3D MODELING AND RESTORATION: FROM METRIC TO THEMATIC SURVEY. 
THE CASE STUDY OF SAN FRANCESCO AL PRATO IN PERUGIA 
G. Tucci a , F. Algostino 3 , V. Bonora a , F. Chiabrando" 
a Dept, of Scienze e Tecniche per i Processi di Insediamento, Politecnic of Turin 
Castello del Valentino, Viale Mattioli, 39 10125 Turin, Italy - grazia.tucci@polito.it 
KEYWORDS: Cultural Heritage, Digital Photogrammetry, 3D Modeling, Conservation 
ABSTRACT: 
A multisensor approach seems to be the most suitable solution for complex environment modeling. In this paper such an approach was 
exploited to generate a model, capable of covering different interesting aspects of the church of San Francesco al Prato, in Perugia (Italy). 
It is one of the main churches of the city, reconstructed around the middle of XVIII century over a previous church, and enriched in the 
centuries with masterpieces by such artists as Perugino or Raffaello. Unfortunately, due to some earthquakes its structure suffered several 
major lesions, as for example the loss of the roof, and the recent decision to restore it led to the need for an accurate survey. 
The procedures adopted for documenting this building and some preliminary results are described here. Due to its interesting aspects, 
the work was concentrated on the transept-apse complex, characterized by a complex spatiality, different building materials, a marked 
architectural decay, and a geological instability of the underlying ground that produced cracks and lesion all over the structure. 
Topographic and digital photogrammetric tecniques have been employed in order to obtain complete and accurate data; 3D laser scanner 
has been employed only on the transept. The model has been obtained from the merge of the different acquisition. 
The documentation of such a complex architectural space was planned according to progressive levels of critical synthesis; the surveying 
approach described here followed the same logic and allowed us to obtain a three-dimensional model of the structure under investigation, 
by adapting the space resolution to the complexity of the surface or to the historical and artistic interest. Such tradeoff involved also the 
optimization of fundamental resources such as acquisition time or computer memory space, that have to be carefully taken into account 
in order to make any high complexity surveying project feasible. 
1. INTRODUCTION 
The approach to restoration planning in order to give specifications 
needs more than an accurate geometric survey, although this is 
an unavoidable starting point. It needs to base itself also on a 
critical survey and often has to consider diagnostic studies in 
order to produce thematic mapping related to chronologic events, 
materials, pathologies, that will be considered in order to look for 
best solutions. 
3D modelling of heritage monuments nowadays has increased 
significantly but the accurate and fully automatic capture of all 
details, for all types of application, remains, at present, elusive. 
So the requirements of the model depend, each time, on the 
application. 
The effort to determine the 3D model requirements able to 
represent a thematic survey and able to be useful for a restoration 
planning is one of the aims of this work, still in progress. 
To obtain data, geometrically correct, with suitable details and 
realistic representations, the main approaches today are interactive 
and automatic photogrammetry and terrestrial laser ranging. The 
other task of this work is to look for an integration between the 
two methods and to estimate the effectiveness and the potentialities 
of laser scanner survey. 
1.1 The church of S. Francesco al Prato, in Perugia 
San Francesco al Prato is one of the main churches of the city; the 
first plan (1251 -1253) at “crux commissa” was aisle-less, with 
three spans, wide transept and pentagonal apse. Unfortunately, 
due to some earthquakes (the first documented one is in 1340) 
its structure suffered several major lesions. Other damage was 
caused by landslides, which happened at the construction of 
external rampant arches (1421). Starting from the XIV century 
family chapels and other monuments were built. In 1527 heavy 
rains caused great cracks and the loss of important paintings. From 
1599 to 1737 there were numerous collapses, after which Padre 
Giuseppe Modestini commissioned the architect Pietro Carattoli 
for the reconstruction of the church. He tore down the damaged 
vaults, reduced the height of the perimetral walls and built a lining 
with new thick walls with their own foundations; he also built a 
dome with a lantern on a high cylindrical tambour at the cross of 
the transept. In 1860 the church was used as a store and suffered 
ornament vandalism. A rapid decay of the structures transformed it 
in a ruin. The facade was reconstructed, in 1926, by Pietro Angelini 
and new hydrogeological study on the area was carried out, but 
the unsafe part of the church continued to decay. Only in 1932 
consolidation works were planned. The eighteenth-century part 
was demolished to restore the mediaeval architecture. In 1968 the 
nave roof caved in, and in 1972 the construction of a new roof was 
begun. A recent decision of restoring the church to transform it in 
an auditorium, led to the need for an accurate survey. The transept- 
apse complex remains without roof, as a ruin, to be used only 
the summer season. Therefor our work seems, in all respects, an 
archaeological survey, with all the difficulties in the interpretation 
of the different constructive phases and with all the representation 
problems linked to the curve plan of the apse. 
2.1 Sfratigraphic survey 
Critical survey was founded on the USM (Unità Stratigrafiche 
Murarie) individuation, with the aim to interpret, on the palimpsest 
walls, the sequence of the works on the building. These are divided 
in positive actions (red in drawing) - e.g. extensions, elevations, 
closed openings - and negative actions (yellow in drawing) - e.g.