## 2 Materials and Methods  
### Survey through Laser Scanner 3D  
3D terrestrial laser scanner (TLS) is an innovative methodology to model and monitor rock cliffs and underground environments. The method allows to survey and reproduce the measured surfaces with an acquisition mesh at sub-centimeter resolution. The outcome is a snapshot of the observed scene at the date of acquisition, expressed by millions of georeferenced points; it is easily navigable and shareable through cloud-computing. The first phase of the survey consists of a careful and detailed analysis of the area, and in the selection of the scan positions, that is the sites where the instrument has to be located; further, based upon the distance between the laser and the object, the resolution of the scan is defined, taking into account the detail required in the output (in turn, depending upon the aim of the work). This activity is mandatory to the survey design, aimed at locating the control points (targets). To reach the best results in terms of final outcome, it is necessary that the scan positions are such that the laser beam is always perpendicular to the main discontinuity sets. In this way, it is possible to have a complete and homogeneous point cloud, without void spaces, and where all the planes of the scanned object could be perfectly recognized when varying the orientation of the model in space. The survey, carried out in the case studies of this work with a Laser Scanner RIEGL VZ400 (Figure 1), allows acquisition of a georeferenced point cloud of millions of points, each one of which is described by the following information: geographical position in space (X, Y, Z), chromatic information (RGB), and reflectance (i). During the survey, multiple scans are taken at each scan position, with variable resolution: initially, an overall scan with wide mesh is obtained to get a file that is easy to manage, and to have an immediate control of the wide surveyed area; then, several other scans are produced with progressively more  
Figure 1: Main characteristics of the Laser Scanner Riegl VZ400. accurate meshes. During the phase of setting the detailed scans, the data regarding photographs are set too, that include the exposition time and the diaphragm aperture, so that the digital images to be linked to the point cloud are properly calibrated, based upon the variable light conditions and the closeness to the survey object. At the end of each scan, before changing the scan position, a detailed scan of the targets is carried out to allow the scanner to localize and record the installed reflectors, which will be used as tie points and ground control points in merging and georeferencing the point cloud. The data acquired with the laser scanner are elaborated with dedicated post-processing software, to merge the scans at the different scan positions, and to clean the model from untrue elements to eventually move toward the 2D and 3D elaborations [51]. The elaboration phase further includes assignment of colors to the point cloud, linking to each X, Y, Z point also the coordinates R, G, B, derived from the images acquired during the acquisition phase. This procedure occurs through alignment techniques, both manual and automatic, between the photos and the scans. At this point, it is possible to move to merging and roto-translation of the several acquired point clouds. This operation is of crucial importance; aimed at obtaining minor errors, it is necessary to merge the cloud points through high-reflectance targets, georeferenced by means of GPS and high-precision total station. At the end of these operations, the error deriving from merging the scans is evaluated. In case the error results to be unacceptable, \"forced\" alignment techniques, defined multi-station-adjustment (MSA), are applied. During the adjustment phase, it is possible to bypass the coordinates of targets, and to use the coordinates of the filtrated point clouds (polydata), properly created for each scan.