(352) 
Austria reports the process is used exclusively since 1950 for control both 
horizontal and vertical between known premarked points. 
Germany employs short bridges of 8 to 10 models between geodetic con- 
trol for 1: 25,000 scale maps. A method for using unidentifiable geodetic points 
by measurements to substitute points is being tried. The method has been ex- 
tended to the 1 : 5,000 scale, tolerating some sacrifice of precision. 
The Belgian IGM uses aerial triangulation only for controlling wooded or 
very rough areas; for surveys of the Belgian Congo aerial triangulation is em- 
ployed systematically for plotting. The Public Works Photogrammetric Service 
controls only one model in ten on the ground for surveys at 1: 5,000 and one 
model in five for surveys at 1:1,000. If there are more than five models 
between known points, intermediate heights are added on the ground. 
Graphic adjustment is made for 1 : 5,000 and analytically for 1 : 1,000. 
Density of Control: 
These densities are only slightly comparable for correct and sure plotting 
in the different countries where the control varies between adjustment by 
ground survey or by aerial triangulation; many countries separate horizontal 
and vertical control, as realized for many years in France, because the number 
of horizontal points needed for scale is much smaller than the number of 
heights necessary, not only for absolute orientation, but for control of the 
flatness of level surfaces. These numbers are on the average two horizontal 
points per model and six vertical points per model at least. 
Of course where the planimetric control results from a spacial triangu- 
lation or from a radial triangulation, the number of points may be readily 
increased, and at least 4 points per model are obtained for planimetry; it is not 
only the number of ground points which counts but also those that are furnish- 
ed by aerotriangulation. 
In countries where the geodetic triangulation is very dense, its stations are 
usable for either horizontal or vertical control. This is the case in Switzer- 
land, where for the national map a density of two stations per square kilometer 
furnishes 8 to 10 stations per stereoscopic model. 
In Italy, the IGM of Florence always measures 4 horizontal points per 
model near the corners and often a fifth point near the center. Occasionally 
points are measured only in azimuth but numerous vertical angles, direct or 
inverse are also measured. 
The IGM of Brussells reports 2 horizontal points and 5 vertical points per 
model; Finland reports 2 and 4 points respectively. 
In France for the 1:20,000 maps the average per model is 2 horizontal 
and 8 vertical points. 
It is evident that with average overlap a vertical or horizontal point well 
placed can serve for two models of the same strip and for two contiguous 
strips. 
Methods Used. 
The methods used for control surveys on the ground are usually triangu- 
lation, direct and by intersection (more rarely stadia traverses) for determining 
both horizontal and vertical coordinates. This is true mostly for maps at large 
and medium scales. (1 : 1,000 to 1:50,000). For maps at very small scales which 
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