WORKING GROUP 1 
CORTEN 
41 
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 
procedures. 
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.