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New perspectives to save cultural heritage
Altan, M. Orhan

D. Mavromati a , E. Petsa b , G. E. Karras a
a Department of Surveying, National Technical University of Athens, GR-15780 Athens, Greece (gkarras@central.ntua.gr)
b Department of Surveying, Technological Educational Institute of Athens, GR-12210 Athens, Greece (petsa@teiath.gr)
KEY WORDS: Archaeology, DEM/DTM, Laser scanning, Orthoimage, Bundle, Non-Metric
Photogrammetric textured representations of archaeological sites, and above all orthorectification, combine geometric accuracy with
visual detail (regarding damage or decay), thus providing a suitable basis for conservation. Yet, orthoprojection in archaeology still
poses difficult problems. At the examples of two ancient Greek sites, certain issues are discussed and illustrated. First, in most cases
sites need to be recorded from considerable heights above ground with special low-cost camera platforms (balloon, modified fishing-
rod etc.). A usual consequence of ‘unstable’ camera elevators is poor control over image rotations, responsible for irregular strip geo
metry; bundle adjustment is further complicated by unknown interior orientation of lightweight non-metric cameras and strong dis
tortions of wide-angle lenses. A second crucial aspect dealt with here is the authors’ approach for precise surface modeling to ensure
products of both geometric accuracy and high iconic quality; this entails surface description through a careful combination of break
lines and densely sampled spot elevations for handling edges and surface discontinuities. Regarding laser scanning, now being exten
sively tried in the context of archaeology, experiments carried out here confirmed that it could indeed replace tedious photogramme
tric 3D modeling in several cases. However, it is rather clear that laser scanning cannot in fact totally replace photogrammetric mo
deling. This is due not only to problems posed by shape, size, location and surroundings of many archaeological objects, but also to
problems emerging mainly with respect to edges. It is concluded that simple means of image acquisition and careful photogrammetric
handling can produce results of high geometric and visual quality, while tiresome photogrammetric modeling can partly (but some
times cannot) be replaced by laser scanning. Functional synergy of the two approaches is a delicate matter to be further investigated.
Among deliverables required by most archaeological services,
line drawings now face an ever-growing competition from raster
products, notably orthomosaics, which are now a standard pho
togrammetric product. Indeed, these constitute powerful textur
ed representations, combining geometric accuracy with a wealth
of detail, suitable for conservation and restoration planning. But
archaeological orthoimaging has its own peculiar aspects when
compared to its conventional aerial counterpart, as recently out
lined by Mavromati et al. (2002) based on experiences from the
collaboration with the Greek Ministry of Culture.
Summarily put, archaeological sites generally need to be record
ed from above or horizontally using a raised camera. According
to location - densely built areas, sites accessible only on foot -
and often under constraints of poor financial resources, flexible
low-cost camera platforms must be devised to meet a variety of
conditions (Karras et al., 1999; Mavromati et al., 2002). Even if
monitors are adapted to cameras, however, such inherently ‘un
stable’ elevators do not allow full control over image tilts, being
thus responsible for irregular strip/block geometry. The same is
true for imaging distances and resulting scale variations, further
aggravated by large object extensions in depth (these also cause
strong differences in perspective between adjacent images). Fur
thermore, mainly ordinary light non-metric cameras, of small or
medium format, are used in this context. The unknown interior
orientation and considerable distortion of wide-angle lenses,
usually used in these cases, add to the typical difficulties facing
phototriangulation tasks in archaeology. At least to the authors’
experience, archaeological recording under such circumstances
appears as a rather ‘generic’ photogrammetric problem.
Not least, of course, among the peculiarities of archaeological
recording one meets precise 3D surface modeling to ensure geo
metric accuracy and visual quality of results. Not only a prere
quisite for orthoprojection, accurate 3D modeling also provides
useful information on morphology or deformation, being a tool
in its own right in evaluation and restoration processes. Unlike
several architectural items, shape of archaeological objects may
often be very irregular (one is tempted to say ‘arbitrary’), distin
guished by extreme changes in relief, ‘breaks’, ‘ridges’, edges or
discontinuities, a substantial part of which have been caused by
damage. This entails modeling of surface patches almost per
pendicular to each other or strongly protruding structures. Sur
face triangulation, under these circumstances, is a highly crucial
issue. The authors discuss and illustrate their photogrammetric
approach in this respect (see also Mavromati et al., 2002).
Of course, photogrammetric surface point collection is mostly a
tedious manual task, as automatic DSM generation in archaeolo
gy still remains an open question (Baratin et al., 2000). Laser
scanning, on the other hand, is a powerful technology, capable
of collecting vast numbers of surface points in far shorter times,
and can thus provide the 3D support for orthoprojection (Monti
et al., 2002). However, apart from the high cost and the obvious
difficulties (large volume of data, difficult to manage; noise), a
laser scanning approach faces problems of post-processing for
creating triangulated meshes suitable for the existing orthophoto
software (Bohler et al., 2001; Balletti & Guerra, 2002); perhaps
the question of surface discontinuities is, among these, the most
important (e.g. Boccardo & Comoglio, 2000). And besides, not
every archaeological site is accessible to laser scanners as it may
be to photography.
The aspects of archaeological orthoimaging, referred to above,
are discussed and illustrated at the examples of two Greek sites.
As also outlined in Mavromati et al. (2002), this is a key issue
in archaeological orthoimaging, irrespective of the mode of 3D
modeling. For reasons mentioned above, recovering reliable ex
terior orientation parameters may well not be a trivial task. Re
garding interior orientation, full self-calibrating bundle adjust-