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The decision as to whether auxiliary data should he used or not will 
not only depend on the balance of costs as shown above, but is also 
governed by the technical specifications of the project and the 
environmental factors of both the mapping object and the photogrammetric 
organisation. 
IV. Methods of application of auxiliary data to aerial triangulation 
In a general discussion of methods in use or proposed, a distinction must be 
made between two main groups: 
A) Methods using auxiliary data for setting pre-computed orientation 
elements in the instrumental phase of aerial triangulation 
B) Methods using the auxiliary data in the adjustment phase of aerial 
triangulation 
The author intends to prove that the general use of auxiliary data in the 
instrumental phase of aerial triangulation is incorrect and illogical, with 
the possible exception of methods requiring a limited accuracy such as 
triangulation with independent models on topographic plotters. 
In general, the use of auxiliary data is only to be recommended in the 
adjustment phase of aerial triangulation, which does not unnecessarily 
complicate the instrumental execution and moreover, leads to the greatest 
attainable accuracy. 
A) Direct use of auxiliary data in instrumental phase 
These methods have evolved with time. In fact, these were the first on 
the scene, which may be the reason that they still appear again and 
again. They are probably based on an insufficient appreciation of the 
character of the accuracy of the auxiliary data or on wishful thinking 
that one is able to obtain auxiliary data which are more accurate in all 
respects than required for the photogrammetric problem. 
The correct view should be that auxiliary data are additional independent 
observations of certain camera orientation elements with other error 
properties than those of the corresponding elements determined during the 
aerial triangulation. The best final result will be obtained if both 
types of observations are used simultaneously in an adjustment procedure 
with a correct consideration of their respective ex'ror properties. 
For reference, see table 3» 
If auxiliary data are used directly in strip triangulation for the setting 
of pre-computed orientation elements in the instrument, the residual 
y-parallaxes must be eliminated with the aid of the remaining orientation 
elements. 
This will lead to discontinuities and gaps between consecutive models.