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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.