## 1 Introduction
### Aim of the Study  
A majority of the presented studies in Section 1.2. were carried out in flat areas with homogeneous terrain conditions (except for the studies by [PERSON] et al. [36], [PERSON] et al. [28], and [PERSON] et al. [48]), where there is less satellite obstruction, better signal reception, and finally conditions for measuring GCPs and VPs. In our research, we wanted to test the accuracies of the photogrammetric products in the case of rockfalls, as the conditions for a survey of high-accuracy data (e.g., GCP positions) are highly demanding,due to the higher terrain roughness and larger elevation changes. It is crucial for surveyors of these environments to know whether it is possible to achieve results comparable to those acquired by using georeferencing, without requiring any GCP assistance. Therefore, we question which georeferencing method is more appropriate and suitable for providing high-accuracy photogrammetric-derived products, taking into consideration all of described environmental challenges and risks of field operations in the event of rockfall hazards. Either it is permissible to use the georeferencing with PPK (BBA_PPK) method, or the use of indirect georeferencing (BBA_traditional) is still preferable and needed, especially in the case of planning technical protection measures where an accuracy of the order of centimeters is required. As we wish to minimize the amount of time that field operators spend on an active rockfall slope, the main goal of the research was to analyze which BBA method (BBA_traditional or BBA_PPK) provides more accurate photogrammetric-derived products (e.g., orthomosaics and DSMs). In the research, we compared the photogrammetric-derived products that have been produced strictly by georeferencing, using BBA_PPK and indirect georeferencing (using BBA_traditional only), to determine the possible applications in the event of rockfall studies.  
The reason that we considered the option of not using BBA_PPK in combination with the BBA_traditional method is that the surveying of rockfalls can sometimes be too risky to place any GCP below, for example, rock walls in the transit corridors of the falling rocks. However, rockfalls located in remote and hardly accessible areas are still valuable and need to be included in past rockfall databases. In our study, we are focused only on rockfall transit and deposit areas, where we are collecting data on past rockfall events needed for calibration and validation of rockfall models. For rockfall modeling, we require data on the volume, location, shape of individual rock deposits, and surface roughness of the slope, among others, and know that the accuracy of photogrammetric-derived products is crucial for rockfall modeling at different scales.  
The remainder of the paper is structured as follows: In the Materials and Methods section, we describe the test site, the experimental setup for comparing the results of georeferencing with two different methods (i.e., AT with solely GCPs and using auxiliary data provided by PPK and no GCPs), the UAV feature and flight plan, data processing, measuring and obtaining the positions of the VPs, and statistical evaluation of the results. In the Results section, we present the differences in the accuracy, using VPs, of the BBA (X, Y, and Z), orthomosaics (X and Y), and DSM (Z), as obtained by the two georeferencing methods mentioned above. In the Discussion section, we explain the significance of the results, draw the main outcomes, and discuss possible future investigations. Due to different applications (e.g., comparison of photogrammetric points clouds and laser scanner point clouds; spatial planning--orthomosaics; and so on) [51], our comparison of the results is carried out separately for BBA, for orthomosaics, and DSM.