## 5 Discussion
Other problems, however, do still exist, the main one being the presence of \"holes\" in shadow areas; further, UAV do not typically go through the vegetation, so that in forested areas the percentage of uncovered zones might become significant (this latter drawback being in part common to laser scanner surveys as well). The first problem is not easily solved [119]: even paying attention to the best time during the day for flying, depending upon the light and cliff exposure conditions, there might be sectors of the areas always in the shade (overhanging walls, etc.). A possibility, which however requires longer times, in both acquisition and elaboration phases, is using data from two or more flights, taken at different hours and in different light conditions. Vegetated areas, on the other hand, can be a real obstacle when it is too dense to obtain an accurate view of the ground. Using Lidar sensors is probably the best way to overcome such drawback. Other negative aspects about UAV are the impossibility to use it in no-fly zones (densely urbanized areas, landing corridors near airports, military areas, etc. ); lower accuracy (1-2 to some cm, depending upon technical features of the camera and distance from the object), also in function of the impossibility to keep a constant distance from the cliff; survey heavily conditioned by the meteoric and climatic conditions (wind, humidity, sun light, etc. ); long elaboration times for post-processing, requiring robust and highly performing hardware (most recent generation of i7 processor, 64 Gb RAM, 6 Gb dedicated video card). Given all the above considerations about individual LTS and UAV surveys, we are convinced that integrating the point clouds acquired by UAV to that produced from TLS allows to get a complete 3D cloud: this may eventually include also data difficult to be acquired from fixed measurement positions (a feature typical of TLS acquisitions), such as horizontal terraces, flat roofs of buildings, and areas hidden by obstacles. The integration is also extremely useful in those cases where it is not possible to perform surveys from the land, or to reach the upper sectors of rock faces, and the top of mountains, ridges, and hills, due to complex morphology and topography. Based upon our experiences, we conclude that:  
Discontinuity analysis by means of remote sensing techniques results in good outcomes through the semi-automatic extraction of data, thus allowing to solve significant logistic problems encountered by the classical survey methods;  
It is possible to perform a geo-structural characterization of the whole area to study, rather than proceeding site per site. This results in a relevant increase in number of data, useful for probabilistic and statistical elaborations;  
The combined procedure allows to provide a large amount of high-resolution data about a variety of parameters that are fundamental for the proper design of stabilization works. Among these,in particular, spacing and persistence of the discontinuities are two main key features in the estimation of the block volume;  
Possibility to highlight the most significant sectors of the investigated areas, where to carry out geomechanical scanlines by geologists/rock climbers, in order to directly acquire parameters such as Joint Compressive Strength, filling materials, etc. ;  
The combined use of UAV and TLS is recommended wherever the logistics do not allow to establish TLS scan positions at different locations, and, even more, for frontal views toward the discontinuity planes;  
Increase in the safety of the operators working in the field;  
Less time required for data acquisition (this is a very significant, if not strategic, point in settings as tunnels and quarries);  
Significant reduction in the costs.