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COMPARISON OF SURVEYS OF ST. MARK'S SQUARE, IN VENICE
Andrea Lingua, Fulvio Rinaudo, (*)
Giovanni Auditore, Caterina Balletti, Francesco Guerra, Luca Pilot (**)
(*) Politecnico di Torino - Dipartimento di Georisorse e Territorio
Corso Duca degli Abruzzi, 24 - 10129 Torino - Italy
(**) Laboratorio di fotogrammetria - CIRCE
Istituto Universitario di Architettura di Venezia - S. Croce 1624 - 30135 Venezia
Email: auditore@cidoc.iuav.it, balletti@iuav.it, guerra2@iuav.unive.it, pilot@iuav.it, andrea.lingua@polito.it,
fulvio.rinaudo@polito.it
KEY WORDS: Architecture heritage, Orthoimage, Digital, Metric, LIDAR
ABSTRACT
In recent years, St. Mark’s Square and the surrounding buildings have been the subject of surveying campaigns that have gradually
used more modem techniques and instruments for both the survey and representation phases. St. Mark's Square represents a field of
application of survey of a complex monument (in that it comprises a group of parts). As a whole, we refer to the composite as "the
square" but it can be divided into different objects that are traditionally handled by different sectors of geomatics, topography, close
range photogrammetry, aerial photogrammetry and terrestrial and aerial laser-scanning, among others. Each of these techniques has
been used in surveying the square and has contributed to the geometric definition of the monument in its entirety. Clearly, the
general reference system must be unique; this leads to problems connected to geo-referencing instrumental systems with diverse
characteristics of precision that allow for survey at different nominal scales. As a result, the surveys conducted have different
characteristics in the form of representation and in their precision.
Some processed data can be compared to certain other categories; one example of this is the survey of the pavement, surveyed in its
planimetry and its altimetry. In fact, in 1993, an altimetric survey was done for trigonometric levelling on a regular grid and
photogrammetric survey of 100,000 stone tiles. More recently, the first survey using terrestrial laser-scanner was conducted (2001)
and a year later, a laser-scanner image was realised from the air. Since the data deal with exactly the same area, it was deemed
useful to compare them in order to gain insight on the expediency of using a certain technique, conditional on the scale and the type
of representation.
In terms of planimetric representation in raster form, photoplanes, orthophotos and precision orthophotos have all been produced.
This processed data use diverse digital reference models acquired by solid modelling from numeric cartography, digital
photogrammetry by means of matching, and from aerial laser scanners. Another important issue in all this is comparison of products
by placing them in relation to the use intended of them, in order to verify the possibilities of economies and optimisation.
Similar considerations can be made for the elevations of the buildings, such as the Basilica.
The paper presents the various experiments done and illustrates the methods and results obtained, the processes of integration and
the difficulties encountered.
St. Mark's Square consists of a group of monuments which
studies in recent years have been found to be technically very
different.
In general, for the purposes of representation, we usually divide
the field of survey into two broad categories: architectonic
survey and cartographic survey.
In the particular case of the Square, as this is considered a
single complex monument (composed of parts), it is useful to
use single system of reference common to all the surveys.
Different instruments and techniques produce diverse
representations not only in the form (vector, raster, orthogonal
projects, 3D models) but also in the accuracy.
It is clear that the cartographic coordinates in the national Gauss
Boaga system are the general benchmark. The Square contains
an IGM95 cornerstone whose coordinates are known in the
WGS84 and the Gauss Boaga systems. The uncertainty of this
cornerstone, used to geo-reference the local networks of each
individual survey operation, can be estimated to ±2,5cm
accuracy, much higher than what is required in the architectonic
surveys, such as for example on the Loggetta del Sansovino at a
1:20 scale where the uncertainty is ±4mm. We have to
introduce the idea of local accuracy, which must be diverse that
the general accuracy in the sense that it is associated with
procedures and techniques used.
In the case of the Loggetta, the technique used is
photogrammetry supported by markers detected
topographically. Local precision is on the 1:20 scale, although
after georeferencing in the absolute system, this precision will
deteriorate in the direction of the accuracy of references used
for the actual georeferencing procedure.
At this point, we have to ask ourselves if it is logical for an
operator or professional (architect, engineer, restorer, or
historian) or a citizen to compare an element of the Loggetta
with an example of the Basilica or its floor with the uncertainty
of a few millimetres.
This is where the issue emerges of the purposes for which the
survey is being conducted, namely for whom and for what
reason. Both contribute to the decision of the nominal scale to
use, therefore in practise, the accuracy of the form of
representation (orthogonal vector projection, orthophoto, 3D
model, rendering, solid model). In connection with this
accuracy, geomatic has taught us much about surveys
