## 4 DSM, Orthomosaic and VR Scenario Building
### UAV Survey, Pictures and Ground Control Points Collection  
We set out the 3D reconstruction of the Metaxa Mine by applying the SfM photogrammetry techniques; pictures were collected using drone surveys (e.g., [27; 57; 58]), with the camera oriented both in Nadir and Oblique settings, as shown in Figure 5a,b. In this work, we used the DJI Phantom 4 Pro, a versatile quadcopter equipped with a 20 Megapixels camera and an integrated Satellite Positioning System GPS/GLONASS (referred to the WGS84/Geographic coordinates reference system). 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.  
Figure 4: (**a**) The first version ofs Virtual Reality (VR): the Sensorama, images are free of use from [PERSON] (1962), US Patent #3,050,870. (**b**) Example of a geologist surveying the territory using GeaVR (Credits: [PERSON]). (**c**) Navigation modes selection during survey in Immersive Virtual Reality (IVR) using GeaVR software, from the left to right: walk, drone and airplane mode. (**d**) All tools available in GeaVR, regarding mapping, field survey and Global Position System (GPS) tools.  
The first set of pictures (\(n=3896\); Figure 5c) were collected with nadir camera orientation (orthogonal to the ground; Figure 5a), following the workflow presented by [PERSON] et al., (2019, 2020) [27; 58], where the flight missions over the target area are designed and managed using the 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. We set up each single mission, defining the areal extent, flight path direction, flight height (m above the take-off point), camera parameters and pictures overlap, and directly checking data acquisition on the iPad display. The overlap ratio has been set to 90% along the path and 85% in the lateral direction (e.g., [27; 32]). Pictures have been captured using the equal time interval mode and using constant flight speed [27; 58]. The flight height was set up to 25 m, to reach 1 cm/pixel of ground texture resolution; the flight speed was set up automatically by the software in compliance with all other settings. All the above steps  
Figure 5: Example of pictures captured during the drone survey with the camera oriented in Nadir (**a**) and Oblique (**b**). (**c**) Summary of the overall surveyed area: each subarea is represented by white boxes, Nadir and Oblique captured images are represented by blue dots and yellow squares, respectively. Reference system: WGS84/UTM35.  
were managed directly in the field. We completed eight flight missions/areas (Figure 5) for this step; each was run using one UAV battery, totaling 2.5 h of flying time; further details are shown in Table 1.  
The second set of pictures (\(n=1898\); Figure 5c) were captured with oblique camera orientation (Figure 5b), to add details to the vertical cliffs of the mine, resulting from excavation processes. In doing this, the quadcopter was manually operated, maintaining a constant speed of about 1-2 m/s, and the pictures were automatically taken every two seconds using the DJI GO 4 app ([[http://www.dji.com/](http://www.dji.com/)]([http://www.dji.com/](http://www.dji.com/))). The camera was oriented from mostly orthogonal (oblique) to vertical cliffs (between \(-45^{\circ}\) and \(-70^{\circ}\)). This task was performed in four missions, totaling 1.2 h of flying time (details in Table 2).  
Finally, we collected 34, uniformly distributed, Ground Control Points to co-register the photogrammetry outputs (3D model, DSM and Orthomosaic) to the WGS84/UTM35 reference system (e.g., [59; 60]). This task was performed using Emlid Reach RS single frequency receivers in RTK configuration (with centimeter-level accuracy), expecting centimetric accuracy (e.g., [61; 62; 63; 64]).