17 
ground control points were needed, of which two at each end and two in the middle. They 
are signalized with white 30 X 30 cm hard-board plates and determined by ground survey 
methods, such as traversing, etc. New permanent points to derive from the aerial triangula 
tion are monumented by stones or iron pipes and signalized. Also existing known points 
are signalized. The stones are whitewashed if they have a size of at least 15 X 15 cm. In 
several cases the terrestrial points are marked by means of three signals with a mutual dis 
tance of 10 to 30 m as an insurance against loss of one by for instance agricultural activities. 
After the tape comes from the ecomat and is thoroughly checked and corrected, the 
coordinates of the strips are transformed into one system with the use of an electronic 
Zebra computor. For this linear transformation more than two common points between 
adjacent strips are used which leads to a least square adjustment. Afterwards the trans 
formed machine coordinates of each strip are transformed by means of two points on the 
terrestrial coordinate system. With the remaining four terrestrial points corrections to this 
last transformation are calculated. Then the coordinates of identical points in the overlap 
between adjacent strips are automatically averaged by the computor when their differences 
are smaller than 50 cm with a scale of photography of 1 : 10,000. 
Regarding precision, the Cadastral Service mentions in its report to Comm. IV that 
the standard error in the coordinates of points determined by aerial triangulation is com 
puted with the formula: 
(raphot cm) 2 = d(L km) 3 + (0.0010 Sphot^ + I^L cm) 2 
in which L = length of the strip, a — a coefficient depending on the scale S'phot and 
cIl = standard error in the terrestrial coordinates of the six ground control points. 
The reporter mentions that in practice a standard error of 5 cm was found. 
In 1963 the Cadastral Service studied the possibility of applying a method of block 
adjustment developed in 1961 by Zonnenberg in his thesis for his degree of geodetic 
engineer under Prof. Roelofs. (This method is similar to that developed by KLM Aero- 
carto). It was, however, only partly used, because in the meantime the group Van den 
Hout—Eckhardt—Van Leyden of the ITC had finished the development and program 
ming of the very fast anblock method of least square block adjustment in planimetry. 
The latter method has the advantage that it can handle rather large blocks in the Zebra 
computer with arbitrary positions of ground control points (see III.3). In the beginning 
of 1964 this method was made available on special conditions to all Netherlands Govern 
ment services. Since then all block adjustments for the cadastral service have been carried 
out on ITC’s Zebra computer, using this so-called anblock method. 
D. Aerial triangulation by the Cartographic Service of the Ministry of Transport and Waterstaat 
In table 3 the characteristics of the air photographs triangulated in this service between 
1960 and the beginning of 1964 are given in more details than in table 1 by KLM 
Aerocarto. 
It is, with a few exceptions, the same type of photography and all triangulations were 
carried out in the same Wild Autograph A7. Table 4 shows the block adjustments carried 
out with different methods. 
The empirical method is the same as described by the author in an article in Photo- 
grammetria 1940, No. 1, p. 22-33. The mechanical method applied to number 4 is that 
with the use of the ITC-Jerie Block Adjustment. At present only the anblock method is 
used with ITC’s Zebra computer. Since recently a Wild EK5a has been installed at the 
A7 for coordinate registration. The tape with the proper numbering of points can be used 
directly as input in the Zebra.