CIPA 2003 XIX th International Symposium, 30 September-04 October, 2003, Antalya, Turkey
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Figure 7. A 3D view of the TIN, density l-2cm
The next stage included orientation of all stereopairs and DTM
extraction at the SSK Z/I Imaging digital photogrammetric
workstation. Several attempts were made for performing
triangulation adjustment, absolute or relative, of all the
stereopairs at each part (upper and lower) of the sculpture’s
body. Yet, the fact that the pre-signed control points were
different at each stereopair in addition to the difficulty in
recognising the natural control points from various views, did
not allow estimation of the parameters with sufficient accuracy
(rms of resulted coordinates was less than 8mm). So, each one
of the 22 stereopairs was oriented separately. The DTM
extraction was made manually, with point density of 1 -2 cm at
the scale of the sculpture, since the automatic procedure fully
failed in most cases, and a large number of breaklines was
restituted. The outline of the surface was made with irregular
TIN net (Figure 7).
The merging process of all stereopairs was performed through
characteristic tie points or natural control points, taking
advantage of the relatively large overlapping of the stereopairs.
Merging was not always easy to accomplish and finally
systematic errors were introduced, as was confirmed by the
comparison between the unified model with the equivalent
derived through the laser scanning data. Also, despite the
multiple photographic coverage of the sculpture there were
some parts of the object which were not covered
stereoscopically (for example, part of Hermes’ right ear).
4.2 Laser Scanning Data Processing
Editing of the acquired laser data comprises mainly the tasks of
aligning and merging the scans. Alignment is a critical process
to perform in order to bring all the scans of the statue to a
common coordinate system. The registration of all scanned
images was performed by applying the Triangle Mesh
Registration, a variant of the Iterative Closet Point algorithm
(ICP), which does not need targets in order to achieve high
accuracy registrations. During the merging process, integration
of the registered sets of surface measurements into a single 3D
model was earned out using a hybrid approach of mesh
integration with volumetric hole-filling. For these tasks, in-
house software developed by Archaeoptics Ltd. was used
(Tsakiri et al., 2003). Figure 8 shows two 3D views of the
produced merged model of the statue. It is noted that these
models are georeferenced to the same system as the
photogrammetric models.
Figure 8. Typical 3D views of the merged model of the statue
4.3 Combined Use
The common products of digital photogrammetric procedures
are vector (line drawings, DTM) and raster (orthoimages) data
which are produced with high accuracy. On the other hand, the
emergence of laser scanning has benefited cultural heritage
applications in that is a fully automated process, but without
necessarily having directly out the above products. However,
the cloud points form at once a 1:1 scaled 3D geometric model
of the object compared to the initial scaled model produced by
photogrammetry. It is therefore un uncomplicated task to
obtain, in a time-efficient manner, all the products such as
profiles and sections, lines and polylines, etc expected by the
end-users, in addition to solid 3D model production in a variety
of formats (many of them acceptable from CAD environments).
Once the 3D models of objects are developed, the user can
easily perform some basic interpretation using a variety of
freeware viewers (e.g. 3D-Exploration of Right Hemisphere,
SpinFire Professional, etc) which include easy-to-use tools that
allow creation of sections with levels at any inclination and
measurement of distances on the object. Figure 9 gives an
example (not to scale) of such an editing of the Hermes 3D
solid model.
Through the combined use of photogrammetric and scanning
data, the geometry of any kind of object can be fully captured
