CIPA HERITAGE DOCUMENTATION - CIPA - Icomos

Common photogrammetric records are based on imagery
and total stations. Planning of network for acquisition and data
capture are time consuming; furthermore, post-processing
work for image-based architectural surveying is tedious and
requires a very high level of expertise. However, 3D laser scanners
have proved to be more efficient in three-dimensional
dense modelling. Although the local accuracy of measurements
taken with robotized total stations is higher, laser-scanner
surveys are often more appropriate for urgent interventions
as they provide massive 3D volumetric information. Information
processing provides metric data which are very useful to
support conservation and restoration policies.
Since the 2001 edition of CIPA, there has been a very
large number of works published, focusing on the laser scanner
and its application to heritage recording subject. [Boehler
(2002), Malinverni (2003)].
On the other hand, a large number of developments in
Computer Graphics and Machine Vision for complex rendering
of historical buildings have been performed along the last ten
years. Modern scientific visualization offers the possibility to
inspect and evaluate structural properties of buildings and
other information relative to their environment. Thus, it is necessary
to develop multi-resolution and hybrid approaches for
architectural and environmental surveying (see e.g. El-Hakim
et al, 2002). The development of interoperability tools between
Surveying and Information Systems, including advanced
visualization techniques models is the next challenge
to face. Advanced Visualization must allow the exchange,
processing and analysis of metric information with object-oriented
data and metadata bases relative to the information
with a 2D/3D support.
For dealing with such a massive data source, it is necessary
to combine volumetric segmentation with 3D matching to different
level-of-detail (LoD). This combination provides a semiautomatic
identification of primitives appearing in buildings for
each LoD. The coarsest level of the hierarchy involves the identification
of geometric elements (dominant planes, quadrics,
e.g.) whereas the finest level deals with complex primitives (or-
Figure 2: Scanning the Church of Santiago outdoors and indoors.
namental details, pinnacles, ledges, sculptures, e.g.). The automatic
identification of complex primitives from basic geometric
primitives is still a challenge for semi-automatic 3D recognition
and modelling with a large number of applications to CAD,
Architectural or Archaeological Surveying, computer animation
and simulation, including VR/AR, among others.
Currently, the LFA-MoBiVAP research group is developing
information systems for Cultural Heritage Recording by means
of advanced visualization systems. Post-processing of 3D information
arising from sampling of huge clouds of points makes it
possible to compare the current state of certain areas with their
simple planar and quadratic primitives. Multi-resolution models
have been applied in the visualization and virtual reconstruction
of archaeological sites; examples of these are the
analysis and ideal reconstruction of the roman theater of Clvnia
(Burgos, Spain) as illustrated in [Finat et al 2005] and an
extension for urban environments, in [Fernandez, 2006].
The main goal of this work is to contribute to the semi-automatic
3D modelling and design of architectural primitives
corresponding to damaged buildings from range data files
from scanning. To achieve this goal, simpler PL-models (piecewise
linear) are generated from huge unorganized clouds of
points captured with the laser scanner Ilris 3D (Optech). Sections
of the point-clouds (understood as equispaced slices
with constant width) and projections of selected ones onto reference
planes are the result of the performed projective operations.
These partial intermediate models are obtained with
low computational cost and they are enough to identify coarse
structural damages before urgent interventions.
Methodology
According to the methodology planned the steps performed in
this and other precedent works is resumed as follows:
1. Data capture and construction of a global model by means
of multiple scans alignining.
2. Global orientation of the model using ground control
points. A high precision levelling traverse has been done.
42 CIPA HERITAGE DOCUMENTATION BEST PRACTICES AND APPLICATIONS Series I, 2007 & 2009