## 4 Discussion and Conclusions  
This study has tested the possibility of using SfM photogrammetry augmented with an RTK quadcopter for monitoring coastal topographic complexity at the beach-scale. The survey method had to respond to both resolution and time constraints, with final DEM resolutions of 0.1 m for a realistic representation of the topo-morphological features from submeter dimensions and a survey completed in two hours around low tide to cover the intertidal zone. The method implemented had to achieve effective measurements in a challenging environment characterized, for instance, by large topographic variations, differences in bed cover, such as rough surfaces alternating with textureless and reflective surfaces, such as sand, and the presence of water as thin patches or deeper puddles and channels, which can all represent important difficulties for photogrammetry. To evaluate the effective measurement resolutions and precisions achieved by the survey method, and, hence, to be able to identify suitable protocols for collecting and processing the data that surpassed the above constraints, purpose-built SfM workflows were applied to aerial images collected at two coastal field sites (La Palue and the breach at Sillon de Talbert) and tested against different ground truths. Of interest for this application, because of their impact on the data collection time and DEM quality, we specifically assessed the effect of image resolution and using GCPs in addition to camera information during BBA. As much as possible, the error evaluations were diversified to enhance the spatial  
Figure 8: Application of RTK quadcopter and SfM photogrammetry for very-high-resolution measurement of topographic roughness and bedform mapping at the beach-scale at La Palue field site. (**a**) Orthophoto and the different zones investigated in panels (**c**–**h**); (**b**) 0.1 m resolution DEM detrended from metric topography (cf. explanation in the text); (**c**–**h**) 0.1 m resolution orthophotos and detrended DEMs cropped to the areas of interest. Detrended elevations below 0.02 m are not shown to help with figure comprehension. The colorbar is same for all DEMs. coverage and the level of error detection afforded and, hence, to increase the chance of obtaining reliable error statistics. Ideally, complete DEM comparisons should be undertaken to fully characterize the error magnitude and spatial distribution as it is now well-known that photogrammetric measurements can be affected by complex 3D deformations (e.g., the dome effect), which may remain undetected when using sparse ground controls [67; 68]. In the absence of suitable ground truth at this scale, we analyzed the DoDs for different processing scenarios. This strategy proved effective for assessing the DEM reliability due to variations in the GCP number and distribution, showing that, in addition to aerial controls, at least five GCPs were necessary at Sillon de Talbert to achieve optimum quality that minimized measurement bias and random errors. Under this configuration, the global vertical precision (RMSE) improved two-fold in comparison to a processing workflow using all the targets as GCPs (\(n=21\)) but no aerial controls. The benefit resulting from using RTK image positioning resided principally in an improved 3D geometry of the model, particularly at zones with limited to no ground controls. Using aerial controls only (akin to direct georeferencing), our results showed that the photogrammetric results were affected by vertical bias, explaining most of the error. The presence of vertical bias in models obtained using DG has been reported before and was explained by inaccurate IOP calibration (e.g., Refs. [49; 52; 56; 69; 70]). Likewise, our results showed that the addition of a single GCP was enough to reduce the vertical bias to the GSD level together with RMSE \(\sim\)2 GSD, but the addition of other GCPs during BBA further improved the photogrammetric quality until a plateau was attained from 5 GCPs, with the RMSE mostly below 1 GSD.