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(475)
In photogrammetry, the *neighbour accuracy" within small picture sec-
tions plays an important role. The investigation of several local sections of 4 X
4 cm“ with a density of 25 points each showed an average irregular shrinkage
of + 3 u. For a distance of 600 m at a picture scale of 1 : 10.000 this would
correspond to a linear error of 4.2 cm. Due to the fact that the main portion
of irregular shrinkage consists of a systematic or affine part, the linear error
will be reduced proportionally in small ranges. This is of the greatest impor-
tance in the application of photogrammetry for cadastral purposes: Short in-
stances will be more accurate than long ones.
If we investigate the chronological variations of shrinkage which corres-
pond to atmospheric changes, the so-called “breathing” of the film, we find
that this is within = 0.85% with respect to regular shrinkage. Allowance for
regular shrinkage can be made in the plotting process in a very simple and
accurate manner by a corresponding modification of the image constant.
Within the range of irregular shrinkage, however, “breathing” is only + 2.0 u,
in other words, it is so small that we cannot indicate any definite period as
being most suitable for the plotting of the pictures or the production of dia-
positives.
In order to ascertain whether film shrinkage is adversely affected by freez-
ing temperatures and low moisture content of the air, a test flight was per-
formed at a height of 8,100 m. This experiment accurately duplicated the con-
ditions prevailing during photographic flights. The air to which the film was
exposed during this experiment had a temperature of —9 ?C and a specific
humidity of 2,1 g per kg. The results were as follows:
Average irregular shrinkage +4u
Maximum extreme value Qu
A comparison with the investigations performed by Lacmann and Block
in 1933 showed that the acetate film used nowadays has only half the irregular
shrinkage of the nitro film used at that time.
This brings us to the following conclusions:
The irregular film shrinkage of + 5 u is within the limits of the measuring
accuracy of first-order plotting instruments. The film is well suited for photo-
grammetric work of high precision. This is confirmed by practical experiments
which have been carried out at the Bundesamt für Eich- und Vermessungs-
wesen in Vienna by Prof. K. Neumaier. During comparative tests with aerial
photographs which have been performed there with both film and plates, the
film had a mean coordinate error of + 6.3 cm and plates of + 5.2 cm. This
difference is so small that the use of plates does not appear justified".
Discussions about this paper are delayed till after the next paper by Prof.
Kasper (Switzerland) about "Investigations on the distortion of photographic
emulsion on film and glass plates".
Text see page (486)-158-1.
Discussion.
Mr. Hárry: Practical use of photographical material (price, weight, use
in the camera and in the plane, transport, development, etc.) makes that we
surely prefer film to plates. Only for the application of photogrammetry to
surveyings of great precision (Cadastre, precision polygonation, engineering,
