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.