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Another example of triangulation work with limited precision is that
executed by the S.D.M.P.W. in the Wild A 6 for tropical areas for the
B.P.M. These triangulations have been carried out on photographic material
of different quality, in many cases even on paper prints. It is obvious that in
such cases only a limited precision can be obtained.
At first the purpose of this kind of triangulation was only the determina-
tion of the altitude of photogrammetric control points, the x and y co-ordinates
being determined in some other way. In some cases these x and y co-ordinates
had been determined many years ago by radial triangulation or triangulation
in a Stereolpanigraph or a Wild À 5 as a basis for a geological map. In other
cases x and y co-ordinates had been determined from a slotted template lay-
out. In a following stage of development a complete x-y-z triangulation wa-
carried out in the À 6, providing the control points necessary for the plotting
of a topographic map of the area with a limited precision. This method, now
normally applied to this problem, is as follows:
After pair 1—2 has been orientated in the À 6, we read the altitudes of
the necessary control points and the x, y co-ordinates of those points are
plotted on a sheet of non-shrinkable material. Then the complete plate-holder
with photo Nr. 2 is transferred from the right to the left plate carrier and
photograph Nr. 3 placed in its stead. The lateral tilt w of photograph Nr. 2,
as measured in the right plate carrier, is set in the left plate carrier; the relative
orientation of pair 2—3 is carried out by means of the swing x and the
longitudinal tilt ¢ of both photographs and the lateral tilt «o of the right plate
carrier. Now we determine the scale, following the usual method, by changing
b; until the altitude of a sharp identifiable point, situated close to the common
principal point, shows the same value as determined in the preceding pair. A
check and the possibility of adjustment is provided by measuring and com-
paring the y co-ordinates and the altitudes of common points situated on both
sides of the photograph in the vertical plane through the common principal
point. A comparison of the altitude of those common points on both sides of
the images indicates the presence of a difference in lateral tilt between the
new and the preceding pair. This difference in lateral tilt is adjusted by the
common rotation of « to an amount of half the difference. Experience has
taught that the mean value of this difference is about 0.109.
It is obvious that when continuing this operation we shall find values for
the altitude of all principal points and necessary control points and that the
x, y co-ordinates will be determined graphically. In the tropical regions to be
plotted in this way, only very few ground control points are available. The
experience gained with large areas has shown that in such cases a complete
study of the drainage system of the terrain furnishes very useful data and
provides a sound basis for the complete computation of the altitude.
In order to obtain those data, the “Preparation” section of the S.D.M.P.W.
5
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