4 
the interface equipment a distinction can be made between systems incorporating: 
- versatile and flexible interfaces with specially designed mechanical components 
(e. g. the AP-C and AS-11 versions [6] [7] ); 
- simple special interfaces, linking the already existing equipment components 
(e. g. D.S. Digital Stereocar tograph [8] ). 
However, between these two limiting cases various intermediate solutions are 
possible. 
The off-line systems have been classified into two main categories: 
- Conventional plotters supplemented by an off-line computer. 
The recording unit (digital, drafting or orthophoto printing) can be controlled by 
the same or another computer; 
- Comparators supplemented by an off-line computer. The recording unit (digital 
or drafting) can be controlled by the same or another computer. 
Another useful classification of the equipment is based on operational characteristics 
(fig. 2). In this classification a distinction has been made between stereo and mono 
systems. 
Fig. 2 
Classification according to operational characteristics