## 3 Methods
### Drone Surveys and Data Collection  
Our approach, based on the application of SfM photogrammetry techniques through drone surveys (Figure 2b), has been made possible thanks to the DJI Phantom 4 Pro (Figure 2a), a versatile quadcopter, which, according to previous studies [17; 19; 63], has been proved to be suitable for surveying volcanic terrains with difficult logistic conditions (e.g., outcropping lavas). This UAV is equipped with a 20 Megapixels camera and an integrated Satellite Positioning System GPS/GLONASS (referred to the WGS84 datum). Metadata of the collected images are stored in EXIF files (Exchangeable Image File Format) which include information such as shutter speed, apertures, ISO and GPS coordinates (Datum WGS84). Figure 1: (**a**) Volcanic systems of Iceland (in red), modified after [62]: EVZ = East Volcanic Zone, CVZ = Central Volcanic Zone, NVZ = Northern Volcanic Zone, WVZ = Western Volcanic Zone. Reference system: WGS84, geographic coordinates. (**b**) Geological map of a portion of the KFS where the study area is shown as a dashed white rectangle, based on [39]; LGM = Last Glacial Maximum; reference system: UTM28-WGS84. Before starting all UAV surveys, a reconnaissance of the selected area has been done to locate any potential terrain obstacle and to evaluate the potential flight path accordingly. After this first visual evaluation, we created a polygonal shapefile in a GIS environment covering the target area, tailored on a satellite image, and we divided it into nine subareas that covered all the features that we wanted to map: extension fractures and normal faults affecting the hyaloclastites and post-LGM lava units (Figures 1 and 2c). The extension of each subarea has been defined to use a maximum of two UAV batteries for each area, considering a flight height between 80 and 95 m to reach a ground resolution of 2-3 cm for the orthomosaic. Shapefiles were converted into KML format and imported in DJI Ground Station PRO ([[https://www.dji.com/ground-station-pro](https://www.dji.com/ground-station-pro)]([https://www.dji.com/ground-station-pro](https://www.dji.com/ground-station-pro))), a free iPad app for DJI drones operations. This app allowed to generate nine different missions and to define and change for each of them the flight path direction, the flight height, as well as to fix camera parameters and directly check data acquisition on the iPad display. In fact, for all the surveys, fixed acquisition parameters were set (Table 1) to obtain proper images alignment and to reduce the distortions on the resulting models: (i) The overlap has been slightly decreased, with respect to previous authors [17; 64; 65; 66; 67], to 85% along the path and 80% in lateral direction, in order to save time during each flight mission; (ii) photos have been captured using equal time interval mode and using constant velocity [17; 68]: these two parameters are automatically managed by DJI Ground Station PRO in order to guarantee the set-up photos overlap at the given flight height. Additionally, a series of environmental conditions have  
Figure 2: (a) The quadcopter used in the present work. (b) Overall workflow used in the present research to generate the 3-D model, the DSM and the orthomosaic using SfM photogrammetry approach. (c) Unmanned aerial vehicle (UAV) flight paths (yellow line) above the nine subareas surveyed between 80 and 95 m height. White dots indicate the 37 GCPs collected to georeference the model. Satellite image is from Sentinel 2 (European Space Agency—ESA, [[https://sentinel.esa.int/web/sentinel/missions/sentinel-2](https://sentinel.esa.int/web/sentinel/missions/sentinel-2)]([https://sentinel.esa.int/web/sentinel/missions/sentinel-2](https://sentinel.esa.int/web/sentinel/missions/sentinel-2))), reference system: UTM28-WGS84. been taken into account: for example, the sun orientation, which is the same for all surveys (sun to the zenith) since they have been performed in time sequence, and the light conditions, which were always optimal and suitable for the camera ISO range (100-1600).