studies. The design model was reconstructed analysing Bernini’s original
drawings. The representation model collates and summarises the data pro-
vided by the measurement and theoretical models; it selects the appropriate 3D data as well as the 2D images of plans, elevations
and sections so that the drawings are correctly represented.
3.1 The measurement model
The measurement model has been created applying the “Inte- grated Survey” method. This method uses various systems: of
an instrumental, topographic, photogrammetric and traditional direct type, which are integrated thanks to a common georefer-
encing system. The topographic survey, which uses a total station, generates a
reference polygon with the topographic measurement stations at
Figure 7. The design model of the cupola Figure 5. The survey model of the cupola
Figure 6. Groin costruction model of cupola. In evidence the isocurves that set the surface
Figure 4. Photo of St. Thomas cupola
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its vertexes, and this forms the basis for georeferencing of all the survey models obtained using the various methods. A survey
of the object is then carried out, while at the same time the pre- paratory drawings are prepared and then a manual survey of all
the details moldings, decorations, etc. etc.. All the preparatory drawings are inserted in the general reference system as they
contain the “known points” and carry information on dimen- sions for control and veriication purposes. The next phase is
the photogrammetric survey. Using a reproduction software, it is possible to use the photographs to reconstruct a photogrammetric
model in which pairs of photos taken using the orientation in
the general reference system by importing the “known points” make it possible to measure all the objects that are not detected
using the other methods. The result of this process is a topo- graphic and a photogrammetric model that form the basis upon
which to create the design model.
3.2 The survey model
Right from the early stages of work a double problem was en- countered. On the one hand there was a problem performing a
proper survey on the roof and cupola of the church due to access or visibility questions, and on the other hand there was a problem
representing the curvature of the cupola and groins properly. For this reason it was decided to create a complete three-dimensional
model of the drum, the cupola and the entire decorative system in
the clerestory. The model, which is constantly veriied using the few points that can be surveyed, has been divided into four main
parts: the drum, the cupola, the groins and the clerestory. The cu- pola has been constructed as surface of revolution, which by dei-
nition requires an axis of rotation and a generatrix curve. The axis has been identiied starting from the horizontal section plan of
the church, using the topographic survey model. Various circum- ferences have been built on three points passing through the
centre of the four pilasters that support the drum, obtaining in this way a projection of the axis itself. Construction of the generatrix
required use of the photogrammetric survey. Thanks to this in- strument it was possible to draw all the elements with great preci-
sion, in particular the joints in the lead sheets that cover the outer surface of the cupola ad even the nails in the sheets themselves.
In particular the drawing of the nails, which are almost perfectly
aligned, allowed deinition of the outer proile both of the cupola and of the groins. The drum has been modelled as a surface of
revolution using a procedure similar to the one described for the cupola, with the difference that the generatrix was detected us-
ing the direct method thanks to the ability to access a walkway around the perimeter. The clerestory and groins, which cannot
be accessed and are dificult to measure using the topographic instrument, were measured and recreated from the photogram-
metric model. Construction of the groins in particular involved a special study relating to their geometry. The curvature of the out-
er proile is different from that of the inner proile, which rests on the crown of the cupola. Furthermore, the initial section and the
inal section of the groin are different not only in size and propor- tions, but also in geometry. As a result it was necessary to build
a bicubic surface interpolated between four edge curves Coons surface, dividing the groin in half as illustrated in ig. XX.
The clerestory and its decorative system has been modelled as a surface of revolution whose axis coincides with that of the cupo-
la. The generatrix in this case derives from the topographic sur- vey model and from the preparatory drawings made on site. The
composite columns that support the groins rest against the central body of the clerestory. The position and dimensions of the nails
on the lead sheets on the cupola have also been obtained from
Figure 8. Represented model: Front view with in situ section Figure 9. Represented model: plan view
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the photogrammetric model and inserted in the design model to complete the work.
3.3 The design model
