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encourages the use of slower and therefore less grainy
films. Meanwhile film manufacturers also claim to have
improved their emulsions. Thus all recent trends have
encouraged photogrammetrists to expect sharper, better
images than ever before. Moreover, for the surprisingly
demanding task of detecting and surveying change while
minimising the need for subsequent field completion,
reducing the scale of photography has not proved to be a
sensible way of exploiting the improved geometric
accuracy of modern photography and measuring
systems. Instead, ease of interpretation has become
the dominant requirement. Throwing all of this away in
exchange for the privilege of viewing lower resolution
pixel-based images on a computer screen requires an act
of faith or a dramatic cost advantage. Nevertheless
Stirling (1995) has reported that users of digital systems
do get used to the comparatively fuzzy screen images
and that the loss of information is not severe.
This introduces the question of the trade-off between
pixel resolution and data volume which affects the cost of
capture, storage and transmission of data. The
standard was set by the Zeiss/Intergraph PhotoScan,
which allows a smallest pixel size of 7.5 um. This
implies about 600Mb of data per aerial image. Much
discussion of the minimum practical resolution has
ensued, without a firm consensus yet emerging.
However it seems likely that most users will eventually
settle for pixels in the 15 to 25 um range, or some 100Mb
per image. Whether or not the next generation has
difficulty with the storage volumes involved (terabytes per
year for the national mapping of a medium sized
country), processing speeds and transmission rates will
certainly be critical, as will the disciplines of managing
such quantities of data.
Until very recently the idea of replacing the conventional
aerial camera with a digital system in the aircraft seemed
ludicrous. Such sensors have now already flown. IGN
(France) has been seeking collaborators for further
development of their experimental digital camera.
Meanwhile in North America systems are in use for
environmental monitoring and are producing images
which show some potential even for urban map revision
(Monday et al, 1994). At the same time several long-
heralded commercial satellite-borne high-resolution
imaging systems may be launched in 1997 and will then
yield results for discussion at the next round of ISPRS
symposia. With planned pixel sizes of one to two
metres on the ground these will certainly be of interest for
small scale map revision. They will surely not replace
airborne imagery for the update of large scale databases,
although some contribution to automated change
detection, even for large scale work, may be foreseen.
5. HARDWARE AND SOFTWARE FOR DPWS
At its most basic level the DPWS has monocular viewing
only, of a single image whose geometry is corrected by a
good DTM. This is the solution now being adopted by
several respected organisations. However, any
monocular system deliberately discards one of the most
crucial benefits of photogrammetric surveying, namely
stereoscopic viewing. Of course this serious
disadvantage may be outweighed by the obvious
601
simplicity and economy of monoplotting systems. If not,
it will be necessary to wait for the hand-held field
computer to combine monocular photogrammetry with
the surveyor's eye view at ground level before the loss of
stereoscopy will be tolerated by most photogrammetrists.
At the next level of present-day development, with the
addition of stereoscopic viewing, the DPWS replicates
the functionality of an analytical plotter. Like the old
universal analogue machines, in its most advanced form
it will be able to be used for all known photogrammetric
processes. All DPWS will have the facility of viewing the
existing vector or raster map data, together with any new
data captured from the current aerial image. Many
processes will eventually be automated.
For revision purposes its capture and edit system must
integrate easily with the existing database or GIS. This
integration points to a major advantage of digital
photogrammetry in terms of quality management : real
world geometry will always be maintained, or at least
departures from it will be immediately apparent.
Along the way the DPWS will be able to generate DTMs
automatically and will allow them to be edited in a user-
friendly way. From the DTM orthophotos will in turn be
generated automatically; for them to be of serious use in
urban areas for map revision or indeed any other
purpose it is essential that they should show the tops, as
well as the bases, of buildings in their correct planimetric
positions. ^ This obvious requirement had long been
overlooked by researchers and system vendors, so it
seemed a remarkable breakthrough when Meister & Dan
(1994) described just such a process during the last
Commission IV symposium. Within months at least one
leading vendor (Leica) was including a similar procedure
in a DPWS although the onus remains on the user to
capture the necessary three dimensional building model
manually in order to exploit this capability of the
orthophoto software. ^ Now, everyone is tackling this
problem vigorously and it will soon seem incredible that it
was ignored for so long. Once this "building lean" is
eliminated automatically, the floodgates will open for the
acceptance of orthophotos for urban mapping. Since
currency is also crucial in the urban environment, this
will immediately affect both expectations and methods for
urban database update, perhaps ultimately even ending
the dominance of the line map as the preferred portrayal
of urban areas.
6. PROGRESS AND PROMISED ADVANCES IN MAP
AND DATABASE REVISION
In this section | will review progress in map and database
revision reported earlier in this four year session of
ISPRS. | hope that speakers at this Congress will
present us with real progress towards the goal of
automation, which will allow this increasingly cost-
conscious world to contain the cost of database
maintenance in the future. Meanwhile almost all of the
processes used in practical map revision today still rely
on the human operator. The excellence of the human
eye and brain, or what soldiers call “the Mark 1 eyeball”,
at tasks of image understanding and pattern recognition
is even harder to reproduce in a computer than it is to
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
