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Rational Adjustment of Blocks of Aerial Triangulation 
by A. J. VAN DER WEELE, Delft. 
Par. 1. Introduction. 
Because of the accelerated development of large parts of the earth, the demand for 
small scale maps of these terrains is increasing considerably. 
Only in exceptional cases photogrammetrical procedures are not used for these maps 
but, still, the results of some of these photogrammetrical methods are not in keeping with 
the efforts put into them. 
During the solution of the problems one always meets a number of conditions that may 
be contradictory. E.g. a maximum of precision, completeness and speed is asked for, at a 
minimum of cost. The precision can be increased by the application of higher-class 
triangulation- and compensation-methods or by enlarging of the number of terrestrial 
data. However, both means would require more time and money. The completeness of the 
maps can be improved by enlarging the photoscale but this involves a greater number 
of photographs and would also require more time and money. More examples of these 
contradictions could be given, but the above should suffice. 
It will be clear that for each special job the best compromise between what is wanted 
and what can be paid for should be found. 
The suggested method described on the following pages should not be considered as 
universal, but still its application might yield savings in some cases without loss of quality 
in the results obtained. 
Par.2. Theoretical foundation. 
The method is based on two considerations, namely: 
1. For the map-user a good relative accuracy is more important than the absolute accuracy. 
2. Heights can be determined separate from the planimetry. 
Let us consider these two points more closely. 
ad 1. Each map-user, whether he be a civil-technician, a geologist, a soil-scientist or 
anybody else, will only work on a small part of the terrain at one time and only requires 
therefore that the map represents the details in correct relative position. If he cannot 
include distant points directly in his measurements or considerations, his accurate position 
with respect to these points will not interest him. 
A correct representation of the terrain details with respect to each other, in a small 
area, is therefore more important than the correct position with respect to any remote 
coordinate system. In other words, for practical purposes the relative accuracy of a map 
has to fulfill more severe requirements than the absolute accuracy. 
This approach has been applied unconsciously many times in practice. This can be 
illustrated by the fact that only in exceptional cases a map-user will take into account 
the deformations in his map caused by the method of map-projection, although these defor- 
mations may have considerable values. 
ad 2. The requirements for the planimetric precision of a map are in direct relation to its 
scale. For the heights this relation is less applicable, especially if the terrain is flat. This 
results many times in more severe requirements for the accuracy of the heights. In addition 
it is easier to determine terrestrial heights than planimetric positions (spirit levelling or 
barometric levelling versus triangulation or astronomical observations). For these reasons 
a separate treatment of heights and planimetry will be logicital. 
The most accurate means for the determination of heights with a minimum of terrestrial 
data is aerial triangulation. The choice between a first- or second order instrument for the 
triangulation will depend on the ratio between the cost of field-work and the cost of the 
instruments. Where transport in the field is difficult, the use of first-order instruments 
will be logical. Together with the heights, this triangulation yields X and Y coordinates of 
a number of points, the accuracy of which will generally be much better than necessary 
for the map. 
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