0 
ere 
be 
ng 
si- 
al 
masts, 
  
^ 
The foundation points of the masts were determined in a way similar to this method. Again 
two or three collars with measuring targets were fixed to the masts and with their help the direc- 
tion in space of the mast axis could be computed. After determination of the coordinates of the top 
and with the known length of the mast the coordinates of the foundation points of the mast were com- 
puted, 
Geodetic determination of the control points 
The next step was the geodetic determination of the control points. This was done by si- 
multaneous intersecting from two concreted points A and B. 
The length of the base, about 5 m, between the points A and B was indirectly determined 
from observations of a calibrated subtense bar, All observations were made with two theodolites 
ZEISS TH 2. For independent control of the first set of intersections the point B4 was displaced 
a few centimeters and the whole procedure was repeated from A and By. Both results were oriented 
against each other and compared by a Helmert Transformation, The computations followed imme - 
diately after the observations were completed with a desktop calculator hp 9100 A of Hewlett Packard. 
For height determination several levelings with a ZEISS Ni 1 and a calibrated steel rod were run 
over the control points, With this method the position coordinates were determined to * 0,2 mm, the 
heights to * 0,1 mm. 
subtense bar 
  
control point 
  
  
  
  
  
  
  
  
  
  
Illumination and Photography 
  
The possibilities of illumination had been explored by preliminary experiments, A light 
background and indirect illumination gave a poor image of the net. The thin wires did not contrast 
against the background, Finally a black non lustrous velvet in direct light proved to be the best 
background, 
The lighting scheme consisted of 6 - 8 photo-floodlamps aligned such that the entire range 
of the model was equally illuminated which was tested by taking Polaroid pictures, The background 
of the fiducial marks of the camera was lightened by laying out white paper. Also a thermometer, 
a hygrometer, the number of the picture, and the indication of the model were layed out and photo- 
graphed as additional information, 
Very helpful for the identifications of complicated areas of the net and around system 
points as well as for partly obstructed details were close-up Polaroid pictures. 
For taking the picture itself the camera was fixed into one end of the base, the glass plate 
was loaded, stop and shutter were set, and the camera was tilted and leveled, After the first expo- 
sure the camera was moved to the other end of the base and the second picture was taken, In order 
to be on the safe side several pairs of photos with different exposure times were made, Because 
of the necessary depth of field aperture 32 was used. The times of exposure varied from 5 to 40 
seconds, 
—57—