4. CONCLUSIONS
4.1 How well have we done?
Any assessment of the success of DPWs must focus on the
issue of automation, which follows logically from the use of
digital images and thus must ultimately distinguish DPWs
from earlier generations of photogrammetric instrumentation.
The extent to which automation has pervaded DPWs is
disappointing. “Easy” automation such as the generation of
orthophotos, of course, is omnipresent. Many DPWs have
some level of automation in their orientation, but relatively
few offer highly automated triangulation. DTMs have been a
research challenge for decades and not all vendors have
developed in-house solutions. Mosaicking, too, has proved
intractable in some respects, but the most remarkable aspect is
tools for automated feature extraction - the huge lag between
achievements in research laboratories and what vendors judge
to be robust enough for the market-place.
4.1.1 Digital versus analytical. Many discussions, as we
have hinted above, tend to focus on the rather inappropriate
debate of the virtues of digital against analytical. Nevertheless,
many of the factors they adduce are well worth noting. The
strengths of digital, for example, include the following:
e more functions and more products
® no repetition of interior or exterior orientation
* models ready to use immediately on completion of
triangulation
e no calibration or wear and tear, i.e. all the expensive, high
precision, mechanical components of the AP are
eliminated in the DPW, as are the measuring components
such as encoders; also, in most cases there are no optical
components of the type found in APs
* development benefits from advances in computer
technology
* easy hardware maintenance using computer
manufacturers’ service networks
colour, stereo superimposition without additional cost
* Scope for extensive automation in interior orientation,
triangulation, DTMs, orthophotos, mosaics and feature
extraction
* endto end workflow, giving greater simplicity.
The demerits include:
additional cost of scanner; still significant if one scanner
feeds several DPWs or if a bureau service is used
* poorer image quality than that traditionally provided by
film imagery and optical trains
* less smooth image roaming than APs, despite expensive
graphics sub-systems
ergonomics less well thought out
narrower range of proprietary and third party packages for
feature extraction and GIS
very large data volumes
* greater computer literacy required on the part of the
operator
* more complex software, less easy to use, than APs
* lack of software for project management (APs do not have
this either, but the need is less pressing)
* lack of standards, especially for image formats and quality
control.
Dowman (1996) and Nwosu (1996) retain some doubts on the
efficacy of DPWs vis a vis APs and the debate goes on. Two of
their arguments are centred on the high cost of
photogrammetric scanners and the lack of automation for
feature extraction. The cost issue is complex, however,
because DPWs ought to be compared with APs with colour,
392
stereoscopic superimposition and the cost of the scanner
should be distributed if it serves several DPWs or if a bureau
service is used.
4.1.2 Trends in the market-place. Bearing the above in
mind, let us consider briefly what has actually happened. After
the 1992 Congress there was a hiatus, as we commented
earlier. Perhaps the aggressive, pioneering stance of vendors at
the 1992 Congress in Washington, D.C., caused prospective
customers to delay and contemplate deeply on their choice of
technology. The first result was a lull in all sales during this
pause for thought. The second was that many then elected to
be conservative and choose analytical plotters, sales of which
have remained buoyant to his day! Since about 1994, however,
numerous agencies have taken the plunge into digital
photogrammetry. It must not be our role to compare the
different vendors, but it would hardy be biased to state that in
the workstation world Intergraph, Leica-Helava and
Vision/ERDAS have led the way, alongside ISM on the PC.
The overall scene is heterogeneous, with adoption not only
geographically disparate but also spread through various
market segments. Big internationally aided projects have been
one source of early adopters, but numerous national and
provincial mapping organisations have been quite adventurous.
The private sector has adopted the new technology with
enthusiasm too and a spate of advertisements and published
papers indicates how many firms of all shapes and sizes plan
to eam significant slices of their livings from digital
orthophoto work, both as final image map products and for
populating raster layers of their clients" GIS systems.
It is risky to introduce commercial, business issues into a
scientific paper, but these have been important in the last four
years of DPWs. Marketing clout, for example, may have been
just as important as technical expertise in the success of some
products. Worldwide presence, heavy advertising, participation
in trade shows, high profiles in conferences, introductory
offers, installation of evaluation systems at nominal cost,
resources to participate in complex procurement exercises,
etc., have been real influences on development. Vendors have
a built in advantage on sites where their own equipment is
already installed, benefitting those with a history of analogue
or analytical workstations, analytical upgrades, software for
map compilation, etc.
4.1.3 Users. The behavour of the users is another source of
insight into the evolution of DPWs. The majority of adopters,
of course, have mixed systems consisting of analogue,
analytical and digital workstations. A few organisations have
jumped directly from analogue to digital, by-passing APs
altogether.
Most users still scan film aerial photography in
photogrammetric scanners. The majority possess an in-house
scanner, sometimes from the same vendor as their DPW, yet
many use one of the increasing number of scanning bureaux
companies which have sprung up, especially in North America,
often by expanding existing business in photo processing.
These firms often scan round the clock, gaining extensive
experience and felicity with the scanner hardware and
software, to offer a satisfactory, cost effective service. Most
users produce DTMs, orthophotos, mosaics and image maps,
often employing desk top publishing software like PhotoShop
for map finishing. A lesser number use DPWs heavily for
feature extraction. While all users have done some
triangulation on DPWs for years, the appearance of the
automated triangulation packages noted in section 3.3 above
has spurred interest in exploiting automation for quite large
blocks, despite the huge storage demands and the fairly
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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