39 
ANNEXURE E 
BLOCK ADJUSTMENT 
by S. G. BERVOETS 
Senior Photogrammetrist Central Mapping Authority 
Department of Lands, Sydney, Australia 
INTRODUCTION 
In most cases of photogrammetric mapping the area is covered by parallel strips 
of aerial photography. The position and elevation of a minimum of two pass points 
in each photo are established by aerial triangulation. The adjustment of each strip 
to control can be handled in a number of different ways. 
Following present techniques two distinct problems arise: 
(1) To ensure consistency between runs. 
(ii) To obtain the final position of points within the accuracy specified by 
accepted mapping standards. 
Concerning the first condition it is well known that single strip adjustment is 
weak, in that sometimes it shows rather large discrepancies on the tie points. The 
situation cannot really be improved by adopting more complicated formulae for 
the correction of each strip, unless so many control points are put in that there is 
hardly any need of aerial triangulation at all. Moreover, the method of providing 
nearly every overlap with sufficient ground control has to be rejected in most cases 
for economical reasons. The answer seems to lie in a different technique of adjust- 
ment, which gives better agreement between the various strips, and at the same time 
requires fewer control points to yield a map complying with the accepted standards 
of accuracy. 
The following procedure of adjusting the strips simultaneously has been devel- 
oped and tested by the Photogrammetric Division of the Central Mapping Authority, 
Department of Lands, Sydney. The principle is that the tie points, instead of being 
used as subsequent checks on the quality of the result, will rather form the basis 
of the initial stages of the adjustment. The idea of incorporating the additional 
information provided by the tie points is certainly not new. Other publications have 
also dealt with this subject, suggesting various solutions to the problem. It seems, 
however, that the possibilities of a semi-graphical approach have not been fully 
explored. 
This paper will discuss such a method which does not require any special 
equipment such as automatic computers. The whole process is indeed based on 
correction-graphs and calculations to which the average computor should be 
accustomed. 
Customarily, the determination of position and elevation is dealt with separately 
in surveying. In the next paragraph a theory is advanced for the planimetric adjust- 
ment of aerial triangulation. At present, the vertical adjustment is also being exam- 
ined with a view to the possibility of using a simple technique along lines similar 
to the horizontal determination of pass points. 
  
THEORY 
The adjustment will be carried out in two stages: 
l. The simultaneous correction of all strips in such a way that the dis- 
crepancies between the runs will be eliminated. 
2. The final transformation of the internally adjusted block as a whole, 
in order to fit it to ground control.