## 3 Methodology
### Geovisualization  
The analysis of the 3D model also depended on the spatial resolution of the primary data. The 3D geometry of the digital object was formed by a triangulation irregular network to which the UAS acquired images then attributed a photorealistic texture. Each surface was lined with a part of the images that corresponded to the specific surface [59]. Therefore, the desired GSD resolution of 3 cm was crucial for the detailed geovisualization of the geosite at a scale of 1:400. Thus, the 3D model chosen for visualization was derived from Scenario 4 (altitude 80 m, 90% and 70% overlap) and its corresponding panorama.  
Then, for the geovisualization of the geosite, the textured 3D model of the gorge and the panorama image were used. The 3D model was uploaded in the Sketchfab software [70] and acquired texture from the jpg file created from the high-resolution images collected with the UAS. One of the special features of the geosites' 3D models was that they represented outdoor areas. Therefore, the area of interest, when located in the outdoor space, was illuminated using as light sources (i) the sun, (ii) the sky, and (iii) the environmental lights [71]. The variety in landscape and the material reflectance created heterogeneous light sources that contributed to the light's total intensity that reflected and lightened the mapping area in the 3D space.  
For this reason, the panorama of the area was chosen to act as environmental lights and was converted to a high dynamic range image (HDRI). A sphere produced from the  
Figure 10: Results from the application of cloud-to-cloud absolute distance to 3D point clouds: (**a**) scenario 2 vs. scenario 1, (**b**) referenced point cloud, (**c**) scenario 2 vs. scenario 3, (**d**) scenario 2 vs. scenario 4, (**e**) scenario 2 vs. scenario 5, and (**f**) scenario 2 vs. scenario 6.  
panorama had its center of gravity in the 3D model of the geosite. The HDRI takes the pixel's values and uses them as a light-emitting source, partially simulating a surrounding environment. Thus, the geosite 3D model was illuminated in the same way as it was illuminated at the time of the UAS data acquisition. Figure 11a shows a part of the 3D model that has been automatically visualized, where the shadows captured in the images were maintained in its texture. After using environmental lights in Figure 11b at the same part of the geosite's 3D model, the respective shadows appeared smoother and softer.  
For the geosite's visualization in a virtual environment, and for creating the augmented map, the Sketchfab software [70] was used. The scene created in the virtual space consisted of the following: (i) 3D model, (ii) texture, (iii) lights, and (iv) primary camera ([[https://skfb.ly/oqGYY](https://skfb.ly/oqGYY)]([https://skfb.ly/oqGYY](https://skfb.ly/oqGYY)) accessed on 7 January 2021). The main camera defined the observer's position and the field of view from that exact position. We used the method of third-person viewer for the observer to illustrate the gorge's 3D model in the correct cartographic scale (1:400) on the augmented space. The observer-model distance was set to 80 m to achieve the best visualization of the geosite without unnecessary detail. Finally, an orthophoto map was used as a level on which to project the geosite's augmented 3D model (Figure 12). The AR gorge geovisualization presented in Figure 12 is accessible from any mobile phone as long as the SketchFab application and the 2D map of the study area are installed. The 3D model of the geosite is available to the public in the SketchFab repository and can be selected. Using the mobile phone's camera to point at the projection level (2D map) enhances the 3D model.  
Figure 11: Part of the 3D model of Voulgaris gorge: (**a**) illuminated by environmental lights calculated by a panorama of the area, and (**b**) without lighting source.  
Figure 12: Augmented reality map of Voulgaris Gorge, geosite (scale 1:400).