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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. 
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