3D Documentation and Risk Assessment of Sicilian Fortified Heritage: Integrated Low-Cost UAV Imagery and Videogrammetric Protocols

Sicilian fortified heritage is a rich and widespread system, yet at the same time fragile and exposed to risk conditions determined by the geomorphological complexity of the sites, local slope instability, erosion, difficult accessibility and the progressive decay of masonry structures. Castles, towers and defensive outposts are often located on elevated ground, urban margins, steep slopes, rocky outcrops or coastal fronts, in close relation to the historic landscape and to territorial settings that are now subject to increasing vulnerability. These conditions require rapid, sustainable and replicable documentation procedures, capable of supporting knowledge of the monument and a preliminary assessment of conservation issues. The paper proposes an integrated low-cost protocol for preliminary 3D documentation and for supporting the risk assessment of this valuable heritage. The protocol combines low-cost UAV imagery, non-professional video sequences, critical frame selection, SfM-MVS photogrammetry, data quality control and the production of knowledge-oriented three-dimensional models. Particular attention is given to the reuse of images and videos produced by non-specialist users, a vast, still underexploited visual archive that, if interpreted through explicit procedures of selection and validation, can support the 3D documentation of monuments at different stages of transformation. The experimentation concerns three case studies: Castelluccio di Gela, Castello di Agira and Rocca di Aci Castello. These were selected because they represent three different conditions, allowing the transferability of the workflow and the usefulness of 3D models to be tested for the preliminary reading of structures, decay phenomena and vulnerability conditions.