To satisfy a "complete" digital stereo-compilation system, the following 
fundamental requirements were considered: 
- The digital photogrammetric map compilation must not be more time- 
consuming, more tiring (physically or psychologically) for the operator, 
or more subject to blunders than usual graphical methods. 
- The system should support on-line digitizing, graphic display and editing 
capabilities not inferior to graphic methods. 
- The system should have multiple entry facilities and be easily expandable 
both by the system designers and by the user. 
- It must retain the similarity and the ease of operation currently used in 
photogrammetric practice, so that the change-over will be as 
transparent as possible. 
The photogrammetric digitizing station consists of the photogrammetric 
instrument, the microprocessor interface terminal, graphic display and edit 
terminal, and the proper interfaces to the system computer and I/O devices. 
2.1.1 Photogrammetric Instrument Attachments 
For digital stereo-compilation, the measuring system of the photogrammetric 
plotter should be equipped with a digitizing device which picks up the co-ordinates 
in the form of video pulses. The present available digitizing devices operate in 
the following modes: incremental or absolute. In the incremental type, the 
positions are coordinated in terms of the total count of distance measurements 
traversed from an initial starting point. In the absolute type, the positions are 
coordinated in terms of fixed values for each axis. 
Due to the wide variety of photogrammetric instruments available, the choice of 
the optimum digitizing device attachment is dependent on the mechanical 
arrangements made for measuring the X, Y, Z coordinates in the model space of 
the instrument, and the desired coordinate digitizing resolution. 
2.1.2 Microprocessor Interface Terminal 
This interface provides the link between the photogrammetric instrument and the 
computer. The microprocessor interface unit tracks the stereoplotter by counting 
up or down the X, Y and Z counters as a function of the amount of encoder shaft 
rotation which is proportional to the linear distance traversed by the 
stereoplotter. The X, Y and Z model coordinate readings are then transformed to 
ground coordinates by the microprocessor, and are sent serially to the computer 
where the data is entered into the data base. 
2.1.3 Graphics Display 
Over the last two or three years, there have been major developments in graphic 
displays, largely a consequence of the development and growth of integrated- 
circuit and microprocessor technology. As a result, there has been an increased 
capability and sophistication in graphics display hardware accompanied by a 
reduction in its cost. The first generation of graphic display terminal used in the 
Branch was based on the use of stroke-storage CRT. For the past few years, 
graphics terminals which we acquired were all based on the use of graphic 
screens, of the vector-refresh, or the coloured raster screen type. 
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