Programming Computer-Controlled
Photogrammetric Instruments
RH. B. FORREST
A. E. WHITESIDE
J. A. HORNBUCKLE
This paper describes techniques used by the computer programmer and the photo-
grammetrist in programming real-time computer-controlled photogrammetric instru-
ments. Special techniques are used for both the real-time and service programs; an
interesting example is the "parameter perturbation" method of relative orientation.
The continuing development of more powerful and flexible computers offers many
opportunities for more complete exploitation of photogrammetric instruments by
both the computer programmer and the photogrammetrist.
INTRODUCTION
When electronic computers are applied to opera-
tions in a particular field of activity, close communi-
cation is necessary between the specialists in that
field and the specialists in computers. This is espe-
cially true in the use of computers to control photo-
grammetric instruments: in few other computer ap-
plications is there a closer relationship between man
and machine. Such a close association requires the
computer programmer and the photogrammetrist to
be aware of each other’s problems and requirements.
The photogrammetrist must describe the special
needs and processes of his instruments to the com-
puter specialists. Moreover, he must properly evaluate
the potential for expansion of these needs. A basic
knowledge of programming methods and computer
limitations is very helpful for this problem definition.
The computer programmer has his own problems in
turning the stated needs of the user into operating
programs for a control computer. Some photogram-
metric needs require the development of special
programming techniques. Restrictions in processing
time and memory usage contribute to the program-
mer's difficulties and increase the necessity for effi-
cient programming.
Unlike the techniques used for general automatic
data processing, the problems and methods for con-
trol-computer programming have not been widely
described to photogrammetrists and others outside
the area of computer technology. This paper de-
scribes some of the methods used for control-com-
puter programming, with emphasis on the special
techniques developed for photogrammetric instru-
ments. *
*Many aspects of the computer programming discussed in this
paper are associated with photogrammetric control systems developed
by Bendix Research Laboratories under sponsorship of Rome Air
Development Center, U. S. Air Force.
BENDIX TECHNICAL JOURNAL SUMMER 1968
PHOTOGRAMMETRIC CONTROL AND
PROGRAM REQUIREMENTS
Helava's invention of the analytical plotter 10
years ago marked the beginning of the use of stored-
program digital computers to control photogram-
metric instruments. Today, computers and computer
programs are integral and essential parts of the
manual and automated analytical plotters, the auto-
matic comparator, and the hybrid stereoplotter. The
programs for these instruments must perform exten-
sive computations of the kind familiar to photogram-
metrists: | model-to-photo transformation, relative
orientation, interpolation from computer-stored lens
distortion tables. In addition, the programs must
control equipment operations. Information must be
processed and sent to and from instrument compo-
nents: photo-viewing unit, coordinatograph, and
possibly electronic scanning and correlation circuits.
Moreover, the programs must provide for control-
panel communication with the instrument operator.
Several kinds of input-output data must be processed
by the programs. A simple manual stereoplotter re-
quires position inputs from two handwheels and a
footwheel, and output positions for four photo-car-
riage axes and two coordinatograph axes. In a more
complex instrument, many more inputs and outputs
must be processed: the AS-11B automated analytical
plotter has 27 input-output positions and adjust-
ments, and most of these quantities must be sampled
or changed 25 to 100 times per second.
All computer programs can be divided into two
groups, real-time and service programs, based on their
execution-time requirements. Real-time operations
are greatly time-dependent and require either some
maximum computation time or some fixed rate of
