varying brightness and contrast or switching between stereo
models. The Intergraph device operates on a digitising tablet
cursor developed for the P3 AP. DAT/EM, too, offer such a
device. Matra brought the distinctive roller ball and
thumbwheel controls of the Traster 77 to the T10 and Leica-
Helava have simply utilised the hand controller from the
SD2000/3000 line but more recently have switched to a newer
device based on mouse technology, very similar to that offered
for some time by Vision International. Helava have had access
to studies on control devices carried out for military purposes
and, although some of the military systems incorporate
specially built force sticks, the standard offering by Leica-
Helava is mouse for Xy and a simple trackball for z! Most
vendors offer hand wheels and foot disk as an option, though
only ISM has displayed these devices frequently.
It would not be difficult to demonstrate that the overall
working environment of DPWs has been the subject of less
thought than in the case of APs, mainly because of longevity
and because the latter are heavier and have a rather inflexible
relationship between the operator and the oculars. But many
vendors do offer adjustable computer tables, usually with
either a raised area or moving platform(s) to accommodate the
big monitor(s) in the position(s) preferred by the operator.
Many of these tables can be moved vertically by means of
mechanisms on the legs and further degrees of freedom are
incorporated in the chairs.
2.5 Networking aspects
The large data volumes mean that data transfer is just as
critical an issue as data storage. On the scanners, for example,
it is usually faster to scan to local disk than to a disk on the
destination DPW. Similarly, DPWs can use file servers as big
data repositories, but local disk access is faster. Technology
exists for RAIDs to be shared by more than one host computer.
In most cases so far, conventional Thin-Wire or 10-BaseT
Ethernet has been used for digital photogrammetric
applications, though the transfer of big digital images delays
most other network traffic. 100-BaseT is available as an option
on most systems and as standard on one or two. A few users
opt for FDDI networking but as yet rather few digital
photogrammetrists have explored ATM.
Since many DPW purchasers have existing company networks
connecting their analogue plotters, APs, printers, plotters, etc.
and since high speed network links are expensive, there has
been a slight trend towards mixed networks, where 10-BaseT
and 100-BaseT links can be mixed through switching hubs.
Often it is necessary to accommodate more than one protocol,
for example a group of Unix DPWs communicating with
TCP/IP may also have to use DECnet protocols to be
understood by an existing in-house network.
Printers and plotters are typically connected by parallel or
serial line to one workstation and accessed by the others
through the network. In one or two cases, such as the FIRE
2240, the device is SCSI-2. The laser raster plotters often have
their own host computer containing the RIP, so this also is on
the network. Naturally it is sensible to select a DPW without
expensive stereoscopic viewing if one station is apt to be
heavily committed to printing or plotting.
2.6 Output devices.
Vector plotters are not unsuitable for DPWs: they are perfectly
good for displaying the vector data Which is all that a number
389
of users extract on DPWs. Similarly, raster devices are
suitable for outputting vector data from either APs or DPWs,
but are essential for image products. These devices range from
inexpensive inkjet plotters from companies like Hewlett-
on film and are even used for proofing. More expensive and
also higher resolution are thermal transfer and dye sublimation
devices like the Tektronix Phaser III and Kodak XL7600,
though these are typically limited to A4 or A3 format. Colour
laser printers and plotters are emerging now from companies
such as Hewlett-Packard and Xerox.
Superb performance is available from three different types of
plotter at the high end of the price range. IRIS's variable inkjet
technology produces superb plots on paper or film and is often
used as a proofer in top end systems. For the production of
masters to be used as the basis of published maps there are
film recorders, which are devices producing continuous tone of
limited format, such as the FIRE 2240 from Cymbolic
Sciences International, from which final documents are made
by means of enlargement and half-toning in a conventional
cartographic process. Alternatively, large format laser raster
plotters are highly sophisticated systems in which the raster
image is numerically convolved with the equivalent of a dot
screen by a custom built hardware and software raster image
processor (RIP) and the result streamed out to the laser
plotting head. These plotters are produced by specialist
companies in the print industry such as Barco Graphics,
Dainippon Screen, Linotype-Hell and Lüscher. Like the film
recorders, these devices are expensive, leading to increasing
use of bureaux rather than in-house services for fine outputs.
3. SOFTWARE
This section commences with an overview of the operation of a
DPW, which enables a proper understanding of the basic
software. It continues with a discussion of each of the main
photogrammetric application areas.
On the software side, a schematised workflow is used as a
yardstick to assess to what extent the different vendors offer
the complete range of digital photogrammetric operations. It is
found that almost all DPWs are capable of emulating
analytical plotters and of generating orthophotos, but fewer
include functions like automated generation of digital terrain
models (DTMs), mosaicking and perspective scenes. Map
finishing and sophisticated project management are even less
common.
Few vendors have written all their software themselves and
imported components tend to recur, just as car manufacturers
use one another's engines to obviate development costs. Two
vendors have purchased small software houses specialising in
packages for map compilation, to enlarge their capabilities in-
house. The capture of data directly into the popular
MicroStation CAD package is offered by most vendors.
3.1 Overview
The fundamental operation of a DPW is rather similar to an
AP. The projective geometry and mathematical models in a
DPW are very similar to those of the AP with object
coordinates primary, a concept explained by Petrie (1990).
The inputs, in other words, from the operator via the control
devices, are construed as XYZ motions in object space and are
transformed into changes in the positions of the cursors on the
individual images. This transformation is performed with the
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
