UAVs for the analysis of geometrical deformation of fortresses and castles. The case study of Sora Castle

The use of 3D digital survey techniques, such as laser scanning or digital photogrammetry, today represents a real advantage for managing the complexity of the architectural heritage at different levels of detail and scales of representation. As widely recognized, SfM techniques allow obtaining detailed information on architectural objects thanks to the return of reliable geometric models to perform various analyses, including any structural deformations. Furthermore, the UAV tools can also detect architectural elements, which would require much time with a traditional approach, thanks to the possibility of reaching places that are difficult to access, such as ruins perched in disadvantaged areas. In addition, as highlighted by the Restoration Charters, in enhancing a cultural asset, the analysis and understanding of the close relationship between the architectural object and the surrounding landscape are fundamental for defining its main peculiarities. To this end, aerial photogrammetry with drones is of great help. It also lets us investigate and model the landscape of which the architectural object is an integral part, offering the possibility of fast and automated acquisition of three-dimensional data. This paper aims to analyze complex and coherent architectural structures, such as castles and fortresses, and the landscape context in which they are to define their relationships. The procedure is applied to the case study of San Casto castle in Sora, Lazio. The castle takes its name after the mountain on which it stands, Monte San Casto, an arduous place on the historical border between the Kingdom of the Two Sicilies and the Papal State. An interesting defensive system characterizes this border, composed of numerous castles and fortresses, whose spatial conformation depends on the territory's orographic characteristics and the particular “visual game” between them. San Casto castle was the fulcrum of this defensive system around which were satellite fortresses, such as that of Balsorano, Roccavivi, Isola del Liri, and to a lesser extent, Vicalvi and Alvito. It helped to integrate the defenses of the passage of Sora, known as the door between the two Kingdoms. Today, some of the forts of the middle Liri Valley are totally neglected. Unfortunately, many of them present a significant gap in the historical-architectural data. The paper describes the first results of the analysis of the castle. In particular, the architectural analysis is performed through a geometric/metric algorithm that integrates the UAV cloud point model's longitudinal and transverse sections, defining the bastions' geometric deformations. Moreover, the landscape analysis obtained with the aerial drone survey shows the relationship between the territory's spatial conformation of structure and orography. The procedure adopted in the case of the castle of San Casto will be applied in the future to the other fortresses of the defensive system.