## 3 Results
### La Palue Field Site  
Figure 4 presents the results of the topographic measurements using RTK-GNSS mounted on a bike. The raw data counted 2036 survey points separated on average by approximately 2.5 m (sigma = 0.75 m), showing that the data acquisition was carried out at an average speed of 9 km/h (2.5 m/s). Not shown in the figures, comparing the survey points with immediate neighbors (maximum distance of 0.2 m) enabled the point confidence to be evaluated at 39 locations, indicating a mean and maximum point confidence of 0.007 m and 0.015 m, respectively. As one may expect, the pitch-related errors due to the uneven terrain affecting the measurements were larger at the turning points (e.g., beach ends) since the bicycle was then moving up or down the beach (i.e., cross-shore), and where the local topography was suddenly changing (Figure 4b,c). The maximum forward slope recorded was just below 4\({}^{\circ}\), traducing to maximum horizontal (dx) and vertical (dz) pitch-related errors of 0.04 m and 0.01 m, respectively. Filtering the data based on pitch-related errors (dx) resulted in the rejection of 158 points. The final number of survey points retained for comparison with photogrammetry thus amounted to 1878 (Figure 4c). Figure 4: Verification of survey points obtained using a bike-mounted RTK-GNSS at La Palue, with (**a**) the chronology of point acquisition; (**b**) pitch-related horizontal error (dx), where marker size is proportional to error magnitude (\(\times\)1000 magnification factor) and (**c**) the elevation of survey points retained serving as ground truths for photogrammetric evaluation. Changing the accuracy setting during photogrammetric image alignment had major repercussions on the data processing time, as well as on the measurement density and quality (Table 6 and Figure 5). The image alignment time (1407 images) varied by two orders of magnitude, from 27 min (\"Low\"), through 43 min (\"Medium\") and 105 min (\"High\({}_{1}\)\" and \"Highest\"), to an outright maximum of 2721 min (-45 h, \"High\({}_{2}\)\") when \"source\" pair preselection was unused. The number of tie points after the alignment was the maximum using the \"Highest\" accuracy setting, followed by \"High\({}_{1}\)\" and \"High\({}_{2}\)\", with \(\sim\)1:5 variations overall (from 550,000 to 2,500,000 points). However, after the automatic filtering of less reliable tie points, the number of tie points decreased from a maximum of -435,000 points with \"High\({}_{1}\)\", through \"Medium\", \"High\({}_{2}\)\", \"Highest\" and, finally, \"Low\" accuracy settings with \(\sim\)80,000 points (again, 1:5 variations overall). The comparison of the photogrammetric models obtained (DEMs at 1 m resolution) with the ground truths provided by RTK-GNSS shows a similar tendency, with DEM quality (Table 6) in decreasing order for \"High\({}_{1}\)\" (SDE = 0.042 m or 1.6 GSD), \"High\({}_{2}\)\", \"Medium\", \"Highest\" and, finally, \"Low\" (SDE = 0.125 m or 5 GSD) alignment accuracy with 1:3 variations overall. The photogrammetric deviations from the ground truth were essentially to be related to the measurement precision since no significant bias (i.e., ME \(\sim\)0) was detected in the results. However, Figure 5 shows that the systematic errors, although of small magnitude, may still be present in the DEMs, taking the form of undulations. The color grading shows an error magnitude of \(\pm\)0.04 m for the better-quality models (\"High\({}_{1}\)\", \"High\({}_{2}\)\", \"Medium\"), eventually exceeding 0.1 m at some locations when the \"Highest\" and \"Low\" accuracy settings were used. Using ground targets (GCPs and ChkPts) for error evaluation produced error statistics with no appreciable differences between the scenarios tested (differences were found to be non-significant at \(p=0.05\)), and, for this reason, they are not presented. In comparison to previous tests, changing the depth filtering setting during dense model reconstruction had lesser effects on the DEM quality.