A multi-scalar approach for the study of ancient architecture: Structure for Motion, laser scanning and direct survey of the Roman theatre of Nora (Cagliari, Sardinia)

The study of ancient buildings and architectural complexes starts necessarily from a detailed and geometrically correct graphic documentation of their state of conservation that allows a complete analysis and understanding of their planar and volumetric development, as well as of their structural, material and constructive features. This is the case also of the Roman theatre of Nora, a Phoenician, Punic and Roman city situated on the southern coast of Sardinia, about 30 km west of Cagliari. The theatre dates back to the beginning of the Imperial age (1st century CE), and was unearthed in the last century by Gennaro Pesce. Despite being the only Roman theatre found in Sardinia until now, it has never been studied in depth. For this reason, in the last few years, the University of Padova has carried out a new research project finalized at realizing a detailed architectural survey of the ancient building, providing precise representations of its layout and state of conservation to be used both for research and conservation purposes. The most modern survey methodologies have been selected by a multidisciplinary team and calibrated considering the architectural features of the theatre, the context in which it is inserted and the objectives of the research. Different geomatic methodologies (terrestrial laser scanning, close range and aerial photogrammetry) have been applied by, both recovering and re-processing TLS scan data from old campaigns (2007–2008) and executing new surveys (2019). In fact in 2020 the Covid-19 pandemic implied a period of forced stop for new survey campaigns which proved to be necessary to continue the research activity also through the re-discovering and valorization of old digital data. The resulting models have provided a 3D documentation of the building, which permitted the production of plans, sections and orthophotos of the structures, and a photogrammetric architectural elevation of the external part of the cavea. All these data have then been integrated by the archaeologists with direct survey with the aim of mapping the building materials and techniques used in the theatre. Thanks to these activities is now available a set of graphical 2D and 3D data of the theatre of Nora, representing its state of conservation in the last 20 years, which could be used for the architectural study of the theatre and for future projects of conservation, restoration and virtual reconstruction of its original layout. The multi-disciplinary and multi-scalar approach applied to the study of the theatre of Nora has proven to be essential in this kind of studies, from the planning stage to the data acquisition, processing and evaluation. This contributes to set the most appropriate questions and to select the most appropriate survey methodologies, as well as to optimize the different expertise of the researchers involved in the project.