(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 analytical methods. 
- The strips were adjusted in the block, not 
only with analog or empirical procedures but 
also with analytical procedures using 
polynomials (second and third degree) and 
least squares method. 
- Transverse (tie) strips were used in the 
ad justments. 
- Most desirable disposition of control points 
were investigated with concluding ideas on 
precision, economy and time related 
efficiency considerations. 
- Comparative studies were made between 
procedures using models formed with 
comparator observations against those 
established with analog plotting instru- 
ments. 
- 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 1966) 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 by 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 replaced 
by computational procedures and was thus able to 
improve the precision by way of eliminating 
instrumental errors occurring in instrumental 
bridging. By so doing, only the formation of 
individual models was done at the instrument 
(analog or analytical) whereas the bridging, forma- 
tion and adjustment of the block was being 
performed off-line at a computer. Numerous tests 
were performed world-wide. One can refer to the 
works of F. Ackermann, G.S. Schut, G. Inghilleri, 
E.H. Thompson, G. Togliatti, S.K. Ghosh, C.W. King, 
V.A. Williams and H.H. Brazier, to name a few. 
Yet, further block triangulation studies continued 
(Ackermann 1966, van den Hout 1966). One found, 
however, that the more a block is subdivided into 
the elements, the simpler the equation structures 
became. On the other hand, the problem of 
obtaining the adjusted values became more cumber- 
some. Thus, the various methods of adjustment 
procedures would not be basically different in the 
theoretical formulation of the fundamental 
equations, but they would differ in the 
computational procedures needed to handle a large 
amount of data and this in order to solve systems 
with unknowns of other kinds, and to elaborate 
procedures for evaluating the relative and 
absolute precisions of the adjusted coordinates. 
This also required the skill of the computer 
technologist rather than that of the photogram- 
316 
for: tricks" in 
than in the 
of necessity, 
metrist. People were looking 
the computer utilization rather 
photogrammetric procedures. Thus, 
people were yielding to the computer. In the 
program  ITC-Jerie  Anblock, the adjustment of 
planimetry is completely different from that for 
altimetry (van den Hout 1966). Obviously such 
approaches were inspired by previous works of 
recognized experts (Ackermann 1964, Jerie 1964). 
By the end of 1960s one finds that the use of 
analytical photogrammetry was no longer limited to 
research institutes (academic or national mapping 
related organizations). It began to be used (due 
to the operational ease, obtainable precision 
and rapidity of production) in the private 
sector together with the commercialization of 
programs developed at the institutes. For example, 
the Stuttgart University program developed under 
the direction of Ackermann reached world-wide 
diffusion, as well as subsequent programs like 
RABATS developed by J.F. Kenefick associates and 
SPACE-M (or PAT-M) developed by the Canada 
Department of Energy, Mines and Resources. 
B. Analytical Aerotriangulation 
The major thrust of completely analytical 
aerotriangulation has been in the U.S.A. Inspired 
and initiated by people like Schmid (1959), the 
basic approach involves the observation of image 
coordinates only. The elemental unit is not the 
model any more but the photogram and the implied 
condition is that of collinearity of the optical 
ray containing the perspective center (camera 
station), an image point and the corresponding 
object point. During the development, however, 
there have been several digressions. 
Much of the work during and immediately before 
World War II in the USA was done at the Tennesse 
Valley Authority. One of them, Ralph O. Anderson 
(1947) proposed a scheme in which orientation of 
photographs would be done semi-graphically while 
the main scheme of control extension would be done 
analytically. This, however, could not compete 
with pure analogical procedures primarily because 
of economic reasons. 
During the war and the following years, the US 
Naval Photographic Interpretation Center developed 
a series of analytical solutions for camera 
calibration, space resection, interior and exterior 
orientation as well as relative and absolute 
orientation of stereo-pairs (Merritt 1951). 
At the US  Ballistic Research Laboratories, 
Aberdeen, MD. as a consequence of research directed 
towards  ballistic camera operations in which 
several cameras may observe an event 
simultaneously, the application of these procedures 
into strip and block  triangulation followed 
immediately. These were primarily the efforts of 
Hellmut Schmid (1951, 1959) who later joined the US 
Coast and Geodetic Survey. The principal features 
of Schmid's work are a rigorous least squares 
solution, the simultaneous solution of multiple 
photographs and a complete study of error 
propagation. Schmid (1974) was successful in 
extending his ideas in performing a  three- 
dimensional geodetic triangulation by using passive 
(reflecting surface) earth satellites observed with 
ballistic cameras from 45 stations around the 
earth. He was probably the first photogrammetrist 
to look for solutions in anticipation of the use of 
high speed computers (off-line). His early reports