120 THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, HELAVA 
era, are given; some others, such as the elements of relative and absolute orientation, 
are determined during the process. When determining the parameters we are dealing 
with diserete quantities, parallax observations at a number of points, corresponding 
coordinates, etc. In all tasks of this kind numerical treatment is an ideal solution. 
The data to be processed contains information which may be divided into two cate 
gories. The first category, which we call the quantitative information content, contains 
the dimensional information. The second part is qualitative information, which makes it 
possible to identify the objects that appear on the photographie plates and also the homo- 
logous points on two photographs. Therefore, two different identification problems exist 
and they must be separated due to the vast difference of requirements they set to the 
processing system. The first is an interpretation task and at present it can be succesfully 
performed only by a human operator. The second requires recognition of similar details 
on two or more photographs. This is a much simpler task, and at least one useful electro- 
nic solution already exists. 
The dimensional information is extracted from the photographs by a processing SyS- 
tem that is in essence a special purpose computer. This computer may be based on optical 
mechanical, or electronic analogies, or on numerical computation. In every case an opti- 
cal path must be provided for the visual display that is necessary for identification pur- 
poses. When the optical analogies are used, the computation process and visual display 
may overlap because the same optical parts are used, Mechanical and electronic analogies 
and numerical methods require a separate optical path. In this respect the use of the 
principle of the analytical plotter does not lead to solutions drastically different from 
what exists in present plotters. 
Customarily the photogrammetrie data processing is based on the analog principle. 
This approach was justified because of the analog character of the original data and 
because of the fact that until recently there were no digital devices capable of perform- 
ing the operations required in the processing. Today, however, the digital devices are 
fully capable of doing whatever has been done using the analog instruments in the past, 
and more. The analog character of the original data still remains and has been presented 
as an argument against the numerical methods. We shall now study the significance of 
this feature. 
In many data processing operations and in all problems of automatic control, the 
original data is in analog form. The reason is that practically all the physical measure- 
ments yield the answers in an analog form despite the ultimate “digital” character of 
all things. The analog form is said to have one valuable property which is of importance 
also in photogrammetric plotting. This is the property of “continuity”. However, a closer 
study of the problem shows that the analog information is continuous only as far as the 
sensitivity of the system, or — to use a photogrammetric term -— its "resolving power” 
permits. Here this familiar term is meant to include the entire system, including all the 
imperfections of the plotter, dead motion, reading accuracy, etc. The smallest interval 
that may be correctly measured represents the resolving power of the analog system. If 
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a competing digital system is based on increments smaller than that, its “continuity” is 
superior. 
3. Special problems in the application of the new principle. 
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3.1. Mathematical formulation. 
The instruments based on the simulation principle have been dominant in photogram- 
metry for about forty years. One reason for this may be that the problem of mathemat- 
ical formulation on which these plotters are based is solved in an exceedingly straight- 
forward manner: the central projection is used as its own analogy. This solution needs no