2
The scale of the S.A.P. can be determined from the
existing aerial survey photos. The scale control procedure
js illustrated in Figure 3, diagrams a, b and c. Diagram 3a
shows the 35-mm negative projected onto the survey photo
covering the same area. The enlarger is adjusted until the
projected detail coincides with that in the survey photo.
When the enlarger is in this position, the scale of the S.A.
photo is the same as that of the survey photo (356) and an
enlargement can be made. The interpretation marked on this
enlargement can then be transferred onto the map with some
type of "sketchmaster" (3c).
TECHNICAL CONSIDERATIONS
Camera
Any camera is. suitable for 8S.A.P. if it is equipped
with a good-quality lens and has reasonably fast shutter
speed (preferably 1/1000 sec). The equipment naturally will
set a limit on the range of application. Motorized assem-
blies are available which make possible an adequate overlap
even at low altitudes. The following considerations are
made for the purpose of obtaining optimum results.
The resolving power of the lens used is most impor-
tant in this operation because the final product will be
magnified considerably.
The normal focal length for 35-mm cameras is 50 to
55 mm. These, however, provide a narrower angle of view
and a larger scale than is used in conventional photography.
The flight altitude must therefore be greater in order to
obtain the same coverage as in conventional photography.
Wide-angle lenses are the most practical in S.A.P.
because they can simulate conventional photography from the
same altitude (Figure 4).
Pl
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Figure 4.—The above diagram shows the identical focal Length
to picture format ratio for both 24-mm and 150-mm lenses.