## 5 Discussion
In addition, horizontal displacement is shown to be lesser and sinks more the further south the geographical position of the rock glacier (Figure 10). Up to this point, it has been the dynamic \"advancing\" behaviour of the rock glacier that has been analyzed, but its retreat must also be considered. This matter appears to be contradictory, but glacial retreat is measured in terms of the loss of surface, which can be  
Figure 10: Planimetric and altimetric comparison of mean displacement of the rock glaciers of Doesen in the Alps, Posets in Pyrenees and Corral del Veleta in Sierra Nevada. determined by measuring the glacial perimeter, using GNSS where there is no appreciable change. This retreat can also be determined through the evolution of points on the front of the rock glacier (Figure 7). Points that initially had behaviour similar to any other point on the rock glacier may lose their dynamism after some years and therefore no longer form an active part of it. Therefore, the frontal retreat taking place in the rock glacier is checked. The geophysical survey in 2009 indicated a frozen mass thickness of around 10 m (Figure 6), but since then the total station and GNSS have established a mean loss in thickness throughout the rock glacier of 2.3 m. At the end of August 2019, therefore, the Corral del Veleta rock glacier should have a thickness of 7.7 m. With this thickness and mean losses in the future similar to the annual mean since 2014 of \(-0.45\) m, in 17 years (2036) the rock glacier would become inactive. However, if global climate change predictions prove to be correct, the time it takes to reach this state may be shorter and the inactive state could come sooner. In addition to the behaviour of some elements or the volumetry of the rock glacier of Corral del Veleta, other experiments have also been carried out, such as the five profiles of the terrain using GNSS, and these same profiles using \"Restitutor\" photogrammetry, TLS, and UAV. This study is useful for checking the precision of the digital elevation models (DEM), and a comparison was made of the five profiles (Figure 5a). Specifically, the MDEs of the photogrammetry were tested using the Restitutor programme, TLS, and UAV for the measurements from 2019 under the consideration that the data collected in the field with GNSS have acceptable precision (\(\pm 3\) cm). Figure 11 shows these comparisons with profile 2, in which the errors are of \(\pm 30\) cm using the photogrammetric technique, and so the precision of the model is more than acceptable for the purposes of drawing up a photogrammetric cartography on a 1:1500 scale. Although the maximum uncertainty of the data collected by TLS is \(\pm 3\) cm and by UAV is \(\pm 4\) cm, it should be pointed out that there are areas from which information is not collected, owing to their being occluded among the rocks, with the logical result being that they fail to be measured. There are differences of \(\pm 15\) cm between the real model taken in the field with GNSS and that obtained using TLS or a UAV. Since carrying out the analytical restitutions in 2003 and later the automatic digital restitutions using \"Restitutor\" in 2008, through to the present use of MDEs obtained by UAV, the precisions of the MDEs have improved considerably and their production costs have fallen greatly.