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Specifications are regarded as necessary in the majority of the answers. 
In many cases there are standard specifications which can vary considerably 
from one organization to another. Sometimes variations occur within one 
and the same organization according to the purpose of the mapping project. 
As an average the following tolerances are found: 
Direction of the camera: maximum deviation from the vertical direction 3° 
(in some cases 2° and 5°). In one case the maximum deviation for a single 
photograph is 39, the average of ten photographs 2? and the average of all 
photographs of a project Jo: 
Maximum deviation in direction of the flight (between adjacent photographs) 
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Maximum deviations of the flying altitude: Is per cent of the average 
flying altitude. In some answers the tolerance is 2 per cent and in other 
cases average deviations 2 per cent and maximum deviation 5 per cent. 
Longitudinal and lateral overlap: 60 t5 per cent and 25 * 10 per cent, 
respectively. In some answers longitudinal overlap between 53 and 60 per 
cent and lateral overlap between 10 and 20 per cent. In other cases the 
lateral overlap tolerances depend upon the elevation differences on the 
ground. 
2.2 How is the filter chosen? 
24 answers. 
In general the filter is chosen according to experience. Minus blue seems 
most usual. Recommendations from the manufacturer of the camera are 
followed in many cases. 
2.3 Are there special conditions for aerial photography with respect to the 
purpose of the mapping project (length of the shadows, vegetation etc.)? 
27 answers. 
In general there are specifications with respect to the purpose of the mapping 
project. The specifications refer in particular to solar altitude, vegetation 
(leaves on the trees), clouds, cloud shadows, snow, flood conditions. 
The solar altitude shall in general be above 30° but comparatively large 
variations are found: Desert areas are sometimes photographed as early 
as possible. Vegetated areas covered with decidous trees are usually 
photographed during the spring or autumn. The sky must be either complete-