The photo-flight should be planned in such a way that the 
strips fit the frame of the final mapsheets so that the 
number of models to be used for mapping, and of the dia- 
positives to be projected as orthophotos, will be as small 
as possible. For highway design the strips are usually 
flown along the center line. For preliminary planning and 
location studies in smaller-scale photography this can be 
realized by a high longitudinal overlap of 80 % or 90 %. 
In case of large photo scales 1 : 4 000 or 1 : 5 000 for 
the final design, large longitudinal overlaps often are 
not possible because of the low flying altitude (600 m 
to 900 m) and the corresponding short exposure intervals. 
If the models are ill situated to the final map sheets 
some orthophotos have to be fitted one to another by 
standard mosaicing techniques. 
Lengths and directions of shadows should not vary too much 
in adjacent photographs, otherwise the appearance of the 
final photomap will be disturbed. Therefore, the photo- 
flight has to be planned in such a way that adjacent 
strips are flown at about the same sun position, even if 
the photo-flights have to be carried out on different days. 
A time difference of more than two hours between neigh- 
bouring strips must be avoided. 
For the production of orthophotomaps, as for any other 
photogrammetric mapping, control points on the ground are 
  
necessary for setting-up the models. The quality of the 
control points directly influences the accuracy of the 
orthophotomap because the connections between models can 
not be corrected during the automatic projection. 
For setting-up the models only control points determined 
by field measurements or aerial triangulation and exactly 
identifiable in the photographs may be used. The coordinates 
of these control points can also be taken from existing maps.