It was intended to use a Wild RC 7 Automatic Plate Camera with normal-angle Aviotar
lens, f - 170 mm, at 500 m above ground. It was desirable to take the photographs
on an overcast day so that undesirable shadows and irradiation (image spread) of the
signalized points would be avoided.
Unfortunately, on the chosen Sunday a cold front produced an unexpected change in
the weather and the flight had to be made in a thunderstorm and pouring rain. The
Twin Pioneer aircraft and its experienced crew proved equal to these extraordinary
conditions and the photographs were successful.
Measurements with the exposure meter showed that the sensitivity of the plate emul-
sion was not sufficient for the necessarily high shutter-speed even at maximum aper-
ture. It was therefore necessary to fly an extra strip with the Wild RC 5a Automatic
Film Camera with the wide-angle Aviogon lens, f = 152 min, picture format 23 cm x
23 cm, at a flying height of 430 metres above ground. This flying height was main-
tained by the pilot with an accuracy of + 2 m in spite of strong turbulence. A speed
of approximately 144 kilometres per hour, the ,,single-engine safety speed“, gave an
interval between exposures of approximately 6,5 seconds for a 60 76 overlap. (Base/
height ratio 1:1,75). The photo scale was 1:2830. (See Fig.4.)
As a result of the poor weather the photographs were flat and lacked contrast. Owing
to the high shutter speed of 1/1000 sec. and the low velocity of the aircraft no
blurring occurred through image motion and the photographs were very suitable for
plotting.
3. Field identification
The objects and points to be plotted were identified in sectional enlargements, at a
scale of 1:560; at the same time the numbers of the tracks and switches were recor-
ded together with the tangent points of the curves. The points to be plotted were
also numbered. The work was completed without difficulty and without hindrance to
traffic operation.
4, Passpoint measurements
There were sufficient triangulation- and traverse-points for the orientation of the seven
models, but since they were not coordinated in a uniform system, the chosen points
were connected by a precision traverse executed with a Wild T 2 theodolite with for-
ced centring and the Invar subtense bar. The computation of the hybrid net by the