Whereas expensive, custom built solutions had been very 
acceptable in the military systems of the 1980s, there has been 
a shift towards more modestly priced hardware and, indeed, 
the graphics engines offered by workstation manufacturers in 
their standard configurations have become sufficient. Such 
devices are required for stereoscopic viewing, unless split 
screen is used, because the graphics cards supplied as standard 
with most workstations either have too slow refresh rates (over 
100 Hz is required) to display the left and right images 
alternately on the screen in fast enough succession to please 
the human eye, or because there is no convenient way of 
handling two images. Helava and Intergraph led the way with 
VITec-30 and VITec-50 processors and boards from companies 
like 3Dlabs, Peritek and Teksource have been used too. Note 
that while some of these processors have on board VRAM, 
others make use of imagery held in main system RAM. So in 
the case of the Creator 3D graphics of the Sun Ultra, for 
example, Leica-Helava recommend at least 128 MB RAM. 
These graphics sub-systems also help with image roaming, 
though skilful use of their firmware is required to achieve 
smooth motion at sufficient refresh rates. In many cases, 
image roam is still achieved only piecewise, i.e. within the 
imagery stored in the VRAM or the system RAM allocated to 
graphics. Smooth roaming over an entire stereo model with 
heavy graphics overlays of vector data or contours has barely 
been achieved with off the shelf hardware. Naturally, the 
fastest transfer of data is critical between VRAM, RAM and 
hard disk. It is interesting that Sun has moved off its long 
established SBus for the graphics of the higher end Ultra 
models and SGI's reputation for fast graphics has been founded 
to some extent in its excellent use of the PCI bus. While it is 
not unreasonable for users to demand the same smooth 
roaming all over the image that they have enjoyed for two 
decades with APs, it is worth noting that this is a real issue 
only with feature extraction - in most other digital 
photogrammetric operations it is of little significance. Many 
operators, too, having tried the DPW roaming and found it to 
be inferior to APs, change to fixed image moving cursor mode 
and become quite content. 
Special hardware has also been used for compression. The 
JPEG board offered as an option by Intergraph, for example, 
has been very successful, though the latest workstations can 
perform software compression at more than one MB/sec and 
are fast enough for decompression on the fly during processing. 
2.2.2 Storage and archiving. It has already been stated that 
the large data volumes make heavy demands on storage. The 
host computers are usually supplied with at least one internal 
hard disk of 1-2 GB and, usually, one or more external hard 
disks of 2-9 GB each. Thus many users take delivery of 
systems with 5-10 GB per workstation and in many cases 
expand on this soon after. In most cases the SCSI-2 standard is 
preferred but fast and wide SCSI-2, offering 20 MB/sec, i.e. 
twice as fast, is available as an option on most workstations 
and is standard on some models of the Sun Ultra. Power users 
sometimes opt for RAID technology, not so much to take 
advantage of its data security features but to obtain high speed 
and very large capacity with a single SCSI-2 device. RAIDs of 
10-60 GB are quite common. 
The data volumes preclude the use of floppy disks, even 
modem high capacity models, for other than control files, 
orientation results or vector data. QIC tapes, with capacities 
typically below one GB, are also rather unsuitable. Despite 
capacities currently of around 650 MB, CDs have become 
quite popular, especially as a means for users of DPWs to 
supply final deliverables to their customers. CD juke boxes 
ameliorate the problems, but 650 MB is sufficient to contain 
387 
only one stereo pair of 12.5 um images. Optical disks, which 
are currently increasing from 1.3 to 2.6 GB, have rather more 
potential than CDs, but appear to be handicapped by a 
multiplicity of standards and formats. Old fashioned reel to 
reel tape drives are becoming less common but still find use as 
input devices for remotely sensed data, of which they were the 
standard distribution medium for many years. 
The industry seems to have settled for the moment on high 
capacity tape cartridge technology for off-line storage and 
archiving. The popular DAT and Exabyte equipment, for 
example, uses helical scan technology for writing and reading 
the data. Exabyte capacities are 7 -14 GB per tape depending 
on the effectiveness of in-built compression firmware. More 
recently, vendors such as Sun have announced even higher 
performance DLT (digital linear tape) units with 20 GB or 
more per tape. In most cases juke boxes are available, but it 
must not be forgotten that back up and restore are lengthy 
operations: at one MB/sec peak rate, for example, it may take 
several hours to read one tape. 
2.3 Viewing systems 
2.3.1 Monoscopic or stereoscopic? Photogrammetrists have 
been preoccupied with stereoscopic viewing for decades and 
this has been a big issue with DPWs too, but it is necessary to 
pause to review the situation and discern the directions for the 
near future. Many operations in digital photogrammetry are 
not stereoscopic at all and in this respect DPWs differ from 
APs: project management, image import, automated aspects of 
triangulation, DTM generation, orthophoto computation and 
mosaicking, perspective scenes and image map production are 
all entirely possible monoscopically. Only three operations are 
inherently stereoscopic: (i) measurement of ground control 
points, where a single monoscopic measurement followed by 
image matching for transfer to all the other images may be 
satisfactory for targetted points but is likely to be a source of 
error in all other cases; (ii) editing of automatically generated 
DTMs; and (iii) feature extraction. The last two of these, 
however, will become increasingly automated, as the 
discussions in sections 3.4 and 3.6 below conclude. Moreover, 
the trend towards head-up digitising on digital orthophotos 
seems to be a strong one. Thus stereoscopic viewing will be 
needed on a decreasing proportion of the DPWSs at each site. 
2.3.2 Monitors. Whether there is stereoscopic viewing or not, 
almost all DPWs have at least one monitor on which several 
windows appear, one or more of them containing imagery. In 
some cases this imagery may be 24 bit colour, which is best 
displayed in 24 bit colour. Thus a high quality monitor with 
high resolution is mandatory. The use of 640 x 480 VGA 
displays is a thing of the past! Probably 1280 x 1024 is the 
most popular resolution, but values around 800 x 600 are 
sometimes used with entry level systems and Intergraph 
showed the way with its “two megapixel" screen used from the 
early days of the ImageStation at the beginning of the decade. 
The systems which include a second monitor for the stereo 
display almost invariably offer a high quality 1280 x 1024 
unit, but note that the number of “stereo pixels” generated by 
the graphics-subsystems is smaller than this. The Leica-Helava 
systems, for example, have offered a range from 640 x 480 on 
VITec-30 and 640 x 492 on SGI XL to 960 x 680 on Sun ZX 
and Creator 3D and probably 1024 x 768 on SGI High Impact. 
The floating mark is created by using of a cluster of pixels to 
form a cross, circle or other preferred pattern. This moves, of 
course, in single pixel increments, which can be a little 
disturbing in that to an experienced AP operator movement 
may appear to be jerky or insufficiently precise, though for 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996