15 
Instead of carrying out the field definition of the cadastral boundaries 
by annotation on simple enlargements of the original photography, however, 
it is recommended to produce photomaps first and use these for the definition 
in the field. This has the significant advantage that gross errors in the defini- 
tion of cadastral boundaries (which may occur when there are several almost 
parallel features close together, only one of which represents a cadastral 
boundary) can be checked by simple measurements on the photomap. More- 
over, invisible boundary points can be plotted directly onto the photomap 
from the complementary field survey observations, also providing a direct 
check. 
1.5.6 Choice of Photo Scale and Camera Type 
For choice of scale of the aerial photography, in the first instance the 
economic criterion mentioned in section 1.4.1.2 above, namely that the 
accuracy of the boundary survey need be no better than the accuracy of 
idealisation, may be used. If, in a certain region, idealisation errors in the 
definition of boundaries are: of the order of, for instance, 20 cms, then when 
accepting a ratio of 1:1 between survey accuracy and errors in idealisation, a 
photo scale of 1:11,500 is found from the condition that 17.5 u at photo 
scale = 20 cms on the ground. 
If a reconstruction of a disputed boundary is required, then by applying 
numerical stereo-restitution from the 1:11,500 photography, the boundary 
can be re-established in the terrain with an accuracy of 20 4/2 = 28 cms 
(neglecting here the errors in the stake out of distances and/or angles as 
computed from the transformed model co-ordinates; the stake out will in 
any case be executed with special care). 
Other economic considerations (limited budget, reduced value of the 
land, etc) may, however, lead to the choice of a more economical and thus 
smaller photo scale. This applies particularly to cases where idealisation 
errors are very small. On the other hand, it might be that in the case of our 
example of 1:11,500 photography, the total cost (of aerial photography and 
photogrammetry plus field completion survey) can be reduced by selecting a 
somewhat larger photo scale, because in that case less complementary field 
surveying is required. In case of uncertainty about these factors, the execution 
of a pilot project is recommended.