## 5 Impact of Flight Scenarios on Bowl Effect  
For each configuration, the results of the different processings are assessed using the 18 check points (Figure 3a--blue targets, not used in the photogrammetric workflow), comparing their position on the reconstructed DEM and orthophotograph to their position measured by DGPS. Considering the rapidly changing nature of a beach, the uncertainty on RTK DGPS measurements and on GCP pointing, we assumed that a Root Mean Square Error (RMSE) lower than 5 cm is sufficient for the resulting DEM. Using 19 GCPs widely distributed over the study area, the total error varies from 1.7 cm (for Flight 2 processed with PhotoScan) to 4.0 cm (for Flight 1 processed with MicMac) (Table 2). Thus, whatever the flight plan and whatever the software tool, all the computed DEM meet the criterion of RMSE lower than 5 cm. For these three flights with 19 GCPs, the errors appear a bit smaller (less than 1.6 cm of difference) with PhotoScan(r) than with MicMac(r), which is possibly due to differences in filtering algorithm or in the minimization algorithm during the step of bundle adjustment. With the \"5 GCPs\" configuration, the errors are considerably larger (up to 161.8 cm for Flight 3 with PhotoScan and up to 146.8 cm for Flight 1 with MicMac-Fraser), the vertical error being significantly higher than the horizontal error (Table 2). Figure 9 shows the spatial repartition of the error, depicting the error on each check point as a function of the distance to the (MN) profile (Figure 8). As expected, the error increases with distance away from the control region. The extremity of the area (on Cp. 18) is located over 160 m away from the control region. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Flight 1** & \multicolumn{2}{c}{**Flight 2**} & \multicolumn{2}{c}{**Flight 3**} \\ \hline  & \multicolumn{3}{c}{**“Classical”**} & \multicolumn{2}{c}{**Oblique Pointing Camera (40\({}^{\circ}\))**} & \multicolumn{2}{c}{**Varying Altitude**} \\ \hline  & \multicolumn{3}{c}{**With 19 GCPs**} & \\ \hline  & **PS** & **MM-F15P7** & **PS** & **MM-F15P7** & **PS** & **MM-F15P7** \\ \hline XY error \({}^{1}\) (cm) & 1.4 & 1.1 & 1.1 & 2.2 & 1.5 & 1.4 \\ Z error \({}^{1}\) (cm) & 2.2 & 3.8 & 1.3 & 2.5 & 1.9 & 3.6 \\ Total error \({}^{1}\) (cm) & 2.6 & 4 & 1.7 & 3.3 & 2.4 & 3.9 \\ Std. deviation (cm) & 1.4 & 2.1 & 0.7 & 1.2 & 1.1 & 1.4 \\ \hline RMS reprojection & & & & & & \\ error (pix) & 0.84 & 0.69 & 0.87 & 0.93 & 0.82 & 0.85 \\ \hline  & \multicolumn{3}{c}{**With 5 GCPs**} & \\ \hline  & **PS** & **MM-F15P7** & **MM-Fra.** & **PS** & **MM-F15P7** & **MM-Fra.** & **PS** & **MM-F15P7** & **MM-Fra.** \\ \hline XY error \({}^{1}\) (cm) & 30.0 & 3.5 & 6.4 & 20.1 & 16.6 & 18.5 & 16.2 & 12.0 & 7.8 \\ Z error \({}^{1}\) (cm) & 55.6 & 138.7 & 146.7 & 19.2 & 79.7 & 18.0 & 160.9 & 17.7 & 32.1 \\ Total error \({}^{1}\) (cm) & 63.2 & 138.8 & 146.8 & 27.9 & 81.4 & 25.9 & 161.8 & 21.4 & 32.9 \\ Std. deviation (cm) & 42.8 & 100.4 & 106.0 & 18.2 & 53.3 & 14.4 & 117.1 & 12.9 & 19.6 \\ \hline RMS reprojection & & & & & & & & \\ error (pix) & 0.83 & 0.68 & 0.67 & 0.83 & 0.93 & 0.92 & 0.79 & 0.85 & 0.84 \\ \hline \end{tabular} \({}^{1}\) RMS error estimated using the check points.  
\end{table} 
Table 2: Comparisons of the performances obtained for the different flight plans (PS: PhotoScan; MM-F15P7: MicMac with F15P7 distortion model; MM-Fra. : MicMac with Fraser distortion model). Figure 8: Overview of the targets spatial distribution with a reduced number of GCPs. 5 GCPs in the South-Eastern part of the study area are kept for the configuration “5 GCPs”. These GCPs define a “control region”. The cross-shore profile (MN) defines the starting point of longshore measurements in the study area. For the \"classical\" flight plan (Flight 1), with a limited number and limited distribution of GCPs, PhotoScan(r) gives better results than Micmac(r), with respective mean Z-errors of 55.6 cm for PhotoScan(r), 1.39 m for MicMac(r) \"F15P7\" and 1.47 m for MicMac(r) \"Fraser\" (Table 2).