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 modern 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