## 5 Discussion
### Methodological Aspects  
Even though the application of the method can have some limitations (e.g., the age of structures should be kind of'recent', the research area should be well exposed with a low vegetation coverage, basic geological researches should be completed ahead of the survey), our work confirms the usefulness of the UAV-based SfM as a support for classical fieldwork to collect massive amounts of high resolution quantitative data, here aimed at reconstructing late Pleistocene-Holocene deformation in an active volcano-tectonic region, being here a key area within the active Krafla rift. At a general level, the use of UAVs in geoscience has increased over the past decade [92; 93; 94; 95; 96; 97; 98; 99; 100; 101]. The affordability of such method is one of the main reasons why the geosciences community broadly adopts it: UAV data acquisition represents, in fact, a less expensive alternative to other techniques, such as Terrestrial Laser Scanning (TLS), Airborne Laser Scanning (ALS) and LiDAR [102; 103]. Another reason for which this methodology has been broadly used is the possibility to save time with respect to fieldwork, especially in vast areas and along structures that can be tens of km long [15; 104], and to enable to reach key geological sites that are generally inaccessible because of hard logistic conditions, like volcano edifices affected by caldera or lateral collapse [105; 106; 107]. Furthermore, the rapidity of data capturing, the quick processing and the high accuracy of the output model generated through the UAV-SfM technique allow to observe at a centimetric detail the characteristics of the ground surface and outcrops, for example by catching also micro-topographic variations. As an example of the great amount of measurements that can be collected through this method, [PERSON] et al. [17] were able to collect 1098 measurements at 387 sites in a volcano-tectonic field, comprising azimuth, dilation and opening direction of extension fractures by using a commercial-grade UAV and applying SfM algorithms, thus providing a contribution to the reconstruction of the tectonic evolution in such an active rift zone in NE Iceland. Another advantage of this methodology is represented by the continuous improvements over the years of the consumer-grade UAVs, in terms of flight stability, battery capacity and camera quality, owing to the constant release of new products: for example, in this work we used the DJI Phantom 4 Pro (20 Megapixel and ISO range 100-12800), which has been released less than a year later with respect to the DJI Phantom 4 (12 Megapixel and ISO range 100-1600). This implies that in the next years, owing to such fast improvements of the UAV technology, geological studies will achieve better quality by flying at a higher altitude and covering a wider area. Finally, the SfM technique could be applied with a low-cost workflow, particularly suitable for field research, which allows any user to process the data [108]. Through a methodological point of view, we tested two novelties in our approach: (i) we slightly decreased the image overlap to save time during each single flight mission. This overlap decrease resulted in 30% of saved time. Considering our overall survey, we saved about 2 h, which is plenty of time in areas usually affected by wind and rain. (ii) We mainly used natural, ecological, or already available artificial targets for GCPs collection, this action halved the time usually necessary for this fundamental task. We obtained excellent models as output where we collected very high-resolution structural data, especially regarding piercing points along extension fractures. Of course, both novelties can be applied to further researches and areas. In addition, the area of this research is characterized by an overall morphology that is more complicated than in [PERSON] et al. [17]: in fact normal faults have a different morphological expression due to the age and characteristics of volcanic units.