— 
67 
  
Fig. 3. 
A model on a scale of 1: 35 000 with 60 96 overlap will 
cover approximately 38 Sq.km, i.e. an average of 7 trig points 
per model. Thus, most high altitude models have sufficient 
existing pass points. In locations where points for absolute 
orientation of a high altitude model are missing, such points 
can be determined 
l. by aerial triangulation 
2. by terrestrial survey. 
The necessary number of points for orientation of the low 
altitude models on scale 1 : 5 000 can now be extracted from 
the high altitude models. 
The selected pass points can be transferred to the geo- 
detical system on the absolute orientation sheet for the high 
altitude models. From this, the coordinates of the points can 
be read off in the coordinatograph, and the points can be mar- 
ked on the horizontal projection sheet of the economic map. 
However, especially in the case of areas with wide varia- 
tions of altitudes, it has become more usual to determine the 
selected pass points in the coordinate system of the machine, 
and to transform them to the geodetic system by means of 
electronic data processing. 
Now the grid system of the map sheet, the existing trig 
points and reference points are drawn in a coordinatograph.