## 2 Materials and Methods
### Methods
#### 2.2.4 Terrestrial Laser Scanning (TLS)  
The TLS methodology aimed to obtain 3D information of the mosaic to reconstruct the scanned object. This methodology has, a priori, a number of drawbacks in relation to accuracy. These are due to internal and external factors. The internal ones are due to the type of equipment model used [27]. The external factors are due to the type of object surface, i.e., the material itself, which mainly affects the angle of incidence of the TLS laser [28]. Laser scanners use different technologies for their performance. Some are time-of-flight processors, i.e., they measure the time it takes for the emitted laser beam to travel from the scanner to the object and back, also known as pulse processors. Others work by phase difference, which implies that they have a periodic base signal which is modified depending on the object upon which it hits, and the existing modular difference between the emitted and received phase is then used to determine the distance traveled by the laser. The latest technological advances in TLS have allowed for the development of a technique called Wave Form Digitization (WFD), which is based on mixing pulse and phase difference measurement technology [29]. Lasers with WFD technology emit a multitude of pulses to record a single point, from all the records of that single point they eliminate those whose signal is very different from the majority. The signals that are optimal are added together, thus obtaining the measured point [30]. A Leica Geosystems P20 based on WFD technology with linear accuracy \(\pm\)1 mm and angular, vertical and horizontal 8\" accuracy was used in this study. This equipment is able to record between 50,000 and 1,000,000 points per second (see Figure 8). The cost of this equipment is around EUR 20,000. There were three parking points with the TLS scanner, two inside the mosaic and one on the outside, since all the devices in the area related to the safety of the excavation, such as struts and structure fastenings to keep the mosaic as stable as possible (see Figure 8). The TLS parking points (Est-6 and Est-7) were at the level of the mosaic. The third scan was made from the upper part (Est-5), that is to say at the street floor level, in order to have a record from above, although a posteriori all the errors produced by the mosaic excavation support props had to be eliminated. Figure 9 shows the location of the parking lots for the three scans and that of the four targets used. The targets model GZT21, Figure 9 (D-01, D-02, D-03, D-04), were installed on their corresponding magnetic supports. The  
Figure 8: Data acquisition with TLS from different points of view. coordinates of these targets are listed in Table 2. The resolution was programmed to capture points at a step of 3 mm at 10 m. Scanning conditions are listed in Table 3.  
Once the point acquisition was finished, the points were loaded into Leica's Cyclone program. This software merges the point clouds taken from the different positions with the support of the control points, i.e., the four targets already mentioned. After all the data had been grouped into a single point cloud and correctly georeferenced, it was exported in a format readable by other programs to allow for its management and analysis. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Target** & **X** & **Y** & **Z** \\ \hline D01 & 250,504.887 & 4,165,802.485 & 34.710 \\ D02 & 250,501.280 & 4,165,802.133 & 34.706 \\ D03 & 250,500.799 & 4,165,807.835 & 34.615 \\ D04 & 250,503.795 & 4,165,808.125 & 34.728 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Coordinates of targets: D-01, D-02, D-03, D-04.