scale database, stored on an on-line 300 GB disk system,
is also interfaced to OSI’s Sysdeco TELLUS publishing
system, whereby graphics at any scale froin 1:500 to
1:10,000 may be provided in sheet-free form on customer
demand on electrostatic plotters or digital media.
2.3 The decision for digital
More recently, decisions were made to make major
procurements of digital photogrammetric systems,
initially for the production of digital terrain models
(DTMs), then for orthophotos and finally for vector data
capture for large and medium scale mapping. These
decisions followed from a 15 month evaluation begun in
1992, which concluded in favour of digital rather than
analytical photogrammetry on grounds of. value, in
terms of output per unit cost; stereo superimposition as a
standard feature; orthophotography as a standard by-
product; easier training and supervision; more flexible
work planning; generation of DTMs and contours. The
negative aspects were unit cost of digital
photogrammetric workstations (DPWs) and the cost of
scanners, unless each served a considerable number of
DPWs. The digital systems were to coexist with the
analogue and analytical workstations. The procurement
processes were rigorous, with a period of systematic
assessment followed by bench-marking a short list, and
the selected vendor was Leica-Helava.
3.0 DIGITAL PHOTOGRAMMETRY
The situation until autumn 1995 has been described by
Miller et al. (1995) and Kirwan (1995). There have been
three procurements and a fourth one is imminent.
3.1 Phase one
Although not the first national mapping organisation to
select Leica-Helava systems - Newby (1991) and Han
(1992) give counter examples - OSI invested on a large
scale. The first procurement, in 1993, consisted of a
DSW 100 scanner, equipped for colour and black and
white imagery and running on an 80486 PC, and four
DPWs on SPARCstation 10s with Helava's SOCET
SET software. One of the DPWs was monoscopic, for
project management and to play a server role for the
three stereoscopic units. The systems were connected
with Ethernet using TCP/IP protocols and one of the
DPWs was also connected to OSI’s main network with
its DECnet protocols, using DECnet Interface hardware
and software in the monoscopic workstation.
The foremost application was the generation of DTMs
from 1:10,000 and 1:30,000 black and white
photography for the large and small scale databases,
which were specified to include 1 and 10 metre contours
respectively. It was found that at the larger scale the
DTMs were distorted in places by the effects of features
like hedges, dykes and isolated trees. To some extent this
could be ameliorated by tuning the DTM “strategies”
(Miller and Devenecia, 1992), but the response of the
vendor was to add editing tools capable, for example, of
“bulldozing” the DTM within a corridor around a linear
feature and making the terrain in the corridor blend
smoothly into the surrounding fields. Another solution
was a batch process whereby vector data previously
captured on the analytical plotters could be imported
from the Sysdeco database and used in editing. The
926
Helava SOCET SET contains a rich variety of DTM
editing tools, but at both scales a major stumblingblock,
addressed by additional training, was the tendency for
operators to continue editing until the DTMs were
almost “perfect”, i.e. far superior to the underlying
specifications. Dense DTMs of 150,000-250,000 points
per model are merged across boundaries of 6-10
stereomodels to provide large DTMs of 900,000-
1,600,000 points for final contours. ATLAS is used to
measure some additional spot heights manually.
The other challenge which emerged was workflow
management. OSI was well versed in deployment of
analytical workstations for feature collection, but the new
system was primarily used for DTM generation and also
incorporated a scanning phase. Thus management was
partly a matter of applying existing expertise and partly
of ramping up production as skills in optimising the
output of the new technology accrued. One operator, for
example, was allocated the tasks of scanning and routing
the image data to the destination workstations, setting up
each model and initiating the automated DTM
generation, ready for the longer editing phase. This was
done with the DPW670 and the model set up achieved
with the image coordinate files associated with the DSR1
models input to PAT-MR, together with the results of the
PAT-MR adjustment. After editing, this same operator
would export the final contours to the SysScan database,
after translation into SysScan format.
3.2 Phase two
Several national mapping organisations made initial
procurements of Leica-Helava systems (Armenakis, Dow
and Regan, 1995; Collignon, 1995; Ducloux, 1996; El-
Kady ef al., 1994; Johansson et al., 1995), but OSI went
further. Towards the end of 1993 an additional
stereoscopic workstation was added for the evaluation of
DPWs for collection of vector data in comparison to
analytical workstations. The results of this analysis were
inconclusive, but the need for additional capacity
together with the possibilities of hand controllers the
same as the Leica SD2000 analytical workstation and the
same ATLAS mapping package as installed on most of
the analytical and analogue workstations swung the
balance in favour of a second procurement of digital
systems. The reasons were: similar unit cost to analytical
plotters; OSI’s need for superimposition in the future;
orthophotos seen as a bonus; DTMs and contours; digital
photogrammetry, despite its shortfalls for map
compilation, was the area where future developments
were most likely, given that analytical plotters were
reaching the end of their product cycle; finally, a big
investment should be seen to be made in new technology.
The purchase consisted of a second, much faster
DSW200 scanner and six DPW770s. The system now
totalled two scanners, one DPW670 monoscopic
workstation and nine DPW770 stereoscopic
workstations. The latter were deployed on both the DTM
work and on feature collection. The systems were
supplied with SPARCstation 20 hosts and the DSW200
scans a black and white photograph at 25 um in six
minutes, thus supplying all DPWs with enough data. The
DPW770s were also equipped with Leica hand
controllers and ATLAS, resulting in a familiar,
comfortable, productive environment for map
compilation. An overview of the OSI system at the end of
this second procurement is given in Figure 1.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
