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Transactions of the Symposium on Photo Interpretation

As a general rule, the system of survey navigation and the constancy of the
flight execution shall be such that the closest possible tolerances for overlap
and for side lap can be maintained. A maximum permissible spread of ±3%
longitudinal overlap and ±10% side lap can certainly be maintained in all
normal cases. This means that contract specifications calling for 65% overlap
and 30% side lap - which is very common in contracts covering hilly areas -
may be reduced to a substantially lower value: taking 55% overlap and 10%
side lap as the minimum tolerance, an average of 58% overlap and 20% side
lap can be aimed at if the abovementioned ±3% overlap and ±10% side lap
tolerances can be fulfilled. The saving in the number of photographs that
results from such a procedure is much greater than is commonly expected: the
second case produces less than 3 / 4 of the number of photographs as compared
with the first case; this does not only give a saving in the cost of the air photo
graphy, but it also economizes considerably in all subsequent interpretation
Strip photography is the most common form of flight execution: the photo
graphs are aligned in mutually parallel flight lines but the positions of the nadir
points in one flight line do not have any connection with the positions of the
nadir points in the adjoining flight lines. Considerable improvements are pos
sible by applying what I call “block photography”: the photographs may be
aligned not only in the flight line direction but also in a direction perpendic
ularly to it; in many cases this produces the ideal photo lay-out and will not
only save nearly half of the control points and minor control - in case any base
map should be constructed - but it also speeds up considerably the flow of work
in all interpretation and compilation techniques.
Other improvements in flight planning are also possible, but these are
beyond the scope of this paper. In order to produce such economizing improve
ments in practice, however, it is an absolute requirement that the photo
graphic organisation shall be able to carry out aerial photography contracts in
such a way that the improvements are really achieved. In other words, the flight
execution shall be perfect in order to enable a flight planning according to the clo
sest possible tolerances.
The reputable contracting companies are perfectly competent to execute
photographic flights in the correct way, but experience has also shown that
there is a need for basic and practical training in these fields: many govern
ment and institution agencies still seem to think that air photography can
be carried out by having a good quality camera and chartering a good air
plane and a good crew, although experience during the 50 years existence of
aerial photography shows that this inevitably leads to poor results .The I.T.C.
has, therefore, recently established a branch giving basic education in photo
graphy, in image formation theory, in aerial photography and in survey
navigation, as well as practical training in all these fields.