understand. Nevertheless some progress reported
earlier has pointed to the possibility of at least semi-
automated processes reaching the production arena.
First, | will very briefly discuss the current state of
development of digital map revision at the Ordnance
Survey of Great Britain (OS), arguably the most
experienced organisation in this field worldwide. For
large areas of change on the ground, the analytical
plotter with superimposition of both the corrected aerial
image and the edit menu for the integral workstation has
provided the most cost-effective revision method. For
smaller areas of change, OS continued to use a mixture
of appropriate field methods, often assisted by aerial
photography. Changes in the management of revision
at OS, towards extremely rapid database update for
change on the ground judged to be of first importance to
users, and a more frequent cycle of revision for all
change (or to confirm the absence of change), have
been even more important than technical developments
over the past four years. However current technical
developments do include both softcopy photogrammetric
revision using digital orthophotos in the office, and a
handheld pen computer (without any photographic
assistance) replacing graphic processes in the field.
Both of these have the potential to shift the economic
balance between photogrammetric and field activity,
thereby also affecting the management of revision.
(Newby, 1994, Vincent & Logan, 1995 and Greenway,
1994). The pen computer could perhaps also ultimately
incorporate a digital image backdrop, thus placing a
digital photogrammetric workstation in the hands of the
field surveyor and blurring historic distinctions between
field and photogrammetric surveys.
A notable collaborative project by OEEPE, a regional
member of ISPRS, during this period, was entitled
‘Updating of complex digital topographic databases”.
This was headed by the Ordnance Survey of Northern
Ireland (OSNI) but involved the participation of up to
eight European nations at various stages. The project
report (Gray (editor), 1995) shows that participants made
some collaborative progress, while confirming the need
for individual local approaches integrated within existing
data structures. The project also emphasised the
retention of historic data, whether through the treatment
of time as a fourth dimension in databases or by
accumulating “change only” data, rather than retaining
only the current version of reality on the ground.
Moving now to the quest for automation, one of the less
demanding tasks in image understanding is to recognise
and follow linear features such as roads. Successful
demonstrations have been given using both SPOT and
aerial image data. (See for example de Gunst and
Lemmens, 1991, Solberg, 1992, Sakoda, 1993, Plietker,
1994 and Peled, 1994). Comparison of a new image
with the old network allows the latter to be updated.
Peled outlined a plausible scenario for future progress,
beginning with a semi-automatic process of subtraction
of old and new images followed by noise removal, to
supply the human reviser with candidate areas for his
attention. Next, in what he calls “GIS-driven updating”,
the above process would provide the trigger for
automated recognition of new detail and its extraction in
a hierarchy of themes. Finally, autonomous rule-based
602
Al systems may largely take over from the human
operator. As in most aspects of image understanding, it
seems unlikely that any one algorithmic approach will
yield the required results, but that a combination of
approaches, mimicking the human's intuitive combined
tactics, may eventually be successful. Similar efforts
have been reported in the detection and extraction of
buildings, notably in our own Commission proceedings
by Murakami and Welch (1992) as well as by Fórstner,
McKeown and others.
Thus far | have not distinguished explicitly between two
and three dimensional databases; we may assume that
our map is an attempt at a model of a three-dimensional
world although until recently most GIS developers have
preferred to treat it as two-dimensional. Certainly the
topological structure becomes very much more complex
if the third dimension is allowed to intrude.
Theoreticians as well as practitioners have now started to
wrestle with the question of whether time ought to be
treated as a fourth dimension in a GIS or whether update
merely generates a succession of states of the database
(which may or may not be stored for posterity). In my
opinion we should retain the latter view. This makes it
easier for us to consider practical matters such as
change on the ground which has not yet reached the
database, change in the database as a result of improved
data without any corresponding change on the ground,
and the requirements of users in terms of supply of
complete new versions of the database after update or
merely replacement of updated features within the
database. As data structures become more rigorous
and complex, these matters pose formidable problems of
data integrity for both supplier and customer. They are
necessarily addressed in practice by national mapping
organisations, but have also received theoretical
attention from researchers such as Kemppainen (1994).
7. CONCLUSIONS
Map and database revision as practised today makes
intensive use of analytical photogrammetry as well as
appropriate non-photogrammetric methods. Digital or
softcopy tools are starting to be used and will
undoubtedly be adopted increasingly in future. However
they must compete effectively on cost grounds in
satisfying user needs, while being tailored to interact
with existing database structures and practices, for this is
the essence of revision as opposed to original survey. lt
is not just the storage but also the manipulation and
transmission of the huge volumes of data entailed in
digital images which will demand careful attention. The
question of mono- versus stereo-viewing is an important
issue today but in this author's opinion stereoscopy is a
special advantage of photogrammetry which will not
lightly be given up, at least until developments in data
handling and transfer allow the field surveyor to hold a
(monocular) DPWS in his hand, on site.
Database revision has now become a respectable
discipline within the ISPRS commission structure.
Technical and management aspects rank equally as
worthwhile subjects for discussion. There has been
perceptible and promising progress towards the long-
term goals of automated change detection and feature
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
