Which method to be preferred must be depending upon a technical 
and practical, maybe also an economical, judgement. But, if any method 
should be insisted upon as the supereminently best one, that must be 
proved in an impartial way. 
3.1 Preparatory Investigations 
As mentioned, the infra-material had to be used to an unexpected 
extent. That fact made it necessary to subject the combination of the 
Aviogon camera and the infra-filter to an exhausted examination. 
Checking the material before the commencement of measuring the rail- 
road line, the Geographical Survey Office could establish that the resi- 
dual vertical parallaxes were quite small after the relative orientation, 
carried out by the operator in the ordinary way. Their absolute amounts 
could be compared to those of the pancromatic pictures of the Aviogon 
camera. Unfortunately, the deformation of the stereoscopical model 
could not be subjected to a study due to the lack of control points. 
By means of the aerial photography of the Buchanan Area, however, 
pictures covering the coast line and the mouth of St. John River could 
be selected and studied. The flying height of these pictures is 4 000 m. 
The relative orientation of the pictures in the instrument A 8 gave 
very small residual vertical parallaxes verifying the experiences made. 
Consequently, the stereoscopical model was well-defined and easy to 
measure. These studies gave the information of an essential model de- 
formation, to its shape a depression in the ground. A trial to estimate 
its maximal depth resulted in an amount of roughly speaking 16 m.6) 
3.2 Filter and Lens Distortion 
The filter used is called the Infra-red Meniscous Filter (Infrarot- 
Meniskus-Filter) and has a wave length of 670 mu. If this filter is 
adapted to the Aviogon camera, the camera constant (focal length) will 
increase by 1.96 mm. 
Primarily, it was desirable to check the data of the filter and the 
camera as a unit. According to the points of view under item 3. it was 
evidently possible to follow the laboratory way, examining camera + 
filter, or the calculation way using the stereo-pictures. Owing to the 
importance of this question. I found it desirable to try both ways for 
the determination of the lens distortion curve. 
For that reason, the Wild Heerbrugg Company was called and Dr. 
R. David in the capacity of the head of the laboratory division was in- 
formed. Some days later the lens distortion curve of the unit camera + 
filter could be communicated. The curve of this total lens distortion is 
shown in fig. 2. The curve is plotted on the basis of the following data, 
by courtesy obtained from the Wild Heerbrugg Company 
6) The measurings in the SENTAB instrument A 8 under the author’s manage- 
ment have been carried out by Mr. Fridnert who has also assisted at the investiga- 
tions needed for this subject. 
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