82 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976
With reference to a detailed analysis of
computational (or analytical) photogrammet-
ric systems given by Jaksic (1974) or
Makarovic (1974), only a few basic concepts
and terms are reviewed here. In general, the
existing methods of photogrammetric process-
ing can be classified as analog, digital, and
hybrid. The analog systems represented by
conventional photogrammetric plotters, i.e.,
by special purpose analog computers, are in-
herently on-line processors providing an
immediate feedback from the model to the
image spaces and a continuity of processing.
Conventional plotters interfaced with digital
computers form hybrid systems which com-
bine the basic features of the analog and digi-
tal processing, as shown by Makarovic (1970)
and Dorrer (1972). An equivalent on-line link
in digital systems is provided by a computer
interface to a measuring device simpler than
a stereoplotter, usually a stereocomparator.
If the interface allows information to flow
only in one direction, i.e., from the photo-
graph to the computer, the on-line link is
characterized as an open-loop system. The
collected information can be instantly pro-
cessed, but the operation and its control is in
no way affected by the outcome of process-
ing. Consequently, the function of this sys-
tem is somewhat limited. An example of an
open-loop on-line system is the proposal of
the Image-Space plotter conceptually de-
scribed by Forrest (1971). The first commer-
cial product in this category appeared re-
cently when C. Zeiss Oberkochen intro-
duced the Stereocord G2 based on the well
known Stereotop design supported by an
electronic desk calculator and a small plot-
ting table.
A closed-loop on-line system has a
computer-monitored positioning of images,
so enabling a full, rigorous, and universal
control of the operations on the photogram-
metric model. The system is capable of work-
ing in real-time when the system response
delay is negligibly small («20 ms), or in
near-real-time when more complex computa-
tions are necessary and are inserted as off-
line interruptions ranging from seconds to
minutes, which can still be acceptable. This
category includes large and highly automated
systems, such as the UNAMACE or ASII
Analytical Plotter, as well as human operated
less complicated systems like the NRC
Analytical Plotter (Jaksic, 1974) and the Digi-
tal Stereocartograph (Inghilleri, 1972). The
Gestalt Photo Mapper (Hobrough, 1971)
which represents a special purpose auto-
mated system can also be classified as a
closed-loop on-line setup.
Undoubtedly, the closed-loop on-line sys-
tems are most versatile in the photogrammet-
ric use. The function of the system can be
modified or extended by changes in the pro-
gramming software. The mode of operation
can be made simple or sophisticated depend-
ing on the user’s choice. It allows full free-
dom of applying diverse mathematical formu-
lations for the image and model geometries.
In recognition of this potential the present
paper attempts to discuss the basics of this
type of processing in the field of close-range
photogrammetry. The scope is limited to a
general treatment of the analytical aspects.
Related technical details concerning the de-
sign of individual components, the structure
of the system, or any review of existing prac-
tical applications are not considered.
GENERAL CONSIDERATIONS
On-line analytical reconstructions in
close-range photogrammetry are charac-
terized by the same features which, in gen-
eral, make close-range applications so differ-
ent from procedures used in cartographic
production. A high efficiency of the latter is
achieved by securing uniform conditions and
by adhering to standard solutions whereas
close-range projects cover a broad scope of
individual approaches. These reflect a great
variety of diverse conditions and con-
sequently make any unification or generaliza-
tion very difficult or even impossible. How-
ever, at least some degree of uniformity can
be established in close-range solutions based
on the analytical principle, especially in their
on-line versions. This is achieved by a suita-
ble formulation of the system functions and
by a development of versatile programs
which can be readily modified by the
operator at the time of their execution.
OPERATION CONTROL IN PHOTOGRAMMETRIC
SYSTEMS
Figure 1 illustrates how various photo-
grammetric systems are controlled by an
operator or a computer, to perform their basic
functions. To allow a broader comparison the
flow charts are presented not only for on-line
analytical systems, but also for analog and
off-line analytical systems. The boxes in the
Figure represent individual system compo-
nents: photograph, analog model, graphical
model or map, digital model, etc. In closed-
loop on-line analytical systems the primary
input from X and Y handwheels and Z foot
disk represents operations on a physically
non-existent, imaginary model which is digi-
