CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
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Figure 1. Matching of topographic data (red points) and photogrammetric restitutions
demolitions, opening of traces, etc. - which are recognizable
because constituted of homogeneous materials or processing.
For the most significant USM, material type, processing, mortar
type and stratigraphic relation were defined. As every USM has
a stratigraphic position that defines synchronic or diachronic
relation with the other, it was possible to assign, at first, relative
dating among the USM and then absolute dating (still to complete)
comparing data of the building with historic documentation. It is
possible to summarize five most important constructive phases and
relative wall types; a complete report was draw up for everyone.
Normally, the USM are drawn on 2D geometric survey or, if it is
possible, on image rectification, or, even better, on orthoimage but
the relations among them, in complex cases like this one, is difficult
to represent. A 3D model can be a real evolution in this analysis to
merge reconstruction of consecutive stratigraphic layers.
3.1 Geometric survey and 3D modeling
Survey operations were performed during consolidations works.
Thanks to the present scaffolding, it was possible to plan acquiring
data at different heights, focusing on transept-apse complex to
obtain prospects, vertical and plane sections at 1:50 scale. The
articulated geometry, derived from a mixture of architectonic
elements, structural damage and superficial decay, has required
the survey of many points, imposing the management of
numerous detail drafts to choose the representative elements. This
approach combines the data from different sources: topography,
photogrammetry, laser scanner and image processing (either as
image rectification or as surface mapping) this has been integrated
to obtain, finally, a 3D model. Some considerations on the adopted
procedures follow.
2. DATA COLLECTION
Increasing automation of the acquiring and elaboration of data
processing are essential to widen the use of the 3D model, but
despite this, at present, to represent and to elaborate data, is still,
unavoidably, a selective operation: what changes is the moment
in which synthetic interpolation, to obtain structured data, is
required. With every survey technique we obtain a synthetic
model of reality, but different cognitive approaches are available
derived from different measurement techniques: topography and
photogrammetry data collection phase requires a preliminary
interpretation process, while, laser scanner acquisition collects
redundant data, and synthesis is postponed to a post-elaboration
phase.
2.1 Topographic survey
Topographic techniques need graphic restitution, in which the
object has to be defined with line interpolating measured points.
The operator has to be able to interpret, at the moment of survey, the
object of study. We can also say that with few measured points we
can obtain exhaustive form and geometric representation, because
additional geometric information derives from the operator.
About one thousand detail points has been collected with a reflector
less total station Leica TCR703, after having traced a small net
of arrangement; the adjustment has been performed by using the
least squares method. At the same time, about 100 control points
have been measured - natural points on the transept, 40 targets
on the apse and 17 specific Cyrax targets on the entire scene - to
orient photogrammetric stereoimages and to define a reference
system for range images.
3.1 Photogrammetric survey
In this case, measured data and represented data are approximately
the same, except editing processing. Human critical selection
about what to represent - autocorrelation algorithms, automatic
restitution, furnish interesting results in small scale survey but are
less applicable at the specific requirements of cultural heritage -
takes place simultaneously at the restitution phase, with additional,
well known, advantages derived from the separation between
orientation and restitution phases.
The images have been acquired by means of both semimetric
Rollei 6006 camera (f=40 mm), and digital Nikon D1 camera
(f=24 mm).
With semimetric camera, three stereocouples have been acquired
