## 4 Discussion  
Georeferencing of UAV-acquired images has been studied by various authors, as already summarized in the Introduction section (see the references therein) and in Appendix A. The vertical and horizontal accuracies vary depending on several factors, including the BBA methods used, namely BBA with and without GCPs. The RMSE reported by the majority of studies varies between 0.020 and 0.060 m for horizontal accuracy, and, in the case of vertical accuracy, 0.020-0.090 m, while studies have also reported values of approximately 0.100 m up to three decimeters (in some cases, even higher). The best accuracies in our study were achieved by the BBA_traditional method, when comparing the accuracies of BBA, orthomosaic, and DSM. The accuracies for the BBA_traditional method were, for either BBA, orthomosaic, or DSM, in the worst case, under 0.030 m for X and Y coordinates, and under 0.060 m for the Z coordinate. The BBA_PPK method generally achieved lower accuracy than the BBA_traditional method for all photogrammetric-derived products, but it must be highlighted that they were still under 0.035 m for X and Y coordinates and 0.075 m for the Z coordinate (except at the Mangart site), and, comparing the BBA_traditional with BBA_PPK, the results differed, in the worst case scenario, by 0.013 m for X and Y coordinate, and 0.036 m for the Z coordinate. The following results are in similar ranges as those reported in studies carried out by the authors mentioned in the Introduction [25; 27; 32; 33; 36; 40; 43; 44; 45; 46; 47]. In the case of BBA_traditional, the smallest variations (diff-SD) are present at Kekec and Mangart, achieving maximum SD up to 0.010 m (X, Y, or Z coordinate) when observing assessment of the BBA. With orthomosaics the variations were larger for BBA_traditional and in some cases comparable with BBA_PPK (e.g., Kekec Y coordinate), while maximum differences were present at BBA_PPK. On the other hand, the Z coordinate always achieved the lowest diff-SD in the case of BBA_traditional, including the diff-MAX values, where the differences exceed 0.006-0.008 cm in the case of Kekec and Krnica, and 0.041 cm in the case of Mangart, when compared to BBA_PPK. The Mangart study site was an exception compared to the other two locations, where RMSE\({}_{Z}\) for the BBA_PPK method was larger by 0.093 m (X coordinate) and 0.111 m (Y coordinate), and 0.254 m for the Z coordinate, when observing the accuracy of BBA. The results for the orthophoto and DSM accuracies were in a similar range: 0.077-0.114 m for the X and Y coordinate and 0.231 m for Z coordinate. The major cause of such a large difference in accuracy was the unfavorable satellite position configuration [8; 27; 29] on the day of the UAV survey (more or less three visible satellite positions in the time of survey), compared to that on the day when the GCP/VP survey was performed. Consequently, it is recommended to predict the satellite position quality [29] before the UAV flight, both from the perspective of the final accuracy of the photogrammetric products and for the safety of the flight. Due to the extent of the UAV survey (rapidly changing weather conditions; clouds and fog), and the GNSS survey of the points, both measurements could not be performed on the same day. Even though our study was carried out in mountainous areas with larger elevation changes along the area of interest (rockfall deposit areas), the final accuracies of BBA, orthomosaics, and DSM were in line with the values reported in other studies. The reliability of the results is therefore high, as similar results have been reported for two rockfall locations that we included in the study; however, the third location highlights that the BBA_PPK method still may--particularly in remote areas--cause larger deviations, and GCPs should be used to validate the results.