LASER APPLICATIONS FOR 3D SURVEY OF CULTURAL HERITAGE
C. Balletti°F. Guerra”
"Dipartimento di Ingegneria Idraulica, Ambientale e del Rilevamento - Politecnico di Milano
P.zza Leonardo da Vinci, 32 — 20133 Milano
"Laboratorio di fotogrammetria, CIRCE — Istituto Universitario di Architettura di Venezia
S. Polo 1624 — 30135 Venezia - ITALY
balletti@iuav.it
guerra2@iuav.it
KEY WORDS: laser scanner system, triangle mesh, 3D measurement, orthophoto, digital elevation model, cultural heritage
ABSTRACT:
The possibility to obtain in a fast and usefull way a model of different scale object (from architectural monuments to small statues)
using different scanning laser instruments suggest to experiment a new systems able to produce 3D models and 3D orthophotos of
objects for cultural heritage applications, such as monitoring, virtual museum, digital archives. In these cases, as a matter of fact, it’s
very hard to work with the usual topographical and photogrammetrical methods, expecially in the case of small objects.
The procedure purposed involves to scan and to take digital pictures, with a coomercial camera, of the object. The 3D model
produced by scanning can be use to obtain control points with sufficient accurancy for the orientation of a single close photo.
The sequence of the procedure can be summarized in the following steps: once that an empty image is created, where pixel size is
fixed, a corresponding plane of vertical projection (that one of the orthophoto) is definite in the reference system of the scanning
instrument. Image and plane are put in relation through a roto-translation with scale factor: each pixel is in corrispondence with a
point on the plane. Each normal to plane identify a threedimensional point in the scanned space. The coordinates of that point
provide to find, in the non-metric digital picture, the corresponding pixel, which radiometric value is transferred in the digital
orthophoto.
Beyond generating the digital orthophoto, pixel depth (corresponding to Z coordinate regard to a reference XY plane) is stored.
An orthophoto is the output, where the third dimension is known for each pixel, so that its advantage (to be metrical on the plane)
are joined to the digital elevation model. In this way distances can be taken both on projection plan and on space.
In the research into a more accurate and reliable understanding
of cultural works of art, we are faced with an exciting new
possibility of using optical technology for studying cultural
objects, which opened to us in the recent years by progress
made in the field of computers and optical electronics . The
interest in these techniques lies in the fact that the analysis is
not invasive, thereby damage to the artistic work is avoided,
while understanding of the work is expanded. Ordinarily,
optical techniques are associated mainly with diagnostics and
IR reflectography, UV fluorescence, X radiography and
thermography serve to make a structural analysis of the work, to
study its composition, state of repair and every other
characteristic necessary to define the plan of action and
renovation.
Recently, in the field of cultural heritage protection, techniques
have been widely used that enable acquisition of the shape of an
object, thereby permitting survey of the existing object and
monitoring the evolution of the shape over time, in terms of its
progressive state of decay and in terms of the documents In particular, use of laser scanner tools in architectonic survey is
reporting any renovation and maintenance work. still a novelty that fundamentally entails a change in the way we
Among these are laser profilometry, laser scanning TOF, video look at survey.
projection, structured light projection system and conoscopic Traditionally, in instrumental survey (topographic and
holograpy all of which enable three-dimensional direct photogrammetric) only a few significant points are used to
measurement of the work and construction of a digital model; in describe the object. For example, in rectifying the
Figure 1: Laser profilometry
this case, interest is closely linked to the quality of the
reproduction in terms of resolution and accuracy, obtainable
only with the use of increasingly sophisticated instruments and
methodologies for acquisition and digitalisation of 3-D shapes.
photogrammetric survey of the Scuola Grande di S. Marco in
Venice, the archways are constructed like a graphic entity
through 3,5,...n points on which to trace an interpolating
circumference.
-302—-
