## 2 Materials and Methods
### Methods
#### 2.2.3 Photogrammetry (SfM)  
The photographs were taken from the ground with a CANON Powershot G3X camera, focal length 8.8 mm and pixel size 2.4 \(\times\) 2.4 microns. The cost of this equipment is usually around EUR 500. Data acquisition was carried out following a displacement similar to  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**GCP** & **X** & **Y** & **Z** & **E\({}_{\text{distance}}\) (mm)** & **E\({}_{\text{pixel}}\)** \\ \hline
1 & 250,501.239 & 4,165,808.807 & 30.225 & 1.7 & 0.352 \\
2 & 250,503.802 & 4,165,808.644 & 30.201 & 0.9 & 0.285 \\
3 & 250,504.121 & 4,165,808.976 & 30.199 & 0.7 & 0.175 \\
4 & 250,504.042 & 4,165,806.570 & 30.134 & 0.6 & 0.313 \\
5 & 250,504.003 & 4,165,805.965 & 30.148 & 3.6 & 0.265 \\
6 & 250,503.921 & 4,165,804.518 & 30.208 & 1.5 & 0.424 \\
7 & 250,501.001 & 4,165,804.747 & 30.251 & 0.6 & 0.559 \\
8 & 250,500.937 & 4,165,805.000 & 30.252 & 08 & 0.308 \\
9 & 250,503.342 & 4,165,807.727 & 30.177 & 04 & 0.213 \\
10 & 250,501.945 & 4,165,807.384 & 30.204 & 04 & 0.198 \\
11 & 250,502.596 & 4,165,806.482 & 30.185 & 24 & 0.289 \\
12 & 250,503.031 & 4,165,804.971 & 30.201 & 15 & 0.463 \\
13 & 250,502.005 & 4,165,805.997 & 30.748 & 02 & 0.275 \\
14 & 250,502.448 & 4,165,805.860 & 30.756 & 17 & 0.500 \\
15 & 250,502.529 & 4,165805.234 & 30.747 & 15 & 0.600 \\ \hline \hline \end{tabular}
\end{table}
Table 1: GCP Coordinates (UTM ETRS89) and errors.  
Figure 5: Example of Ground Control Points recorded (O).  
that used in the programming of a photogrammetric flight, i.e., nadir shots (see Figure 6), maintaining the necessary overlap both in rows and columns.  
The conditions for data collection using SfM techniques at the Cantillana site were excellent, since, due to the characteristics of the environment, there was diffuse light and consequently few shadows were produced. Likewise, the features of the object of study, due to its rough texture and varied colors, made it possible to obtain potentially high quality results. A total of 305 photos were taken within 30 min. The result obtained was an overlap of more than 80% (see Figure 6), where the whole mosaic had an overlap of more than 9 photographs. It should be remembered that the usual structure-from-motion (SfM) techniques need, at the least, trifocal overlaps to calibrate the cameras and reconstruct a scene [26].  
Once all the photos had been taken, they were dumped and processed with the Agisoft Metashape program, which is based on a series of automatic algorithms. The processing time for the 305 photos was 14 h. Of these photos, only 15% needed technician assistance, and the others were processed automatically by the software (see Figure 7).  
A virtual 3D representation of the mosaic done with SfM can be found at the link [[https://sketchfab.com/3d-models/mosaico-terramar-o-de-los-delfines-9d38178](https://sketchfab.com/3d-models/mosaico-terramar-o-de-los-delfines-9d38178) adae4403384f9]([https://sketchfab.com/3d-models/mosaico-terramar-o-de-los-delfines-9d38178](https://sketchfab.com/3d-models/mosaico-terramar-o-de-los-delfines-9d38178) adae4403384f9) 38 cf25 cbc51c (accessed on 11 December 2021).  
Figure 6: Data acquisition with SfM: (**A**) Taking photographs; (**B**) Overlapping of photographs.  
Figure 7: SfM Processing workflow.