychologically perceived 
as for what purpose they can be used: a table is not a square wi th 
four legs, but a plane on which one can put a typewriter. (Gregory 
1966 p. 220). Are photo-interpreters themselves aware of carrying 
out interpretation with the purpose of using the final map for 
practical work, e.g. for farming or mining? Do they see the soil with 
these artificial soil classification names, which are as unpsychological 
as they are logical, or do they see grazing cows and harvesting 
machines in their deepest expectations?] 
STEREOSCOPIC DEPTH and HEIGHT PERCEPTION should 
have a place on the board, because it is of 
paramount value for many disciplines using aerial 
photographs; interpretors guess iis value above 
50% of the total data gathering techniques, 
Of interest is the connection of depth-perception and psychology, not 
the fusional problems and the accuracy of stereoscopic measurements, as 
these aspects were mentioned under Fusion. 
At first a general remark: Experiments are very difficult with the 
subjective factors in  depth-perception, They depend on personal disposi- 
tion and experience, and often are modified during testing or uncon- 
siously adapted to fulfil the expectations of the experiment. 
Apparant size of the details influences stereo—depth-perception. 
The practical consequence is that stereoscopes with prism binoculars 
(with a designed larger magnification along the edge of the field of 
view) seem to give a concave, curved model, whereas quick scanning with 
the Old Delft type gives an apparently convex model of a flat terrain. 
In the latter instrument, unsharpness at the far and near edge of the 
model may further enhance this effect, as does haze and iis resulting 
low detail conirast in the shadow areas,