## 2 Materials and Methods
### 3D Data Modelling
#### 2.4.1 Synchronisation  
The synchronisation was performed in the open-source software QGIS (QGIS Development Team) using the geoprocessing algorithm of joining attributes between fields (ID algorithm: 'nativejoinattributestable'). Some secondary data processing was performed using spreadsheets in software such as Excel (Microsoft, Redmond, Washington, USA) and Libre Calc (The Document Foundation).  
The synchronisation followed:  
1. The File Sonar was synchronised with the File SPP via the latitude and longitude fields, the new file was named \"File Sonar + SPP\" (through this synchronisation, the File Sonar was associated with the GPS Time value of the system);
2. The File Sonar + SPP was synchronised with the File PPK via the common GPS Time field. Figure 5a shows a detail of the pre-synchronisation File Sonar + SPP in orange and the File PPK in red. The new file produced was called \"File Sonar + SPP + PPK\" and contained the information on latitude, longitude and ellipsoid height corrected  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Area** & **Transects (no.)** & **Device** & **File Produced** & **Positioning** & **Points** & **Main Data** \\ \hline \multirow{4}{*}{\(\sim\)30,000} & \multirow{2}{*}{\(16+1\)} & \multirow{4}{*}{SimpleRTK2B} & \multirow{2}{*}{File SpP} & \multirow{2}{*}{5098} & \multirow{2}{*}{1659} & \multirow{2}{*}{Latitude, longitude, depth} \\  & & & & & & \\ \cline{1-1} \cline{5-5}  & & & & & & \\ \cline{1-1} \cline{5-5}  & & & & & & \\ \cline{1-1} \cline{5-5}  & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 7: Datasets for 3D modelling of the seabed (natural surface).  
Figure 4: Implementation of the pipeline for 3D modelling of bathymetric data derived from the G \(>\) S \(>\) P Sys.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{} \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 8: Datasets for 3D modelling of the pool (artificial structure).  
to PPK, depth of water and time of point acquisition. Figure 5b shows a comparison between the File Sonar + SPP + PPK in green and the File PPK in red.  
Generally, after the synchronization, the File Sonar + SPP + PPK may be processed as follows:  
1. Correction of the offset between the GNSS antenna and sonar transducer;
2. Correction of the offset between the sonar transducer and keel line;
3. Selection of the vertical reference system, for example, tidal observations (Mean Sea Level--MSL, Lowest Astronomical Tide--LAT, etc.), physical model (geoid) or reference ellipsoid [17];
4. Correction of the water level variations.  
In this specific case study, the planimetric offset between the GNSS antenna and sonar transducer was originally annulled by the floating platform. Concerning the vertical reference, it was decided to use a physical reference on land, that is the tide gauge of Taranto, part of the National Mareographic Network and located in the Mar Grande a few kilometres from the surveyed site. The water level variations corrections were deduced from the tide gauge data. In order to be able to subsequently process the dataset in CloudCompare (DF R&D/TELECOM Pari-sTech ENST-TSI, Paris, France) [39], the GNSS coordinates of the surveyed points were converted into the WGS 84/UTM 33N system with the VERTO software (Istituto Geografico Militate--IGM, Florence, Italy).  
Finally, a random sample of 1000 points (approximately 60% of the surveyed points) was extracted for 3D modelling; the remaining 40% of the points were used in the model validation phase.