241 
ETRY 
Clearly the demands on hardware vary with the operation being carried out and whilst it is clear that 
a single universal type instrument of the Kern DSR type will satisfy most of these requirements it 
techniques, 
DPW to be 
; hardware, 
use and at 
carried out. 
instrument 
should be asked whether the market may not be best served by a number of specialised instruments 
each optimised to carry out a certain task. Four tasks may be readily identified and are shown with 
their requirements in Table 2. 
Table 2. The task required of Digital photogrammetric Workstations. 
mance. 
OPERATION REQUIREMENTS 
CEMENT 
ORIENTATION AND POINT MEASURMENT Mono/Stereo display of points of 
interest 
lay of 
ial area. 
MAP COMPILATION Stereo display with continuous scroll 
nse 
MAP REVISION Mono display if DEM available 
Stereo display if no DEM 
Ly of 
positions 
VALIDATION WORKSTATION FOR Stereo display for checking contours, 
AUTOMATIC PROCESSING control points etc, mono vector 
display for perspective views 
nse 
The market seems to be already responding to this requirement, for example the Helava Digital 
Comparator Correlator System (DCCS) is designed to match and measure image co-ordinates of 
pass points for triangulation and does not need stereo viewing and the images are controlled in 
sing 
image space. Close range systems designed to determine object space co-ordinates of well 
defined points do not need stereo viewing either. 
d real 
PC systems are being developed which provide a subset of requirements at a reasonable cost. 
Welch (1989) describes desktop mapping with a PC, and a microcomputer based general 
photogrammetric system is described by Gagnon et al (1990). These systems do not always have a 
rigorous model and work only on small windows. The Topcon stereo image workstation, together 
with an image scanner provide a complete system but is also limited by the size of display. 
The main difference between the two types of general system is the size of image which can be 
manipulated and the cost. A less expensive instrument can be developed is only small images are 
used and the operator moves about the image rather than the image scrolling. The requirement for 
continuous scroll is still a debateable point: what delays will the operator tolerate in order to obtain 
a cheaper instrument? This point is discussed in more detail below. 
System architecture and performance 
f image 
y 
Gruen (1989) identified three different approaches to design of a DPW: genetic, refering to 
custom built systems; turn-key and modular. These categories are still relevant although it is 
useful to subdivide the modular category according to application and power and to consider in 
more detail the add-on possibilities in all systems. 
f image 
y- 
ind edit. 
An important guide to the power of a system is speed and functionality. Systems with reduced 
functionality have been discussed above and are generally lower in cost, systems such as the Kern 
DSP1 have high performance on both counts and this is achieved by dedicated hardware and the 
use of transputers to give speed for operations such as image matching. A summary of the existing 
sytems is given in table 3.