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General remarks
Although APR is one of the most powerful means of controlling aerial
triangulation, there are obviously a number of obstacles preventing a wide
spread general use 0
1) The equipment is relatively expensive, requires the capacity for
additional payload in the aircraft and, in most cases, an additional
orew-member for operation,, Furthermore, the possibility of technical
failures can increase the operation time for the execution of aerial
photography.
2) The conventional type of APR equipment will render less useful results
in certain types of area, such as accidented and forested terrain.
This difficulty might be overcome to a large extent with the application
of Laser-APRo
Neglecting the influences of the disturbances of the isobaric surface, the
accuracy of APR data permits extremely long bridging distances for mapping
projects with large contour intervals, thus reducing the amount of necessary
ground control to a minimum, (see figo 8) 0
At present, however, insufficient knowledge is available of the behaviour of
isobaric surfaces over long distances. The conclusion of a research project,
currently being carried out at the ITC, involving about 25000 Km of APR
flights, arranged in a network and including runs of up to 65O Km, might
render some valuable information in this respect.
Still more research will be necessary to achieve generally valid information.
One simple and relatively inexpensive method to this end might be to execute
APR profiling during flights over sea. (Possibly executed during the flight
of survey aircrafts to project areas). The APR charts will then directly
offer registrations of the deformations of the isobaric surface, which can
be analysed after the application of the Henry correction. A snag, however,
is the fact that this information will probably be better than that
obtained in flights over land, due to the influence of a more stable
temperature.
Meteorologists might even be very interested in this information, as they
seem to have little knowledge about these phenomena.
Horizon-camera
The only presently available horizon-camera is the Wild HC1 (f=34 mm,
format 8x24 “m) taking photographs in four directions simultaneously with
the survey photography. The horizon camera has to be mounted in a fixed
position with respect to the survey camera.