4 
Œ-no. Display, Graph. CRT | Larger | 
Keyboard Display Term. | Computer 
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| Plotting | Photogram. ; J Lim mio sen of Digital I/O 
| Device Instrument Interfaces + Computer [| Units 
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b | Orthophoto! Drafting — — — — — — — — — d 3 
| Printer | Machine le ——————————7 € e 
figure 2 
overall system configuration 
In fact, such a “complete system" would not be optimal, because 
its components cannot be fully exploited. Some of the boxes in figure 2 may 
comprise more than one physical unit (e.g. keyboard and display units, 
I/O (input-output) units, etc.). A computer, integrated in the system, may be 
autonomous or it may be linked with another, more powerful computer. The 
different variants of the system will be treated in chapter V. The nature of 
interactions with digital components strongly depends on the correspon- 
ding software. A sensibly designed software is essential. Development of the 
software may affect the overall cost of a system more than equipment itself. 
In principle the designs of hardware and software should be considered 
simultaneously. A modular approach to software development is appropriate. 
The different modules can be classified into three main categories, i.e. modules : 
related to: preparation, main restitution, and post-processing. Each of these € E 
categories can be subdivided further. The software system will be treated 
separately in chapter IV. 
The /evel of interactions between each pair of components (vide figure 
2) depends on the type and amount of the information flow. The level depends 
on the number of channels, uni- or bi-directional flow, rate of information 
flow in each channel, time constraints, and complexity of data processing 
related to interactive operation. The requirements pertaining to interactive 
operation are strongly application oriented. A feasible way of classifying the