ANALYTICAL AERIAL TRIANGULATION BY TRILATERATION MICROSCOPE 
H.S.Williams, University of the Witwatersrand, Johannesburg 
Abstract 
Analytical aerial triangulation is possible to "micron" accuracy 
using simple apparatus. By restricting the distances to be 
measured to less than 50 microns accurate plate co-ordinates 
may be determined. Results of experiments with the apparatus 
(called a trilateration microscope) are described. Moderately 
efficiert application of the method is possible if an electronic 
computer with remote access and time-sharing facilities is 
available. 
1. Introduction 
Analytical aerial triangulation is a complex process of point- identification, 
precise plate co-ordinates measurement, daia collection and data processing. 
Generally, accurate measurement to the micron in photogrammetry, stereo- 
scopic viewing of the diapositives (or negatives) during measurement, massive 
instrumentation and absolute stability of equipment have been considered 
inseparable requirements of the analytical method. Exploitation of numerical 
procedures in photogrammetry has thus tended to remain very largely in the 
hands of those institutions and agencies large enough to be able to afford the 
very expensive instrumentation needed. The fallibility of the human operator 
has further tended to entrench the use of the method in the hands of the big 
organisations. Direct digital read-out peripheral equipment has to a very large 
extent removed the human element from the data recorcing operation and while 
undoubtedly providing the most efficient means of data collecting available in 
photogrammetry at the present time (1970) data read-out systems contribute to 
the high overall cost of the instrument configuration. On-line coupling of these 
recording systems to accessible automatic computers provides for the computa- 
tion and adjustment stages of the aerial triangulation with a minimum of human 
intervention. 
Interest in the monocomparator probably first seriously questioned the need 
for stereoscopic viewing of the photo-points at the measuring stage. Harris, et 
al. (1962), (1), describe in detail a system of analytical aerial triangulation used 
at the U. S. C. & G.S. , based on monocomparator data procurement. In the 
majority of co-ordinate measuring comparators the co-ordinate reference 
frames require some form of rigid mechanical definition either as x and y guide 
rails or definition in some other suitable way. For these reference frames to 
remain in adjustment, stringent stability requirements have to be met by the