bicubic resampling suit most requirements. Typically,
orthophotos can be generated in a few seconds per MB. One or
two vendors have added "true orthophotos", in which the
effects of building lean are rigorously removed provided that
the building tops are measured and the bottoms lie on the
DTM. DPWs can easily generate stereomates, but this seems
less important today than it was in the 1960s. Indeed, there
seems to be a gathering force towards simple head-up
digitising from digital orthophotos as an alternative to
conventional stereoscopic methods of feature extraction: the
digital monoplotter of the 1970s and 1980s is undergoing a
revival, though the orthophoto, with or without DTM, appears
to have replaced the raw image and DTM of former times.
While many users attempt the acquisition of pinpoint
photography in order to make orthophotos from single aerial
exposures and thus avoid mosaicking - a reasonable task now
that in-flight GPS is with us, most systems have mosaicking.
While some systems join orthophotos together with
radiometric and geometric feathering, others treat a mosaic as
nothing more than a single orthophoto produced from a
collection of input images, a quite logical approach. In both
cases, however, user definition of seamlines may be necessary
to avoid the unpleasant effects of seamlines going through
buildings.
The addition of marginalia and graphics overlays to the mosaic
to form an image map is provided in many DPW and remote
sensing packages. Some are quite sophisticated, but others are
quite simple, to the extent that a market niche has been
identified for map finishing software: for example, the
Canadian company 2+1, a partner company of PCI, has moved
smartly to fill this gap with its ACE product. Often the
software accompanying high end plotters provides this
functionality also, for example Mercator from Barco.
3.6 Feature extraction and GIS
In practice the last four years have witnessed considerable
activity to make available on DPWs the same software
packages for feature collection that have served their users
well on analogue and analytical workstations. This has
included, of course, DPW vendors porting their own software
to the new workstations. The process was facilitated by two
purchases of software houses: Vision International bought
KORK and Zeiss brought DDI, the producer of CADMAP.
Thus Leica-Helava, for example, offer ATLAS, CADMAP,
DAT/EM and PRO600, Zeiss offer PHOCUS and CADMAP,
Vision offer KORK and DAT/EM, etc. Led by Intergraph,
almost all vendors offer on-line data collection into Bentley
Systems MicroStation.
Arguably, the software for feature collection is not as refined
as on the earlier generation of workstations. The software
choice outlined in the previous paragraph is not as wide as
before. The implementations have not had as many labour
years devoted to them. This situation, of course, is improving
as we write.
There has been a trend, which DPWs have experienced in
common with the previous generation of workstations, towards
the generation of digital data rather than the production of
aesthetically sophisticated maps. Thus many of the most
popular data collection packages are interfaces to CAD
packages, especially MicroStation, which is the on-line
software offered by most of the vendors. GIS packages are less
common and probably GeoCity, PHOCUS and System 9 are
the only ones seen on-line on DPWs.
391
But perhaps the greatest disappointment in digital
photogrammetry is that as yet few DPWs offer any significant
automation in feature extraction, notwithstanding the fact that
this area has been one of the most intense research areas for
decades. True, a valuable result of this Work is knowledge
based extraction of attribute information from imagery, for
example many workers have successful identified most or all
buildings in their sample imagery. But the rationale behind
this work has not been measurement and, arguably, more
useful to the practising photogrammetrist are tools which can
rapidly and accurately measure features identified by the
human operator as requiring to be mapped.
Many vendors are working on this problem. The DPWs sold
by Leica are benefitting from much work being done by
Helava and its parent company, GDE Systems, some of it
originating in US military projects. While attribute extraction
has received much attention, work has also been done on edge
extraction, classifiers, Z finding, snakes, region growers,
squaring, splining, thinning, blunder detection and other
methods. The result is that the product sold in the commercial
market-place now includes tools for building measurement,
feature delineation and area measurement. The first works
from fast, approximate measurement of building corners by the
operator, finds the edges of the roof, computes the corners and
squares the building. Tests have confirmed beyond reasonable
doubt that the use of this tool allows two to four times as
many buildings to be extracted as completely manual
measurement, with a significant reduction in fatigue. The
second tool permits the operator to position the floating mark
at a few points near a linear feature such as a road edge; the
edge is then determined accurately along its length. The third
tool requires the operator to place a seed in the area to be
measured, for example a lake, and two seeds outside; it then
finds the area and delineates it. In all cases the philosophy is
"refinement", ie. improvement by image processing of
information input by the operator. Productivity is further
enhanced by setting defaults, i.e. for each feature code pre-
allocating the most appropriate measurement tool. The next
stage will be to develop edit tools to overcome the most
common errors in the automated processes. Vendors have a
wealth of research work available for productisation.
3.7 Perspective scenes and simulation
The generation of perspective scenes or “birds eye views”
either singly or in animated sequences as “walk throughs” or
“fly throughs” has created considerable interest. Most vendors
have this function and it has proved especially popular with the
remote sensing software houses, for example ER Mapper and
the PCI FLY product, provided that DTMs are available. Earth
Resource Mapping and VirtuoZo have exploited the hardware
strength and geometric engine of the SGI Indigo? workstations
to enable the operator to change the viewpoint in real time.
The GDE Systems RapidScene product achieves the same
through the use of powerful add-ons by Evans and Sutherland
to a high end SPARCstation. In many cases the final products
can be monoscopic or stereoscopic, though most vendors do
not yet have automated flight path creation to enable the
stereoscopic pairs of views to be created automatically. While
the applications of perspective scenes are legion in the remote
sensing area, where the draping of the imagery on the DTM
offers a most informative tool for interpretative purposes, thus
far it has been used rather less in what may be considered to
be its main non-military home - planning. One
counterexample is given by Simmons (1996), where it has
been used in a planning enquiry associated with the UK rail
link from London to the Channel Tunnel.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
