## 3 Results
### La Palue Field Site
Using RTK-GNSS for ground truthing produced nearly identical error statistics for all the settings, shown by SDE = 0.033 m for no filtering and \"Mild\" and SDE = 0.032 m for the \"Moderate\" and \"Aggressive\" settings, respectively. For this reason, Table 7 presents errors evaluated as DEM comparisons using the case of no filtering as a reference. Deviations from this reference DEM were essentially found at steep sections of the study site (e.g., cliff faces), resulting in potentially large elevation deviations over small spatial extents (indicated by the neat difference between MUE and SDE). Elevation deviations increased (in magnitude and occurrence) with increasing depth filtering, until a plateau was reached (\"Moderate\" and \"Aggressive\" have identical error statistics). \begin{table}
\begin{tabular}{c c c c c c} \hline  & (a) Low & (b) Medium & (c) High\({}_{1}\) & (d) High\({}_{2}\) & (e) Highest \\ \hline Alignment time & 27 min & 43 min & 105 min & 2721 min & 105 min \\ \hline Initial tie points & 567,716 & 1,918,509 & 2,102,195 & 1,967,440 & 2,482,173 \\ \hline Final tie points & 79,909 & 412,200 & 435,657 & 402,828 & 320,413 \\ \hline \multirow{5}{*}{Error statistics} & ME = 0.000 m & ME = \(-\)0.001 m & ME = 0.000 m & ME = \(-\)0.002 m & ME = \(-\)0.004 m \\  & MUE = 0.096 m & MUE = 0.038 m & MUE = 0.031 m & MUE = 0.033 m & MUE = 0.045 m \\ \cline{1-1}  & SDE = 0.125 m & SDE = 0.049 m & SDE = 0.042 m & SDE = 0.044 m & SDE = 0.062 m \\ \cline{1-1}  & RMSE = 0.125 m & RMSE = 0.049 m & RMSE = 0.042 m & RMSE = 0.044 m & RMSE = 0.062 m \\ \cline{1-1}  & Max = 0.368 m & Max = 0.145 m & Max = 0.117 m & Max = 0.144 m & Max = 0.213 m \\ \hline \end{tabular}
\end{table} 
Table 6: Effect of changing the image alignment accuracy setting during sparse DEM reconstruction at La Palue field site in terms of processing time, number of tie points (before and after filtering) and measurement error. Deviations between photogrammetric models (1 m resolution) and GNSS-RTK survey points, from which vertical error statistics are extracted, are presented in Figure 5.  
Figure 6 presents the DEM obtained at La Palue using the standard workflow designed for this study (Table 2) and produced on a 0.1 m grid. The measured beach topography shows marked 3D morphologies, particularly on the northern side of Kerdra point and at the seaward DEM boundary, with numerous channels incised in the sand and humps and hollows representing sand accumulations and depressions (Figure 6a). The smaller-scale topography, which is apparent in the orthophoto and is suggested by uneven and rugged elevation contours, is somehow subdued by the general topography when viewed at this scale. The fine grid spacing was advantageous to allow comparisons at a large \begin{table}
\begin{tabular}{c c c c} \hline \hline (a) Reference DEM & (b) Mild & (c) Moderate & (d) Aggressive \\ \hline ME = 0.001 m & ME = 0.000 m & ME = \(-\)0.001 m & ME = \(-\)0.001 m \\ MUE = 0.027 m & MUE = 0.003 m & MUE = 0.005 m & MUE = 0.005 m \\ SDE = 0.033 m & SDE = 0.072 m & SDE = 0.084 m & SDE = 0.084 m \\ RMSE = 0.033 m & RMSE = 0.072 m & RMSE = 0.084 m & RMSE = 0.084 m \\ Maximum = 0.186 m & Maximum = 76.23 m & Maximum = 77.33 m & Maximum = 77.33 m \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Evaluation of depth filtering during dense DEM reconstruction at La Palue field site. Except for the reference DEM using no depth filtering, for which errors are estimated in comparison to 1381 RTK-GNSS survey points, vertical error statistics are the result of intercomparing DEMs with the reference DEM. All DEMs evaluated were reconstructed using “High” image alignment accuracy (Figure 5c) and “High” reconstruction quality for sparse and dense DEM reconstructions, respectively (cf. explanations in the text). Evaluations were performed using 0.1 m resolution DEMs.