Bitelli, Gabrielle 
  
DIGITAL ORTHOPHOTO AS A TOOL FOR THE RESTORATION OF MONUMENTS 
Laura BARATIN , Gabriele BITELLI , Marco UNGUENDOLLI, Antonio ZANUTTA 
"DISTART - University of Bologna, Italy 
laura.baratin@mail.ing.unibo.it, gabriele.bitelli@mail.ing.unibo.it, 
marco.unguendoli@mail.ing.unibo.it, antonio.zanutta@mail.ing.unibo.it 
Working Group V/5 World Cultural Heritage 
KEY WORDS: Analytical and digital plotting, Digital Surface Model, Orthophoto, Restoration. 
ABSTRACT 
For the restoration of monuments it is now becoming normal practice to carry out a preliminary study of the monument 
before starting any kind of work. The metric survey is one of the main operations required. For this reason, 
photogrammetry applications already have a large bibliography. In particular, digital photogrammetry is currently an 
effective system, providing different solutions of both the vector and raster type. 
For an 18th-century gate in the Malta fortification walls, many tests were carried out in order to establish a suitable 
procedure for the creation of digital orthophotos. A photographic metric product, showing maximum detail without 
subjective interpretation of the object and the conditions of the material, could be very interesting in supporting any 
restoration work. The use of automatic procedures is also particularly suitable where large-scale surveys have to be 
carried out on very large objects with homogeneous characteristics, as is the case with the Malta fortification Walls. The 
advantages of digital over manual processing lie in the possibility of measuring a very large number of points without 
fatigue and with a high level of productivity. Digital orthophotos require a digital surface model as input, but the 
production of a high-fidelity reconstruction of surfaces by automatic measurement of points in close range 
photogrammetry still remains an open problem. Particular difficulties are caused for instance by object details that are 
not imaged or with strong shadows, or by complete failure of the image matching algorithms. Discontinuities in the 
surface, very common in architectonic objects, complicate the generation of models and surfaces. 
Within this work, several tests using commercial software were carried out, to define the best parameters for the 
automatic generation of DSM (Digital Surface Model) related to the monument being studied. The initial tests are based 
on the different dimension of the search window and the consequences of this choice on the accuracy and reliability of 
the matching procedure. Further tests are related to the number of points needed to start the matching procedure: firstly, 
points have been entered following a regular grid, then a greater number of points from the analytical plotting have been 
entered. The results are compared, also taking in account practical and economic considerations. 
Finally, different aspects for the creation of representative surfaces of the object have been analysed due to the non 
planarity of the object, presence of linear discontinuities and noticeable overhangs. 
The above-mentioned tests lead to several general considerations regarding strategies for creating orthophotos as a 
suitable qualitative and quantitative tool for the restoration of a monument. 
1 INTRODUCTION 
The increasing use of non-destructive scientific study techniques for the study and conservation of historical buildings, 
constructions and in general in the field of Cultural Heritage, is a symptom of how the operating procedure in such areas 
aims to use a modern scientific and technological approach, deciding beforehand, that is to say, before any work is 
started, which methods will be used, the sequence of procedures and their possible consequences. 
Close range photogrammetry is essential in this context, since it allows metric-morphological reconstruction of the 
work examined, which can be documented and filed on paper or computer. 
In the architectural surveying sector for the restoration of historical buildings, one of the most effective 
photogrammetric product may be the orthophoto derived from digital differential rectification techniques; the 
generation of a photographic image with metric characteristics constitutes a valuable tool for gathering information 
about the object, thanks to his characteristics of immediate implementation, accuracy and flexible usage (Baratin et al, 
2000). A digital orthophoto is an orthographic photograph of objects which reproduces the photo-texture, colours or 
grey tones of the original photographs. It is derived from conventional perspective photographs by simple or differential 
rectification. Each pixel of the image is transformed by means of an orthogonal projection with scaling onto a horizontal 
  
62 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 
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