## 4 DSM, Orthomosaic and VR Scenario Building
### Photogrammetry Processing for Aerial-Captured Photos  
In order to generate a pre-excavation digital surface model of the mine site, acquired from the Hellenic Military Geographical Service (HMGS) historical archive, as they collected national aerial photos, using KC-I(B)sensor for the whole Greek Territory for military purposes.  
As far as the Santorini island complex is concerned, the aerial photogrammetric survey took place on 26 July 1960, a year when excavations had not yet begun, while the area where the Metaxa Mine is located, appears in two aerial photos of 1:30,000 scale (Figure 8a,b). These photos were then scanned by means of a Leica Photogrammetric scanner DSW700 in 1600 dpi having a ground pixel dimension of 0.51 m.  
The whole processing to obtain the DSM and the orthomosaic model of the area was made on ERDAS IMAGINE v15 ([[https://www.hexagongeospatial.com/products/power-portfolio/erdas-imagine](https://www.hexagongeospatial.com/products/power-portfolio/erdas-imagine)]([https://www.hexagongeospatial.com/products/power-portfolio/erdas-imagine](https://www.hexagongeospatial.com/products/power-portfolio/erdas-imagine))). Firstly, inner orientation of the images was restored, using parameters of the lens. Then, six Ground Control Points (GCP) were used to georeference the two images; three were triangulation points recognized on the images, measured with a GNSS receiver in static mode, with 1 cm accuracy in WGS84/UTM35 reference system, and the others were corners of old buildings, recognized at new orthophotos of the area, two of which with 0.5 m accuracy using ortho images with pixel size of 0.2 m and the third one with 2-3 cm estimated accuracy measured with GNSS receiver in RTK mode, for other projects [67]. Respectively, z values were measured according to tide gauge existing in the Port of Thira, having for the triangulation points an estimated accuracy of 3-4 cm and 5-10 cm for the one measured, whereas the rest of the points were estimated through aerotriangulation before being used as ground control points with an error of 0.22 and 0.22 m, respectively.  
The alignment had automatically generated 116 Tie points with maximum uncertainty value of 0.666 m, which were used to orthorectify the created model and produce a DSM with a pixel size of 1 m (Figure 8c). The final accuracy of the produced DSM, from the above processing, is estimated to be at 2 m horizontally and 1 m vertically, while the produced orthomosaic has a pixel size of 1 m (Figure 8d,e). In Figure 8e,f the orthomosaic of 1960s stereo images and generated digital surface model of the area are presented.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & Alignment processing accuracy & High & & & \\  & & Dense Cloud processing accuracy & Medium & & & \\  & & Dense Cloud Points & 132,527,363 & & & \\  & & 3-D Tiled Model Resolution & 0.9 cm/pix & & & \\  & & DSM Resolution cm/pix & 3.7 cm/pix & & & \\  & & Orthomosaic Resolution cm/pix & 0.9 cm/pix & & & \\ \hline  & & & & & **SMM Photogrammetry Processing Time (hours)** & & \\ \hline
**Images Acquisition** & **Alignment** & **Dense Cloud** & **3-D Tiled Model** & **DSM** & **Orthomosaic** & **Overall Processing** \\ \hline
3.7 & 15.68 & 4.50 & 13.10 & 0.10 & 2.50 & 39.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of resolution and time processing for DSM, Orthomosaic and 3D tiled models derived using photogrammetry.