CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
instruments, show "geometric anomalies" that don't
have a logic explanation but simply depend from the
uncertainty of past tracings.
Survey in this case reproduces of the state of fact, pure
knowledge through representation.
Today science and technic are not separated and
autonomous anymore, but they are more and more
interconnected.
Measure is the support of this exchange.
The quantitative knowledge of the architecture,
consisting in its measurement through the survey and
subsequently through its representation, leads to the
model that can be considered a sort of image of the
state of fact to which in these centuries, different
projects and stratified realizations concurred in a
chaotic order.
1.3 Survey and representation
The different spatial dimension of survey requires
different methodologies, precise solutions, and
adequate instruments.
Unlike territorial representation, that cartography well
supports and where planimetry is in general privileged,
in architecture each of three spatial dimensions (X,Y,Z)
have an equivalent importance. In general, unlike
territory, the architectures have an inside and an
outside and the survey for the inside often requires
different techniques and instruments from those
necessary for the outside.
San Lorenzo makes this distinction ephemeral because
the outside seems an inside (niches, undercuts,
recesses..) and the inside space is composed by solids,
vacuums, volumes at different heights that render the
survey difficult independently from the used instrument.
Therefore an external and internal grid reference
system on different levels of the monument that allows
to supply references to its constituting parts in a unique
system is necessary. Thicknesses, distances, volumes,
trends, surfaces are in this way rebuildable starting
from the simple differences of spatial coordinates.
Survey methods can't leave out of consideration (San
Lorenzo is the validation in the field) the "banal"
methods of direct measure, despite of the
sophistication of the current technology.
The object is shattered in a set of punctual
characteristic points such that their possession leads us
to the knowledge of the masterpiece in its wholeness.
The transfer of typical methods of the cartographic
survey to architecture, hoping of obtaining for analogy
the same results achieved in cartography, is not a
practicable path without correctives and principles of
validation. In fact surveying architectures, the
correlation between the purpose of the survey and the
elements to survey, not only is much more selective
and discriminant but also more subjective. It is true
that some type of homologation, at least in the most
diffused scale, 1:50, has been used (vide the first
specifications), but, as usual, foreseeing all the
following necessities is very difficult especially when
problems come out one after the other, caused one by
the other.
The discretization in points of survey highlights the
entire problematic limitation of the method, and the
curious thing is that, while not long ago the problem
seemed to find its limit in the scarcity of points
necessary to represent the structure of an architecture,
today it's the opposite with the scanning-laser.
Obviously the photogrammetrical method survives and
it is not disjoined, at least in part, from the applications
of the new methods admitted by the instruments and
the computer science.
1.4 Forms of bi-dimensional and three-
dimensional representation
There is a consolidated tradition on how to represent
graphically on a plan the three-dimensional objects, but
there isn't a consolidated tradition on how to represent
them graphically on a three-dimensionally plan. But it is
immediately necessary to specify that when you enter
in the three-dimensional field, the photogrammetry is a
sharer technique, with the others, to achieve the final
result. It's clear that here for 3D we mean those shapes
that can have an effective and three-dimensional
representation (i.e. in the bi-dimensional representation
on video or on paper, the points that I see are affected
by three coordinates and therefore their position is
known in the assumed reference system), observable
from different points of view once the model of reality
is carried out.
Whether the model is obtained through the
photogrammetrical method in some cases, or through
new methodologies based on the acquisition in short
times of clouds of points of the object in an opportune
system of reference, the problem of the representation
of the object always remains.
Basically one of the most important advantages of
using the photogrammetrical system is that it has
already been tested many times, therefore it produces
extraordinary results where it is practicable and
convenient, whether generating vector representations,
or raster ones (photoplans) when possible, or the ones
usually called 3D orthophotos. These last ones which
need the construction of the DSM (Digital Surface
Model), which can be partially computerized, work out
the model dressed with the orthorectified digital images
of the surfaces.
The laser scanner system creates the image of the
model using millions of points, the so called cloud, but
it is a representation that can't be used by itself. It
must be depurated from the abnormal points using
sophisticated algorithms because of their high
numerosity. These algorithms have not yet been
adequately tested on big numbers and are run through
software packages created for other purposes and only
recently present versions that help solving this problem.
Once you obtain the "dean cloud"of points, difficult to
achieve in complex cases like San Lorenzo, you can
start the research of three-dimensional representation.
1.5 Survey Grid Reference
Photogrammetry, in its classical feature or in the new
digital othoprojective one, needs the aid of the essential
topographic operations on the territory. These are
made up of a highly precise reference grid that
embraces entirely the monumental architecture; it
builds up the frame within which all the survey
develops, and its purpose is to stop the propagation of
mistakes proceeding from general to particular views
and it is characterized by permanent reference
vertexes.
