## 5 Discussion
### Methodological Aspects
Finally, the presence of the Hituholar volcanic edifice, with an elevation of more than 100 m from the surrounding area, has made it more challenging to plan and execute UAV missions. Even so, we reached excellent resolution and accuracy in all SfM outputs, allowing us to collect all data necessary to define spreading direction and stretch in the studied area, enhancing the importance of this methodology in this kind of studies. The UAV-based SfM also presents a few limitations, such as the dependence on the battery life/flight time, weather conditions and legal flight restrictions. Also the necessity of positioning some GCPs, in order to reference the model accurately, could be time spending and thus could slow down the survey, especially over vast areas. [PERSON] et al. [109] recommended the use of at least 3 GCPs; other authors used a higher number of GCPs (up to 95 across a 100 m grid), but reducing the image overlap to 60-70% [110]. With a greater overlap, on the other hand, a lesser number of GCPs can be used [68; 111]. The number of GCPs can also be reduced if the geometry network results sufficiently strong [72] or by using an UAV incorporating onboard RTK-GNSS sensors that would allow to generate models with centimetric spatial accuracy without placing GCPs. In our work, we used 37 GCPs, both along the corners and at the centre of every area, in order to reduce the 'doming' effect and reduce systematic errors [69; 70; 71; 72]. Another strategy to reduce the 'doming' effect on the resulting DTM/DSM [73], consists of collecting aerial photos with an oblique inclination with respect to the ground [112]. Nevertheless, this strategy results in greater consumption of the battery power and so it is useless for commercial UAVs equipped with low-cost GPS chipset, where GCPs collection is crucial to georeference the model; it is thus more efficient to design a GCPs survey as proposed in the present work. We thus selected our flight strategy to reach a good compromise between battery consumption, survey time, area extension, data resolution and accuracy of the models. In just two days of fieldwork and almost 46 h of processing of about 7000 UAV pictures, we mapped an area of 232 Ha (2.3 km\({}^{2}\)). Moreover, the achieved resolution of the orthomosaic (2.6 cm/px) and the DSM (10 cm/px) resulted sufficiently detailed for surveying extension fractures and collecting plenty of data of opening vectors, down to centimetric values of dilation. This high accuracy opens the possibility to carry out repeated UAV surveys across the same area at regular time intervals, in order to study the present-day deformation and observe its evolution over time. This could be applied, for example in the KFS, by comparing the resulting models (orthomosaics and DSMs) before and after a rifting event, if the superficial deformation is larger than the model resolution. Regardless, additional improvements are suggested to define the most efficient survey approach for DSMs reconstruction, mainly devoted to measuring vertical offset at normal faults. The fast evolution of UAVs, equipped with RTK positioning system, can represent a solution for the above-presented limitations, increasing the accuracy of the model, flying also at greater heights. Finally, the resulting 3-D models can be navigated in the immersive virtual reality, useful both for scientific research and teaching and divulgation purposes [17; 31; 64; 65].