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components involved and their properties, and on input and output charac- 
teristics. The software should be sensibly designed, which requires utmost 
care and profound insight (i.e. experience in photogrammetry and program- 
ming, a thorough understanding of the equipment and of the information 
flow (13)). Software development tends to cost more than the equipment. 
Some properties the software systems should have,are: 
— simplicity and convenience (e.g. accessibility to data); 
— high level programming language (for application software); 
— modular design; 
— compatibility with different types of computers; 
— optimal solutions for specific applications; 
— callable subroutines (from keyboard); 
— implementation of human engineering (to keyboard and display units); 
— distributed intelligence and/or time sharing capability. 
A software system provides for versatility and flexibility of the overall 
system. Software can be structured according to the different operations 
involved in, or associated with, photogrammetric restitution (figure 4). It is 
of the utmost importance to ensure that the corresponding subroutines are 
mutually compatible and thus adapted to the overall needs of the system. 
Another more common classification is into basic and application soft- 
ware. The basic software is needed for basic functioning of a system, e.g. 
data acquisition and recording. Therefore it should provide for a high efficiency. 
Limits between the basic and application programmes are not very distinct. 
They depend upon subjective judgement of the functions to be considered as 
fundamental ones. 
The application software is, obviously, application oriented. Most of 
the routines, which are not implemented in fast, real-time operation can be 
complex, flexible and versatile. They can be written in a high level language 
which permits the users to change and/or extend application programmes. 
Additional software is required when several computer-supported instruments 
are bundled and controlled by a central computer (e.g. a multiple system 
with distributive computer network) (15), (16). The presently available soft- 
ware is still rather limited. However, there is substantial development under 
way.