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condition equations; and (3) The polynomial corrections of point 
coordinates were affected by considering third order in X, and 
second order in Y and Z strip coordinates. It was also felt at 
the end of this stage that the measuring instruments and the oper 
ational procedures needed improvements more than the mathematical 
adjustment procedures. 
Stage 2 : Adjustment of Blocks of Strips 
By the end of 1950s in view of the developments that electronics 
brought about in the computation processes new challenges con 
centrated on simultaneous adjustment of blocks of strips, the 
models of which have been formed by analogical procedures. In 
this regard, apart from numerous individual efforts in the world, 
the one most significant study which would indicate the progress 
is the report on the coordinated group study under IS£ Commission 
III on "Massif Central" polygon (Cassinis and Cunietti 1964). 
There were twenty tests on the whole performed in six countries. 
The following would give the highlights of the tests: 
In six out of eleven cases, the bridging of models was done by analyti 
cal methods. 
The strips were adjusted in the block, not only with analog or empiri 
cal procedures but also with analytical procedures using polynomials 
(second and third degree) and least squares method. 
Transverse (tie) strips were used in the adjustments. 
Most desirable disposition of control points were investigated with 
concluding ideas on precision, economy and time related efficiency con 
siderations . 
Comparative studies were made between procedures using models formed 
with comparator observations against those established with analog 
plotting instruments. 
With the final objective of analyzing the intrinsic precisions, certain 
approaches were studied for the separation (filtration) of random 
errors from the systematic errors. 
Two specific adjustment programs deserve special mention in this 
regard, one developed at the NRC, Canada (Schut 1973) and the 
other at the IGN, France (Masson d'Autume 1960). At this stage, 
however, one could note the closing of the era of aerotriangulation 
by strips (analog aeropolygon or aerolevelling), the opening of 
aerotriangulation by blocks (or sub-blocks) and the appearance of 
a new trend with dismemberment of the strips into its constituent 
unit, the model. 
Stage 3 : Adjustment bv Independent Models 
Among the technological developments of the 1960s there is the 
computer with its exceptional possibilities of logic, memory and 
calculation capabilities which brought forth in photogrammetry very 
important changes not only in the data processing but also in the 
instruments themselves. The first step in this evolution was the 
development of the "semianalytic" triangulation. The instrument 
bridging through coorientation and scale transfer was being re-