applications is according to the type of the output record, which can be: 
numerical, symbolic-graphical or photographic. These categories may obvious- 
ly be differentiated further. 
Some operations, and thus the corresponding equipment components, 
are involved in any photogrammetric application. These components are: photo- 
grammetric instrument, interface unit with displays and keyboard, and often a 
digital computer with input and output devices. These main components will be 
treated successively in chapter III. A digital computer may provide support in 
different phases of photogrammetric restitution. The interactions between the 
components can be structured accordingly. A review of the different actions and 
operations involved in preparation, restitution and post-processing is presented 
in figure 3. The post-processing operations will not be considered further in this 
paper as they are normally performed off-line. The same applies to the ortho- 
photo printing equipment, which is the subject of a separate paper. 
II Equipment 
The basic equipment components of a system have been defined in figure 
2. In the following the pertinent components will be briefly discussed from 
the viewpoint of the overall system needs. 
III-1 Photogrammetric instrument 
The instrument may either be a precision or a topographic plotter. From 
the viewpoint of accuracy precision plotters are advantageous. However, topo- 
graphic plotters are normally faster in operation and their cost is substantially 
lower. With some modifications the accuracy of topographic plotters may be 
raised to approach that of precision plotters. Projection system should be pre- 
cise and stable and the means for orientation should permit accurate settings of 
pre-determined orientation parameters. The model space need not be large; a 
scale ratio photo-to-model 1:2 may suffice. Freehand control of planimetric 
tracking is preferable. Measuring device should permit easy interfacing with 
digital components and driving agents should be adapted to digital control. 
Internal plotting tables are preferable to external types for drafting data during 
digitising. 
III-2 Interfaces 
Interface equipment performs several tasks. The two basic tasks are ana- 
logue to digital (A/D) and digital to analogue (D/A) conversions of data.