to near vertical as well as to convergent photography. This method, also programmed by 
D. Eckhart, has already given a proof of its high quality. It will be published, probably 
by the end of 1960, in the thesis which Ir. v. d. Hout will present for his Doctor’s Degree 
to the Delft Technological University. 
Concluding this series of contributions of Delft to the improvement of aerial triangula- 
tion we should not forget a moderate instrumental contribution which nevertheless is of 
importance: a new device for stereoscopic point transfer. With the more frequent applica- 
tion of block adjustment it is obvious that the identification of tie-points in adjacent strips 
needs a high precision, since errors in identification will increase the residual errors in 
these tie-points to their full amount after the block adjustment. The first method in use is 
to select natural points. In many types of terrain, such as forest and jungle, this is either 
barely or entirely impossible. In more cultivated terrain we can identify natural points, 
but many easy points give only a very low precision. For these points mean square errors 
in transfer between 20 and 40 microns can be expected. These values are much too high, 
compared with the values which can be obtained as relative errors after block adjustment. 
In some kinds of terrain it is possible to select very small black dots of only a few microns. 
Their precision will be sufficient but to find these in the always small areas of the overlaps 
takes much time and these dots require special indication in order to avoid confusion. 
The solution for the marking of points could be obtained in Delft along two lines, both 
already in use there since 1935. The first was applied in the radial triangulator of De 
Koningh (see National Report of 1938 in International Archives of Photogrammetry 
Volume III, fase III). In this the negatives are observed in the normal way as in the radial 
triangulator but with emulsion down, and after the floating mark is set on the ground, 
small holes are drilled with a special drill which comes up from below. This solution has 
been accepted by Wild in its new P.U.G. The I. T.C. has preferred the improvement of 
the other Delft solution of 1935 (see the same National Report 1938) using a free hand 
motion of small transporters with floating marks, observed under a normal mirror stereo- 
scope with Dove-prisms in its telescopes. The floating marks were little holes, through 
which a hole is pricked with a needle in the emulsion at the moment that the floating 
marks are set on the ground. This free hand method gives a mean square error in the 
transfer of points between 15 and 30 microns, depending on the training of the operators 
and the material. P. Dongelmans, chief mechanic of the I. T.C., improved these simple 
devices by replacing the holes in the transparent material of the transporters by very small 
steel spheres pressed into perspex plates. These steel spheres act as floating mark and after 
setting these marks by means of micrometer screws instead of by free hand, they are beaten 
from above with a constant force. The spheres press a small hole in the emulsion. The 
mean square error in the transfer of points obtained with the use of these new trans- 
porters and normal mirror stereoscopes is so far 8 to 12 microns, also depending on the 
skill of the operator and the quality of the photographs. Patents have been applied for 
these transporters in several countries and Zeiss Aerotopograph will bring these small 
instruments on the market. They will be shown at the Zeiss exhibit at the London Congress. 
III. 2. General remarks about the execution of aerial triangulations 
Regarding the execution of aerial triangulations by the various services in the Nether- 
lands we first make a few general remarks in common for about all of them. 
There has been about no calibration at regular intervals of the cameras. In general 
the data provided by the factories was trusted. As mentioned in cHAPTER I, I considered 
this as a weakness of the system, in particular with the use of the Zeiss twin camera. 
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