-8- 
General remarks 
Although horizon data, obtained under favourable conditions, offer a 
relatively inexpensive means for a very effective improvement of the 
accuracy of aerial triangulation, the hazard involved - due to meteorological 
conditions - apparently forms an obstacle to a wide-spread and general 
application. 
Furthermore, the author is of the opinion that the system could still be 
improved along two lines: 
a) by using horizon cameras with longer focal lengths yielding a more 
accurate determination of tilts 0 
b) by execution of further research aimed at finding the optimum film- 
-filter combinations suitable for the depiction of the horizon with 
maximum sharpness. 
Solar-peri scope 
In this system, photographs of the sun are taken simultaneously with the 
survey photograph with the aid of a second camera, rigidly connected with 
the survey camera and directed upwards. 
If the time and place of exposure are approximately known, the direction to 
the sun can be computed and be used as an additional orientation element. 
The problem of the somewhat comjjlicated computations involved in this system 
has been solved by the construction of an analogue computer. 
The computations would be even less of an obstacle, if a digital process 
were to be applied. 
Evaluation of tests carried out indicate a possible accuracy of tip and tilt 
determination of approximately 4-5° (standard deviation). There is no 
indication of continued use of this equipment. This is probably due, amongst 
other reasons, to the technological difficulties caused by the requirement of 
providing a second camera hole in the ceiling of the survey aircraft(not 
permitted in many planes for structural reasons)and by the requirement of a 
rigid connection between both cameras. 
Gyroscope - Inertial Platform 
An equipment, designed for the registration of the camera tilts based on 
gyroscopes, was developed by Nistri (OMl) several years ago. 
In this design, two gyroscopes have been used, their respective primary axes 
situated parallel to the co-ordinate axes of the survey camera. 
Two light beams are projected on the edges of the aerial photograph with the 
aid of mirrors attached to the primary axes and a prism-system and lens. 
The longitudinal and lateral tilts are determined from the deviations of the 
projected Light spots from the zero positions.