## 2 Materials and Methods
### Field Operation and Data Collection
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Processing Step/Scenario** & **Image Alignment** & \multicolumn{2}{c}{**Sparse Point Cloud**} & \multirow{2}{*}{**Model Optimization**} & \multicolumn{2}{c}{**Dense Matching**} \\  & & & **Filtering** & & **and DEM** \\ \hline \hline \multirow{8}{*}{Standard (High\({}_{1}\))} & \multirow{8}{*}{Key points: 60,000} & \multirow{8}{*}{Minimum image count: 3} & \multirow{8}{*}{\begin{tabular}{c} Adaptive camera \\ model fitting \\ \end{tabular} } & \multirow{8}{*}{\
\begin{tabular}{c} Reconstruction \\ quality: “high” \\ Depth filtering: \\ \end{tabular} } \\  & & & & \(\bullet\) & Adaptive camera \\  & & & & model fitting \\  & & & & \(\bullet\) & Rolling shutter \\  & & & & compensation \\  & & & & \(\bullet\) & Camera accuracy: \\  & & & & count: 3 \\  & & & Maximum \\  & & & & \(\bullet\) & Marker accuracy: \\  & & & & reprojection error: \\  & & & & \(\bullet\) & Depth filtering: \\  & & & Accuracy: “high” \\  & & & & \(\bullet\) & \(\bullet\) \\  & & Pair preselection: & & Maximum \\  & & & reconstruction \\  & & & uncertainty: 5 & & error (pixel) on \\  & & & & \(\bullet\) & Camera positions \\  & & & & and attitudes \\  & & & & \(\bullet\) & \(5\) CkPts/N \(-\) \\  & & & & \(\bullet\) & \(5\) ChkPts \\ \hline \hline Low & Accuracy: “low” & - & - & & DEM resolution: 1 m \\ \hline Medium & Accuracy: “medium” & - & - & & DEM resolution: 1 m \\ \hline \multirow{2}{*}{High\({}_{2}\)} & Pair preselection: & & & & \\  & “reference” & - & - & & DEM resolution: 1 m \\ \hline Highest & Accuracy: “highest” & - & - & & DEM resolution: 1 m \\ \hline \multirow{8}{*}{S-GCP} & \multirow{8}{*}{-} & \multirow{8}{*}{-} & \multirow{8}{*}{
\begin{tabular}{c} Camera positions \\ and attitudes \\ unused \\ N GCPs/0 ChkPts \\ \end{tabular} } \\  & & & & \(\bullet\) & Camera positions \\  & & & & and attitudes \\ \cline{1-1}  & & & & unused \\ \cline{1-1}  & & & & \(\bullet\) & N GCPs/0 ChkPts \\ \hline S-RTK & - & - & 0 GCPs/N ChkPts & DEM resolution: 1 m \\ \hline S-RTK-GCP & - & - & N GCPs/0 ChkPts & DEM resolution: 1 m \\ \hline S-RTK-IGCP & - & - & 1 GCPs/N\(-\)1 ChkPts & DEM resolution: 1 m \\ \hline S-RTK-3 GCP & - & - & 3 GCPs/N\(-\)3 ChkPts & DEM resolution: 1 m \\ \hline S-RTK-5 GCP & - & - & 5 GCPs/N\(-\)5 ChkPts & DEM resolution: 1 m \\ \hline S-RTK-9 GCP & - & - & 9 GCPs/N\(-\)9 ChkPts & DEM resolution: 1 m \\ \hline \hline \end{tabular} N is the number of photogrammetric targets available, and - stands for standard parameters.  
\end{table} 
Table 2: Overview of the processing parameters and scenarios used for DEM reconstruction with Agisoft Metashape. Aerial image acquisition for photogrammetry was carried out using DJI's Phantom 4 RTK quadcopter (P4 RTK) and D-RTK2 mobile station. The latter is used for drone RTK positioning. According to the manufacturer, this system allows image georeferencing precisions of 1 cm + 1 ppm (parts per million, i.e., 1 mm per 1000 m) and 1.5 cm + 1 ppm along the horizontal and vertical directions, respectively, traducing to absolute accuracies of around 0.05 m at a flying height of 100 m. During surveys, the D-RTK2 is positioned atop a geodetic mark with known coordinates (explanations above). The P4 RTK is equipped with a 20-megapixel (2.41 \(\times\) 2.41-pixel size) complementary metal-oxide-semiconductor (CMOS) camera (FC6310R) with a mechanical global shutter and an 8.8 mm focal length. Besides precise positioning, the drone attitude (pitch, roll and yaw angles) is recorded automatically for each image using an onboard inertial measurement unit (IMU). For collecting images, we used a single photogrammetric block arrangement with a front and side overlap of 80% and 70%, respectively (Table 1, Figure 2) and a forward-looking camera angle of 6\({}^{\circ}\) off-nadir. The flying height, and, thus, the GSD, was adjusted depending on the field site in order to complete surveys within two hours around low tide.