8 
As the upper four categories of equipment have received considerable attention 
elsewhere, no further mention will be made of them in this paper. 
. 
Digital rectifiers, in contrast to orthophoto printers and digital mono-plotters, 
are applicable only to photographs oi nearly flat terrain. They may be regarded 
as simplified versions of the digital mono-plotter. Hence, in this paper further 
consideration will be given to the lower two categories of the classification scheme 
(indicated by heavy boxes in fig. 2). 
Digital computers perform various operations in different systems. These operations 
concern data processing and/or process control. 
A list of the computer operations together with some associated properties is pre 
sented in table I. The operations are arranged chronologically as they occur in these 
processes. From this table it follows that most operations can be carried out either 
on-line or off-line. The differentiation tends to be more distinct between operations 
concerning data processing and those concerning process control . 
Table II reviews more detailed relationships between the basic operations and the 
digital equipment components. The matrix indicates a rather complex involvement 
of the digital components in photogrammetric processes. It is beyond the scope of 
this paper to threat in detail all these operations and digital components. Only those 
components which appear most promising for future applications will be considered 
further. 
The most important component is the mini-computer. Mini-computers are becoming 
less expensive. Their performance, storage capacity, speed, input/output capability, 
{ . * « 
mass storage, peripherals and software are increasing. Manufactures are providing 
substantial software packages and more application programmes. Some general ad 
vantages of mini-computers are: they need little space, employ modern display equip 
ment, perform advanced functions, implement internal checking, reject incorrect 
data, stop the operation and/or alert the operator, and can communicate with remote 
systems. When Interfaced with analogue components they may enhance and/or expand 
the capabilities of analogue instruments, supervise them, and in some cases replace 
analogue components. Digital computers may generate variables other than those 
• i 
measured. 
. < l - ■' i ■ • - 
One of the principal obstacles to broader on-line application of digital computers 
with analogue components has been the interfaces . As these were, until recently, 
specially developed for each system, they have been rather inflexible and expensive.