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• GIS data interface with mono/stereo superposition of graphic elements and optional 
editing functions, 
• option for direct input of digital image data (e.g. from a CCD-camera) for on-line photo- 
grammetry, 
• stereoscopic image display on one CRT by means of Liquid Crystal Shutter techno 
logy using a second CRT for overview display. 
GRUEN (1988) gave an overview of the different possible approaches to set up a digital station, 
namely the 
• genetic concept that refers to custom-built systems providing great functionality and 
performance at a high cost, the 
• turn-key image processing system concept which allows the use of already implemen 
ted system functions but normally is not extendable, and the 
• modular approach offering extensive options for expansion of hard- and software but 
demanding great expenditure of development and implementation time, especially at 
the beginning. 
According to GRUEN the modular approach has the greatest long-time prospects and a modern 
digital station should consist of a workstation computer combined with several add-on devices. 
By this means highest flexibility and adaptability of hardware architecture could be achieved with 
respect to cost and functionality. As discussed in KOENIG et al. (1988) additional processor 
hardware is required to achieve the performance level needed in photogrammetric applications, 
which means that the use of parallel hardware architecture is indispensable. Modern image 
computing systems make use of this type of architecture, for example PIXAR or AT&T's Pixel 
Machine, which is built around up to 82 Digital Signal Processors. These systems offer a high 
computational power, permit high throughput, and can be configured with parallel transfer disks, 
but are limited in number of processors and are not free expandable in memory. High perfor 
mance systems require open parallel architectures with data and algorithms being distributed 
over many processors. 
The Transputer Approach 
In 1987 INMOS (1988) introduced the 32-bit T800 transputer. It comes with a RISC style CPU, 
4K of fast on-chip RAM, a floating point processor, an external memory interface, and four serial 
links which run at up to 20 Mbits/second. The introduction of the link architecture can be regar 
ded as a milestone in computer science, which allows to construct networks of transputer family 
products by direct point-to-point connections with no external logic. A significant advantage of 
this concept is the design of large multi-processor systems with different physical interconnec 
tion topologies such as pipelines, trees, or 2D arrays (see Fig. 1). 
This also means an extreme flexibility in software development. Depending on the algorithm, 
different topologies can be switched so that a significant increase in performance can be achie 
ved. Moreover, if performance requirements and budget increase, a transputer system can 
easily be upgraded by adding new processors. In this case existing algorithms do not have to be 
adjusted. Despite its major deficiency, the lack of memory management hardware, the transpu 
ter family is suitable to form the basis for a photogrammetric workstation.