DIGITAL PHOTOGRAMMETRY AND LASER SCANNING
IN CULTURAL HERITAGE SURVEY
A. Guarnieri®, A. Vettore“, S. El-Hakim”, L. Gonzo‘,
* CIRGEO- Interdept. Research Center of Geomatics, University of Padova, Italy - cirgeo@unipd.it
"Visual Information Technology, NRC, Ottawa, Canada —Sabry.El-Hakim@nrc-enrc.gc.ca
¢ Istituto Tecnico per la Ricerca Scientifica e Tecnologica, Trento, Italy - Lgonzo(ditc.it
Commission V, Working Group V/2
KEYWORDS: laser scanning, 3D modeling, image-based model
ABSTRACT:
A joint project between the Interdept. Research Center of the University of Padova (C IRGEO), a research group of Istituto Tecnico
per la Ricerca Scientifica e Tecnologica (IRST) of Trento and the Visual Information Technology group (VIT) of the NRC Canada,
in Ottawa, has been undertaken, with the main aim to create a set of 3D models of an historical building by means of
photogrammetry and laser scanning-based surveying techniques. Beside the investigation of their geometric accuracy, a photo-
realistic representation suited for interactive navigation and manipulation in VR environment was a further objective of thie project.
To this aim, the main room in the Aquila tower in Buonconsiglio castle (Trento, Italy) was choosen, as it featured a relative simple
geometry along with artistically and historically very precious frescoed walls. In this paper a description of both surveying and
modeling procedures adopted and results of a comparison test between employed techniques are presented.
1. INTRODUCTION
One of the major masterpieces of international Gothic art
(1350-1450) is the Cycle of the Months, a fresco to be found in
the Aquila tower in Buonconsiglio castle, Trento, Italy. The
main room in the tower, completely frescoed with the Cycle of
the Months, preserves a rare and most precious example of
medieval painting on a nonreligious subject. À joint project,
between CIRGEO (Interdept. Research Center of Geomatics)
of the University of Padova, IRST (Istituto Tecnico per la
Ricerca Scientifica e Tecnologica) of Trento and the Visual
Information Technology group (VIT) of the NRC Ottawa,
Canada, has been established, with the objective to acquire the
most suitable data of the room by means of different survey
techniques to generate corresponding 3D models. The
geometric accuracy of each model will be assessed and
compared with the other models. Another objective is the
creation of textured photo-realistic models for interactive
manipulation. To this aim we considered two different
approaches based on photogrammetry and laser scanning. In
the first case we addressed the 3D modeling of the room by
image-based modeling, a technique suitable for regular
geometric surfaces like architecture and monuments [El-
Hakim, 2002]. The simple and regular shape of the room was
therefore a good candidate. The photogrammetric survey and
the subsequent modeling were performed using a high-
resolution colour digital camera and a commercial modeling
software.
The second kind of approach is a Range-based modeling by
means of laser scanner, which allows to acquire a lot of small
geometric details although the accuracy varies significantly
from one scanner to another. Since scanners with integrated
texture or colour acquisition do not provide the quality required
for a room of this size and detailed frescos, textures have been
acquired with a high-resolution digital camera. This means we
separated acquiring geometry from acquiring texture.
Obviously, digital images should be then properly calibrated
and registered with the geometric data in order to perform the
texturing of the 3D model by laser scanning. For this project
the work has been arranged as follows: CIRGEO team created
the models both from range data, whereas the IRST team and
VIT provided the model from digital images and its texturing.
In following sections, the various kind of surveys will be
described and concerns on both the modeling and rendering
stages will be highlighted. Furthermore, results of a
comparison test between the resulting models are presented
along with some conclusions and future work.
2. INSTRUMENTS
In order to generate both photogrammetric- and laser scanner-
based 3D models of the room, different instruments werc
employed. For the photogrammetric model, a 5 Mpixels digital
camera, Olympus E20P (1920 x 2560 pixels), has been
employed. The small size of the room yielded a limited
coverage of each image (without using a fish-eye lens),
therefore 16 images were needed to cover the walls, 8 for the
ceiling, 2 for the entrance, and 5 for the tower. The average
texture resolution was 2mm x 2 mm area of the wall per pixel.
It should be noted however, that for texturing purposes it is
advisable to limit as much as possible the number of required
images in order to avoid changes of lighting conditions and
subsequent effects of inconsistent illumination. Then, the Riegl
LMS-Z360 laser scanner [Riegl, 2004] was employed to create
the second kind of model. Such scanning system is provided
with a rotating head and two inner high speed rotating mirrors,
which allow to acquire a scene with a large field of view (FOV
= 360° H x 60? V), reducing therefore the need of several
stations. Typically, accuracy such sensor is ranging between
3.2 mm (at 50 m distance) to 6 mm (at 100 m distance) with a
beam diameter divergence of 3 mm at 50 m. Besides X, Ÿ, and
Z point coordinates, the laser is able to register the reflected
beam intensity and RGB colors.
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