controlling the systematic errors of the photogrammetric process. As
mentioned above the combination of photogrammetry and dataprocessing
affords a good posibility of increasing the geometrical quality by adding
corrections for systematic errors, provided that their mathemathical form
is known. A great deal of the errors have been investigated, better film-
bases etc. has also reduced the systematic errors and the photogram-
metric equipments are more stabile than before. The knowledge of the
systematic errors is, however, still far from acceptable. The influence of
the systematic errors can quite obvious be compensated by using a lower
photographing altitude, but then the economy and the efficiency of
photogrammetry will be decreased.
Another problem when using photogrammetry in the final design
arises from the vegetation, bushes, trees etc. The photographs should be
taken without leaves on the trees but in some cases problems still arise.
Therefor it might be an idea to uncover the vegetation before photog-
raphy. That can however become somewhat complicated as the right of
way-area is depending as well on the horizontal as on the vertical align-
ment of the road and some changes may be made due to the detailed data
of the terrain in the final design. Therefor the computing systems must be
so designed that as well photogrammetric as terrestrial terrain-data as a
combination of them will be accepted as input. There must in other
words be included possibilities of completing the photogrammetric datas
with terrestrial ones. This requirement has also been met in most of the
existing system.
Test works have been carried out in several countries and they have
clearly shown that the quality of the photogrammetric data is fully
comparable to that of terrestrial data, when the massquantities are
discussed. Within the different cross-sections the deviations between
photogrammetrically measured datas and terrestrial ones may become
larger than the tolerances of the coordinates. Experiences have
shown that this depends partly on the difficulties of referring
photogrammetric cross-sections to the terrestrial ones and partly to the
fact that the choice of points within a cross-section is essential for the
shape of the whole section. The system of cross-section is unfortunately
not a good method for describing a terrain surface, but it is so far the
principle mostly in use.
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