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In: Paparoditis N., Pierrot-Deseilligny M.. Mallet C.. Tournaire O. (Eds). IAPRS. Vol. XXXVIII. Part 3A - Saint-Mandé, France. Septentber 1-3. 2010 
i&OI 
ON THE USE OF LASER SCANNER AND PHOTOGRAMMETRY FOR THE GLOBAL 
DIGITIZATION OF THE MEDIEVAL WALLS OF AVILA 
Juan Mancera-Taboada, Pablo Rodríguez-Gonzálvez, Diego González-Aguilera*. Ángel Muñoz-Nieto, Javier Gómez-Lahoz, Jesús 
Herrero-Pascual. Inmaculada Picón-Cabrera 
Department of Cartographic and Land Engineering; High School of Avila, 
University of Salamanca C/Homos Caleros, 50, 05003, Avila 
*daguilera@usal.es 
Commission III, WG III/l 
KEY WORDS: 3D laser scanner, cultural heritage, close-range photogrammetry, computer vision, image analysis, surface reconstruction 
ABSTRACT: 
The new technologies are playing a main role in the preservation and restoration of the architectural heritage. In this paper, a 
digitization of the walls of Avila process, by means of laser and photogrammetric technologies, is described. This process includes 
the acquisition of data, the processing and integration of both data sets and finally, the generation of a photographic texture model of 
the wall. More specifically, a terrestrial - time of flight - scanner laser is used besides a low cost aerial imaging device attached to a 
captive blimp. In this way, two earth technologies are integrated successfully while attaining, for the first time, a global digitization 
of this World Heritage monument. 
1. INTRODUCTION 
The techniques for the architectural heritage documentation 
have traditionally been based on topographic surveying and 
photogrammetry. The developing of new sensors and new data 
processing techniques allows us to state that these conventional 
documentation procedures are becoming rather obsolete. 
(Cannatacci, 2003; Barber, 2004). Within the surveying 
techniques based on the use of Total Stations, measures can be 
obtained without the use of a prism while only acquiring the 
most relevant points that define the object geometry. These data 
are then imported to CAD software to render a 2D design. The 
advantage of this method is the accuracy of the model. The 
quality of the results prevails over its quantity. This is due to the 
fact that the data volume aims only at the goal of obtaining a 
sketch of the object, enough to define its structure, and a 
detailed representation of complex or hard to process forms is 
usually neglected. 
With photogrammetric techniques, object restitution can be 
obtained from images taken from a calibrated camera beside 
some surveyed points to ensure the geometric control of the 
model. (Arias et al., 2006; Arias et ah, 2007). In spite of the fact 
that it implies a rather complex and tough work, the results are 
usually better than those provided by the surveying techniques 
as the object is rendered with a more level of detail. 
Nowadays, the trend is to accomplish the 3D documentation 
with a terrestrial scanner laser (Gonzalez-Aguilera et ah, 2008; 
Yastikli, 2007) as millions of high fidelity points can be 
acquired. In a simple fashion, the laser stations are adjusted to 
the object shape avoiding to cope with the highly demanding 
topographic stations. A sequential group of point clouds is 
obtained which must afterwards be unified to a single and 
coherent set. 
Nevertheless, this point cloud is only a simple acquisition of 
points with XYZ coordinates and, in some cases, with IRC 
intensity values or RGB colour. In this way, to achieve a surface 
model or a vector model can be a complex task (Remondino, 
2003). In addition, this step will demand the application of 
optimization strategies (Cignoni et ah, 2004) in order to obtain 
models that can be handled by modern CAD/CAE. 
The goal of this work is to obtain a 3D model with high metric 
accuracy of the outer part of the walls of Avila. This model is 
the result of the integration of the non destructive technologies 
of laser scanner and low cost aerial photogrammetry. In 
addition, the 3D model will permit the generation of some by 
products such as cross sections, profiles, perspective views and 
orthophotos, very useful to technicians and experts related to the 
damage diagnose, the preservation and the restoration of this 
monument, regarded as a World Heritage Wealth. 
The Walls of Avila are the best example of military architecture 
of the Romanesque style in Spain and a unique model of the 
European medieval architecture. (Serna, 2002). The 
construction of its walls and towers is perfectly adapted to the 
relief. The southern parts have little heights as they are built 
upon a cliff that acts as a natural defence. The western and 
northern parts grow higher to reach the highest and thickest 
parts at the east. At this zone, the “Alcazar’' fortress and the 
strongest doors were raised (named of the “Alcazar” and of 
“San Vicente”) and the defence system was reinforced with a 
barbican and a ditch. It must be stressed that it is the only case 
of a medieval wall in which one of its towers is a part of the 
cathedral itself. Some references state that the building took 
place around 1090. Some other researches, instead, point out 
that the works should have persisted over the XII century and 
that the walls were build over and older one. 
This paper consists of three parts: after this introduction, in 
chapter 2, a detailed description of the methodology will be 
developed. In chapter 3, the results will be described and 
finally, chapter 4 will deal with the main conclusions and the 
expected future developments. 
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