## 3 Results
### La Palue Field Site
Figure 5: Evaluation of image alignment accuracy setting during sparse DEM reconstruction showing vertical errors at 177 RTK-GNSS survey points for (**a**) low, (**b**) medium, (**c**,**d**) high and (**e**) highest alignment accuracy setting. For all scenarios tested, “source” reference pair preselection was used, except for panel (**d**). Photogrammetric targets used as GCPs and ChkPts are shown as triangles and circles, respectively. Error evaluation was performed using 1 m resolution DEMs. number of RTK-GNSS survey points (1381 points). The photogrammetric errors (Figure 6b) show a similar spatial organization with the ones identified previously using 1 m DEMs reconstructed from sparse point clouds (Figure 5). DEM precision using the standard workflow was characterized by SDE = 0.032 m (i.e., 1.2 GSD, Figure 6c,d). Four repeat photogrammetric surveys carried out between September 2020 and April 2021 are compared in Figure 7. Using the range and standard deviation of repeat bed elevations, representing the temporal variability in the elevation for each surface cell, shows that large portions of the back beach (i.e., dune and cliff tops), as well as rock deposits and outcrops, can be considered stable over the 7-month period during which the DEMs were obtained (here defined as cells with an elevation range between the four surveys below 0.035 m). Filtering the unstable surface cells left 587,105 invariant cells at a horizontal resolution of 0.1 m, and characterized by an overall (i.e., averaged over all the stable cells) range and standard deviation of repeat bed elevations of 0.025 m and 0.011 m, respectively. Averaging the repeat elevations over the stable cells of the DEM produced a multi-temporal ground truth backing the entire beach over all the sides except seaward, and with elevations spanning over 50 m (Figure 7d). The comparison of the 17 September 2020 survey with the ground truth (Figure 7e) suggests centimeter-scale deformations, which may echo deformations previously identified using RTK-GNSS survey  
Figure 6: Quality assessment of the 0.1 m resolution photogrammetric DEM obtained using the standard workflow. (**a**) DEM overlapped with depth contours every 0.2 m; (**b**) vertical error at photogrammetric targets (GCPs and ChkPts) and 1381 RTK-GNSS survey points; (**c**) probability distribution function (PDF) of pitch-related error (dx) binned at 0.005 m and (**d**) PDF of photogrammetric vertical error in comparison to RTK-GNSS survey points binned at 0.01 m.  
points (Figures 5 and 6). The quantitative comparison shows ME and SDE of \(\sim\)10\({}^{-3}\) m and 0.01 m, respectively. Figure 8 presents the first applications of very-high-resolution coastal monitoring at the La Palue field site, used here for beachface topographic complexity detection and mapping. Three zones are analyzed in more detail, corresponding to regions of return flows, creating narrow channels incising sand superimposed with complex small-scale topographies, such as (mega)ripples and hummock patterns of different forms and dimensions. For detecting submeter bedforms, the 0.1 m resolution DEM was detrended from metric topography by subtracting the 1 m resolution DEM. Doing so left only the small-scale topographies not accounted for in the DEMs with resolutions of 1 m or above (Figure 8b). The qualitative analysis of the orthophotos and DEMs suggests that the approach was suitable for characterizing sub-metric beach topography, creating realistic maps of the bedform arrangement at the beach scale (cf. Figure 8a,b). The submeter-scale topography is most evident at steep sections of the beach, such as seaward of the berm and where there is a sand-cobble transition, but is otherwise widely spread. It includes large patches of ripples and megaripples, channels of varied dimensions, and zones of water resurgence. Quantitatively, the net volume embodied in the sub-metric topography amounted to \(\sim\)600 m\({}^{3}\), which is equivalent to a layer of sand of 0.011 m over the entire beach.