242 
Table 3. A summary of digital photogrammetric workstations. 
SYSTEM 
HOST 
COMPUTER 
DEDICATED 
HARDWARE 
IMAGE SIZE 
Kern DSP1 
Vax 
GEMs 
256 Mb 
Traster T10N 
Array processors 
2048 x 2048 
Teragon ContextMapper 
Sun 
GOP 
96 Mb 
DMS (Welch)) 
PC 
- 
1024 X 1024 
Videomap (Gagnon) 
PC 
- 
? 
Topcon 
PC 
- 
? 
PC based systems are generally less expensive but much more restricted. For example the 
Desktop Mapping System described by Welch (1989) appears to be limited to images of 1024 x 
1024 and not to use a rigorous three dimensional model, but within these constraints can carry out a 
range of mapping tasks. Systems for close range photogrammetry can also use PCs, for example 
the near real time system described by Ruther and Parkin (1990) can detect and measure targets 
imaged by two CCD cameras. The components of these systems will be refered to later. 
By using a more powerful workstation such as a Sun SPARC Station the performance of a DPW 
can be significantly increased to approach that of a system such as the DSP. Table 4 shows the 
characteristics of a number of PCs and Sun Workstations. 
Table 4. Characteristics of PC and Sun workstations 
SYSTEM 
SPEED 
RAM 
DISC 
PROCESSOR 
SOURCE 
IBM PS/2 60 
lOMhz 
1Mb 
40Mb 
80286 
Gagnon 
Welch 
IBM PS/2 30 
8MHz 
8086 
Ruther 
Compaq 386-20, 386-25 
Dell 310, 325 
20-25MHZ 
80386 
Sun SPARC station SLC 
20Hz 
16Mb 
1Gb 
CMOS SPARC 
Sun 
The critical requirement of a DPW is for real time scrolling over the whole of a stereo image. 
Image scanners are now available which will digitise an aerial photograph to 10p.m and thus 
produce 0.5 Gb per image. A stereo pair digistised at that level with 60% overlap would require a 
similar amount of storage. A panchromatic SPOT pair requires 2 x 36Mb of storage. Storage on 
hard disk of this size is not a problem although it should be noted that a single stereo pair may need 
almost 1GB and that several gigabytes would be needed to store several images and relevant data 
sets. A RAM of 4 or 8Mb is now commonly available but one 1000 x 500 pixel image needs 
0.5Mb and a stereo pair 1Mb so that the images fill the screen To allow for continuous scrolling 2 
images of 1500 x 1000 pixels 3Mb will be needed and additional data must be brought in from the 
hard disk as required. To avoid this the alternative use of a fixed image and the ability to bring in 
adjacent images on command can be used.