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122 THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, HELAVA
3.2. Means of computation.
8.21. Analog computers.
The data processing system may be based on any one of the known computational
techniques or on a combination of several. In addition to the optical and mechanical
analogies employed in photogrammetric plotters so far, hydraulic and electronic analog
techniques may also be utilized.
Analog computation has some distinct advantages and disadvantages, e.g., the very
high speed at which they operate is an advantage common to all analog methods, while
their common disadvantage lies in their limited accuracy. This accuracy may be impro-
ved in certain cases. However, the price of improvement becomes very high when the
limits of ultimate performance are approached, and in this respect the economical
standard has already been reached, if not surpassed. In addition to these properties
common to all analog methods the optical principle has the favourable features of being
very reliable and of offering unparalleled possibilities for a simple basic solution. This
solution combines the computations with the visual presentation unavoidable so far.
At the same time the optical method holds the record in inflexibility.
The mechanical analogies are also very reliable in practical operation. Their flexi-
bility is better than that of the optical method, but cannot be considered to be fully
satisfactory. Extremely high standards must be maintained in manufacturing to meet
the present accuracy requirements, and careful calibration and environmental control
are necessary to preserve the accuracy level. In this respect the level of economical
accuracy appears to be reached. It seems that the only further improvement would be
the use of mechanical analogies for the calculation of differences instead of total
values as is done in the projection instruments. However, for any measure of success
this approach requires limitations in the ranges of variables and therefore appears
impractical to the author. This does not reduce the value of such devices for special
applications where flexibility and the ranges of variables are of minor concern. Several
interesting instrumentations of this kind have been proposed by Deker [10]. Undoubt-
edly the mechanical analogies will be utilized in many future plotters at least as means
of deriving some experimental corrections.
The electronic analogies appear to be the most attractive of the analog methods.
Their outstanding advantage over the other analogies is their flexibility. The computer
scheme may be set up relatively easily even for a complicated formulation. An accuracy
that is comparable to that obtained by other analog methods may be reached, although
not without difficulties. High precision manufacture, careful calibration and environ-
mental control are essential conditions for best results. However, in general, the elec-
tronic devices are easily mass-produced and many accuracy problems can be overcome
by improvements in the design. For these reasons the electronic analogies deserve close
attention as a possible way of solving the entire computation problem in a photogram-
metric plotter, [2, 6, 7] or as a component in a kybrid system.
3.22. Digital computers.
About two decades ago all digital computers were based on mechanical principles
The introduction of electronic digital computation techniques has made and is making
tremendous progress in this field. This development has now advanced so far that the
use of digital techniques as the basic means of computation in a photogrammetric plotter
is not only feasible but indispensable for the most advanced solutions.
The advantages of the digital computation are many. Their accuracy is practically
unlimited, and any required degree of accuracy may be obtained by carrying more digits
through the computations. This feature is most important for the basic plotter. It also
means that data may be introduced to the plotter in numerical form and that rigid nu-
