THE CATHEDRAL OF S. LORENZO IN PERUGIA AND THE HYPOGEAL SPACES. GEOMATIC TECHNIQUES FOR SPATIAL INVESTIGATIONS AIMED AT THE KNOWLEDGE AND INTERPRETATION OF THE ORIGIN OF THE TRANSEPT

Abstract. The area of the monumental complex of the Cathedral and the rectories of S. Lorenzo, located on the “acropolis” of Perugia, has been for more than two thousand years the main religious and civil reference site of all populations since the origin of the city of Perugia. The aim of this research was to survey the monumental complex of the Cathedral of San Lorenzo through the use of various geomatic techniques, with particular attention to the areas of the apse, the transept, the attic, the upper courtyard and the lower one, together with the hypogeal rooms of the Capitular Museum underlying the church, including important and impressive ruins as a portion of an Etruscan terracing wall (2nd century BC).

The complex is extremely articulated, so it was necessary to correlate external and hypogeal internal spaces; the site is also characterized by the existence of numerous archaeological traces of various origins, resulting from the succession of events developed over the centuries and countless works that have been integrated, juxtaposed and in some cases replaced by previous ones. All this circumstances contribute to make not easy the understanding of the planimetric and altimetric relationships existing between the different parts and the identification of the real context of the architectural elements is equally complex. The need to connect different areas of the monumental complex located at different levels (from hypogeal rooms and vaults to the church, the attic and the external squares and streets) has required a coordinated and integrated use of geomatic techniques such as precision GNSS positioning (outside the building) and the creation of a very articulated three-dimensional geodetic network connecting the external GNSS vertices with the internal reference points and targets placed in the different areas to frame in a unique global datum the subsequent detailed surveys performed with LIDAR and photogrammetric techniques, so that the single scans and local surveys could be assembled to form a unique 3D model. Among the many aspects highlighted, in particular, it was possible to understand the genesis of the cathedral transept, whose size was dictated by an imposing Etruscan wall. Until now the ruins of the ancient cathedral complex were known - referring to three different buildings, the cathedral, the dodecagonal bell tower and the chapel of Sant'Ercolano - incorporated into the side of the basilical body in front of Piazza IV Novembre. From additional ruins attributed to the structure of the ancient cathedral, it was obtained that the level of the floor of the church has been substantially maintained in the current cathedral.