## 2 Materials and Methods
### Equipment Used  
In Romania, accessing and using GNSS corrections from permanent stations requires access to the ROMPOS state network or another private network from the existing ones. In our case, we used the GNSS corrections received from the ROMPOS network managed by the National Cadaster and Real Estate Advertising Agency (ANCPI) [41]. For additional precision of the results, it was necessary to coordinate the determination of ground control points (GGPs), and for this purpose, a GNSS Geomax (Geomax Widnau AG-Switzerland) Zenith 10 system with the Getac controller was used [42]. In order to obtain a better precision of the coordinates of the points, corrections were received using the GSM modem connected to the Internet to receive the RTK (real-time kinematics) corrections. A number of six ground control points were determined, with known coordinates, three on one side and three on the other side of the river, as shown in Table 1.  
Both the number and the position of the ground control points (GCPs) were chosen taking into account the specialized literature, which describes a minimal number of 4 control points for areas smaller than 20 hectares, as is the case of this study area [43; 44; 45; 46; 47]. The DJI Phantom 4 drone was used (produced by DJI, Shenzen, China), having the advantage of being compact, with high precision, and being frequently used in low-altitude flights. The manufacturer has come up with a revolutionary technology for this model compared to old systems, with the aim of achieving the highest level of precision, facilitating pilots with a reduced number of control points on the ground, compared to earlier solutions (Figure 2). The camera has a 1/2.3\" CMOS sensor effective pixels of 12.4 M, an ISO range of 100-3200 (video) and 100-1600 (photo), an electronic shutter speed of 8-1/8000 s, and an image size of 4000 \(\times\) 3000. The first step while conducting this work was the lifting of the elements from the land. Taking into account one of the most valuable features of topography in the case of floods, namely, the rate of precision related to the speed of results, the authors chose to conduct the measurements for this case study with the help of photogrammetric methods, according to the UAV (Unmanned Aerial Vehicle) technique, a method increasingly used in recent years because speed, accuracy, and precision are the key elements that offer the greatest potential in the development of topography; digital methods offer more possibilities than classical ones [48]. For the good profitability of the work, GNSS-type systems were also used. The connection of these systems to the satellites was performed in advance with the help of the ROMPOS national network. \begin{table}
\begin{tabular}{c c c c} \hline
**GCP No.** & **North (X)** & **East (Y)** & **Elevation (Z)** \\ \hline
1 & 584,306.6158 & 384,245.8419 & 367.2239 \\
2 & 584,301.8881 & 384,517.4902 & 366.8630 \\
3 & 584,680.0783 & 384,706.8169 & 365.8707 \\
4 & 584,211.9466 & 384,228.9209 & 368.1579 \\
5 & 584,207.2164 & 384,231.7458 & 368.0768 \\
6 & 584,337.3075 & 384,685.4259 & 366.7049 \\ \hline \end{tabular}
\end{table} 
Table 1: List of GCP (ground control point) coordinates. Coordinate system: Romania, Stereo 70. Reflective targets were used to mark the six control points (Figure 3). There were two groups of points: the first three control points were marked on the left bank of the Somes (from where the drone took off) and the other three on the right bank of the Somes. In places where the targets could not be mounted due to concrete, a special spray was used to mark the topographic points in a bright color so that they could be easily seen in the aerial photographs taken with the UAV technology. To acquire the data necessary for the case study, it was necessary to make two flights. The first flight covered the area in the immediate vicinity of the dam and the second flight covered the dam itself, it being necessary to perform the simulation of the dam cracking.