a EA ER 
  
  
  
  
  
  
  
  
  
  
   
  
  
  
  
  
  
  
  
  
  
   
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   
  
  
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4 
e. g. that trials with radial triangulation, and especially with mechanical 
radial triangulation, must cover rather great areas (several strips with 
several photos in each). This makes the work very comprehensive for 
a single scientist. On the other hand in practical work with its great 
areatriangulations later stages of the photogrammetric production, e. g. 
the plotting, normally do not allow the delay of the triangulation for 
research. Also in stereotriangulation research the practical triangulation 
work takes its time but usually the evaluation of the results, the adjust- 
ment etc. is the major part of the research work, very suitable for the 
individual research worker. 
Some practical advantages of mechanical radial triangulation 
(MRT) 
The main reasons why MRT (slotted-templet- and star-templet- 
triangulation) has been and still is used to such a great extent are the 
following: 
1:0 The demand for instrumentation is very limited; in many cases 
the punches etc. have been manufactured in the repair shop of the in- 
stitute or survey office in question. 
2:0 Most of the work can be done by rather unexperienced people 
who do not know anything about photogrammetry. Floating line pro- 
duction is possible. 
3:0 The method permits a very fast completion of rather big triangu- 
lation for reconnaisance mapping; the time decreases with the decrease 
of the demands on accuracy. 
4:0 The demands on density and distribution of geodetic control 
points is very liberal. In many practical cases with low demands on the 
accuracy the triangulation is controlled only with map-points. 
Why is the MRT not more developped? 
One reason has already been touched upon. In spite of the fact that 
the Dutch have shown, many years ago, that the numerical radial tri- 
angulation can be used as a practical precision method there are still 
some misunderstandings about the sources of error in MRT. Some of 
them will be treated below. 
The nadir distance (tip and tilt) has an influence on the accuracy of 
the direction angles taken from the centre of the photo (principal 
point). But the average nadir distance in modern carefully executed 
photography from not too low an altitude is only 1**) and in flat or 
slightly broken country this induces an average direction error of the 
magnitude 0,°5*) and a maximum error about the double, which can 
be disregarded. The average nadir distance in 564 studied photos from 
150 strips was 1,51 and the maximum 5, 4, which induced direction 
*) In this article c is used for centesimal minute and g for centesimal grade 
which is 1/400 of the circle.