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thickness or curvature which are difficult to comprehend. Perspectives allow us to have a representation closer to how we perceive 
the three-dimensions of the structure in reality. Nevertheless, they do not allow metric mapping at a precise scale. The survey of a 
building must combine the use of both mapping systems to assure that our knowledge of the object is the most befitting its three- 
dimensional nature. 
2. THEORETICAL AND REAL SHAPE OF HISTORICAL ARCHITECTURE 
Historical architecture evolves along a complex transformation process, the result of the long periods of time over which the 
transformations occurred and which inevitably result in large differences between the theoretical shape imagined for the structure 
and the real shape that has survived to our time. There are many factors which modify the original shape and structure over time: 
flaws and changes in the project at the time of construction, lesions and deformations caused by loads or an inadequate conception of 
the architecture, lesions and erosion from exterior actions - either natural or human - or transformations of the structure due to 
changes in its use. 
The differences between the theoretical and real shape of a building are directly related to the transformation it has experienced over 
time. By comparing these shapes, we can determine questions of its history, construction, pathologies, etc. For example, we all 
understand that a building is usually built with orthogonal geometry and therefore the deformed geometry it presents reveals the 
movements of the building over its lifetime. Nevertheless, the definition of the theoretical shape of a building - whenever there is no 
document or proof of how it was built - is neither a known fact nor is it objective. It must be deduced by using the conserved 
structure and performing studies on its features, history, pathologies, etc. 
In this manner, the study and restoration of an historical building must begin with an exhaustive study of its structure as it appears 
today - with all of its geometrical, physical, construction, mechanical and functional features - and with research on the historical 
process that has spawned it on top of a previous structure, the configuration of which we are unaware. Logically, the starting point 
for this process is the survey of the conserved historical building, objectively and systematically recording all existing edges, the 
contour of the materials and all surface elements such as erosion, cracks, etc. At this point, photogrammetry becomes an essential 
tool of the process, as it allows deformed and eroded geometry to be drawn with precision. This geometry is difficult to appreciate by 
merely viewing it and does not usually adapt to orthogonal geometry. Furthermore, due to its indirect recording system, 
photogrammetric recording also helps to avoid simplifications and falsifications of the shape and real features of the object 
represented. From this viewpoint, surveying becomes the main method to understand a monument and the most important tool of the 
entire research and restoration process. 
The importance given to the entire range of previous studies on a historical building is well known. These allow us to define its 
features (shape, construction and function), its historical development and its problems and pathologies. The information gathered in 
the process on materials, pathologies, building techniques, movements, history etc. has a graphic reflection on mapping in the form of 
thematic drawings. This set of drawings will form a monument atlas similar to those used for territories. Several factors combine to 
form what we have called the MIS Monument Information System, similar to the name used for the Geographical Information 
System (GIS). These are: the systemization and computerization of all of the information on the building into a database, its 
comparison with the survey through a two-way system that links the graphical records with textual and numerical records and a 
management system. 
3. THE 3D ANALYTICAL MODEL OBTAINED BY PHOTOGRAMMETRY 
The mathematical theory of photogrammetry has been three-dimensional since its inception. Nevertheless, the recording of 
photogrammetric plotting has in a practical sense had a graphical (analog) nature and is therefore two-dimensional. This limitation of 
the system required that this graphical record be made using the diedric projection system which was the only architectural mapping 
system that allowed for the metric, to-scale recording of the coordinates (X, Y) of the section being projected. During plotting, only 
the depth of specific points could be added, using the written inclusion of the value of a Z coordinate. Only on occasions were 
attempts made to achieve the three-dimensional mapping of sculpted objects or the geometry of vaults, applying the system of 
contours used in terrain maps. 
As is well known, the computer revolution transformed photogrammetric calculation systems into analytical systems, making use of 
the entire three-dimensional capacity of the equation. At the same time, the development of computer aided design programs (CAD) 
has allowed the three-dimensional result of plotting to be recorded in real time and multiple projection planes can be achieved - on 
screen or on paper - of the plotted model. With adequate topographical support, recording on CAD systems also allows a system of 
unique coordinates to be established and each plotted element can be placed in its real position in space, all within a single drawing. 
In this manner, we can “build” the virtual model of a building as we insert the results of the plotting, until all surface areas 
comprising the structure are completed. The result is in truth a mathematical model: a very high sum of coordinates in space joined 
by lines that reproduce the exact position of the points and lines plotted from their actual position, containing all metric information 
of the building. This model is displayed on screen by the CAD program once the projection elements have been defined (mapping 
system, contour and projection plane). By combining these values, multiple views of the model chosen are available, allowing us to 
select the best one for the problem at hand. 
To obtain a complete 3D model - as we will call it from here on - by photogrammetry of a monument, all visible surfaces must be 
photographed, topographically located and plotted. Logically this requires the number of m 2 of building surfaces that we need to 
work with to be multiplied exponentially, as it is necessary to include all minor, lateral or secondary surfaces that were previously 
excluded from usual analog photogrammetry. These only include drawings of the surface areas appearing in the elevations and