## 4 Case Studies
### Santa Caterina
In this specific case, three georeferenced flat targets (4 cm diameter plates) and four cylinder targets (10 cm diameter) were used. The laser survey consisted of two phases. The first was carried out during the early morning at the pier located east of the cliff, by performing two scans at 80 m distance from the cliff, for a temporal duration of 1 h and 45 min. This procedure allowed to acquire a 2000 pt/m\({}^{2}\) point cloud. Then, a third scan was acquired from the beach, 50 m far from the cliff. This second phase lasted 50 min, and resulted in the acquisition of a point cloud with density of 2000 pt/m\({}^{2}\). The two phases of survey allowed to characterize the eastern side only of the cliff. Then, to get a point cloud adequate to be worked on with geo-structural aims, it should had been necessary to locate the scanner in frontal position with respect to the main discontinuity systems of the rock mass, aimed at avoiding the presence of \"dark points\" within the point cloud. In the specific case, however, it was not possible to have the scanner in the western sector of the cliff, due to logistic conditions. For this reason, the laser scanner survey was then integrated with a photogrammetric survey by SAPR, carried out by means of the UAV Italdron 4 HSE PRO. For this survey, too, the acquisition phase was preceded by inspection visits to investigate the best conditions of light exposition of the cliff, and to establish the strategies to avoid interference with seagulls. The survey was carried out at noon, since this time during the day was the best to guarantee direct light on the cliff, and absence of shadow areas. It was performed in two flights, each one lasting 20 min, to take both frontal and zenithal views of the cliff. The UAV survey, beside covering the western side of the study area (not acquired by the LST survey), overlapped also part of the LST point cloud, eventually resulting in a detailed cloud with density of about 3200 pt/m\({}^{2}\). Regarding this latter survey, to obtain a high-detailed pixel resolution at the ground, needed to perform geo-structural analysis on point clouds, the flight was carried out at maximum distance of 40 m from the cliff. This allowed to obtain a ground resolution of 5.6 mm per pixel. During the survey, both zenithal and oblique images were produced, with overlapping percentage always greater than 80-85%. Flight characteristics are as follows:  
* Sensor size \(35.8\times 23.9\) mm
* Average distance from the cliff: 40 m
* Ground resolution: 5.6 mm/px
* Overlap H %: 80%* Overlap L %: 80%
* Photo height (m): 27.31 m
* Photo width (m): 40.91 m
* Photo spacing (m): 5 m
* Interval (m): 16 m  
In order to obtain a high-definition, properly geo-referenced, point cloud (indispensable condition to perform geo-structural analysis on discontinuity of the rock mass), the manual modality of flight was chosen. This was because experience has demonstrated the higher reliability of this modality against the automatic one. It was also decided to acquire photos with high depth of field; therefore, the shoot of the camera was set with ISO priority, keeping blocked both aperture and time during the flight. In addition, the flight direction was chosen according to attitude of the main discontinuities of the rock cliff. Georeference of real and fake waypoints was carried out. Fake waypoints are represented by a multitude of points within the georeferenced laser-acquired cloud, well identifiable on the screen. This allowed to operate sophisticated roto-translational alignment procedures with the software 3 DReshaper, to obtain a cloud at very high resolution, well positioned in space. In this case study, integrating the point cloud obtained by the laser scanner with that acquired by drone was possible thanks to the very scarce presence of vegetation on the cliff.