Terrestrial remote sensing techniques for the preservation of cultural heritage sites from landslide hazard: the case of San Leo rock slab (northern Apennines, Italy)

San Leo is the most famous among the towns and castles of the historical region of Montefeltro, in northern Apennines (Italy); it combines relevant historical and artistic assets and impressive geological and geomorphological features. This village was built, in medieval period, on the top of a calcarenite and sandstone slab, bordered by subvertical and overhanging cliffs, up to 100 m high, affected by lateral spreading with associated rock falls, topples and tilting. The underlying clayey substratum is involved in earth flows and slides. With the aim to describe the structural feature of the slab and the ongoing deformations a complete Terrestrial Laser Scanner (TLS) survey all around the San Leo cliff was performed, georeferenced by GNSS measurements ; then,a structural analysis of the rock mass has been carried out using the 3D point clouds. Moreover photogrammetric and traditional structural surveys have been carried out in selected areas of the cliffs, to integrate and validate the TLS dataset.