## 4 Discussion
The spatial distribution of the differences between the X, Y and Z coordinates of VPs, as obtained by using BBA_traditional and BBA_PPK methods, did not reveal major spatial patterns, while outliers were located randomly across the rockfall sites. The only location that showed a distinctive spatial distribution was Mangart, and that only for the photogrammetric-derived products produced by BBA_PPK. The differences increase towards the northwestern direction for the X and Y coordinates and towards the southwestern direction for the Z coordinate. The spatial pattern was similar with all photogrammetric-derived products. The spatial distribution of the differences is a result of the lower accuracy of the BBA_PPK method, and it is not correlated with terrain topography. The highest differences in coordinates (X, Y and Z) of the VPs when comparing the VP and GCP locations, were achieved at the Krnica rockfall, which was also the largest area that we surveyed. In our experience, measuring the locations of GCPs and VPs can be very challenging. Under the steep rock walls, there is no signal, meaning that the points cannot be measured with GNSS that has RTK; in such cases, the use of the total station is then needed. Additional challenge presented at this study site was moving the measurement equipment, and in the end measurement of the GCPs/VPs. Namely, not all GCPs/VPs could be measured with the total station from one standing position, since the area was too large. Consequently, the total station was positioned on several locations, and due to the large area that needed to be surveyed, not all measurements could be performed in one day. Moreover, due to the high surface roughness, there were many obstacles (rock deposits that are a few meters of high) that obstructed the measurements with total station, and the GCPs/VPs were in some cases located on scree slope (as there was no other option), meaning that they were not attached firmly to the ground. In latter case, targets were more unstable, and could be moved by simple slope processes or simply by walking past it. Consequently, all this factors could potentially influence the measurements, and result in higher errors at this rockfall site. The RMSE values of the BBA_traditional method were larger at the Krnica rockfall, compared to the other two locations, and comparable with the results of the BBA_PPK method. This was partially confirmed by a non-parametric Wilcoxon signed-rank test that showed that there were no statistically significant differences between the georeferencing methods for Y coordinate when assessing the accuracy of the BBA. However, in the assessment of other photogrammetric-derived products and coordinates, the differences were statistically significant between the georeferencing methods. Taking into consideration this factor, BBA_PPK definitely represents a viable possibility and solution for application in such challenging terrains. Still, for generating products with high accuracy (if possible)--especially vertical accuracy--BBA_PPK should be used at least in combination with few GCPs and possible interferences or obstructions that can influence the final results. [PERSON] et al. [32] have reported that, by using at least one GCP point, the accuracy can be almost as good as with the use of GCPs for horizontal accuracy, and only slightly worse for vertical accuracy. The fact is that the accuracy can be significantly enhanced with the use of a UAV with PPK, especially when the GCPs cannot be properly distributed and measured in the hazardous areas [27; 29; 43; 47; 59; 67]. To improve the vertical accuracy, [PERSON] et al. [47] proposed the use of duplicate flights (double-grid) and geometrically different combinations (different altitudes and camera angle of images); similar conclusions were drawn by [PERSON] and [PERSON] [33]. [PERSON] et al. [68] concluded that including oblique images is advised, as they can improve self-calibration of the bundle block adjustment and increase horizontal and vertical accuracy.