## 2 Materials and Methods
### Setting Up Ground Control and Verification Points  
At each study site we set out VPs, which were later on used for accuracy comparison. Besides the VPs, we also set GCPs, which were used exclusively for indirect georeferencing and were not used as VPs. Before setting up VPs in the field, we have defined the area of interest at each rockfall by digitalizing the known rockfall runout area, using orthomosaics from previous UAV surveys. With the use of the Fishnet function in ArcGIS Pro 2.7.3 [55], we created a VP sample network (locations) for each rockfall. The final numbers of VPs were 42 at Kekec, 51 at Krnica, and 48 at Mangart rockfall. GCP locations were also preplanned and set up such that they were not at the same locations as VPs. The number and locations of the GCPs were set up according to the recommendations in the literature; the main condition was that the points were, as much as possible, regularly spaced within the area of interest and its edges. The horizontal distance between the points was approximately 30 m at Kekec, and 65 m at Krnica and Mangart [56; 57; 58; 59; 60]. We used 14 GCPs at Kekec, and 16 GCPs at Krnica and Mangart.  
The known VP and GCP locations were then imported as shapefiles to the Leica Zeno20 with GG04 Smart Antenna. The points in the field were, thus, set up by using this antenna and the known locations of the VPs and GCPs. As the automatic generation of the VP sample network and predicted locations of GCPs cannot account for the conditions in the field, such that the points cannot be set due to particular terrain features (e.g., a more or less flat area, vegetation, unstable terrain, etc.), the final locations were adjusted when setting the points, based on the predicted locations, while also considering the outline of the rockfall runout area (Figure 3).  
Figure 3: Elevation and slope characteristics of studied areas (**a–c**) with the drawn locations of the verification points (VPs), ground control points (GCPs), and digitalized rockfall runout areas (**d–f**) at Kekec (**a,d**), Krnica (**b,e**), and Mangart (**c,f**).  
VP and GCP targets were squares with a 2 \(\times\) 2 chess pattern (two white and two black squares; Figure 4), as recommended by the Pix4 Dmapper software [61], attached to the ground by a nail through the center of the target. Where that was not possible (as, on the rockfall runout area, most of the terrain was represented by rocks), the targets were fixed with rocks at their edges. At the Krnica rockfall, targets with dimensions 21 cm \(\times\) 29.7 cm were used, while the targets at Kekec and Mangart targets had dimensions of 29.7 cm \(\times\) 42 cm. The targets at Krnica rockfall were smaller than at other two locations, as the terrain was too uneven to position larger targets. As all rockfalls are located within the Triglav National Park, any invasive procedures for placing or making different targets were not allowed.  
All points were then surveyed, using a Leica Viva TS12 total station, while the orientation points and backside locations were measured with a Leica Zeno20 with GG04 Smart Antenna with accuracy of 0.010-0.030 m. Measurements of these points were carried out in two different parts of the same day (average location of coordinates was later calculated), with each coordinate being measured for an interval of 5 min (with a 6-h of difference). The differential correction data were acquired by using a real-time connection with the Slovenian real-time positioning service SIGNAL. The closest station to all rockfalls was station Bovec, which is located 13 km from Mangart rockfall, 15 km from Kekec rockfall, and 21 km from Krnica rockfall. The X and Y measurements were transformed to the horizontal geodetic datum \"Slovenia 1996/Slovene National grid with Slovenia Geodetic Datum 1996\", whilst the Z coordinate was converted to an orthometric height, using the geopotential model EGM96.