## 1 Introduction
### Challenges of UAV Photogrammetric Georeferencing  
The positions and orientations of acquired aerial images must be known in order to provide satisfactory accuracy of photogrammetric products, either by using the indirect georeferencing approach with the use of ground control points (GCPs) or an approach using bundle block adjustment (BBA) without GCPs. The indirect approach performs aerial triangulation (AT) with BBA, using GCPs and/or the airborne global navigation satellite system (GNSS)-assisted controls (i.e., the coordinates, \(X_{0}\), \(Y_{0}\), and \(Z_{0}\) of the perspective centers), while BBA approaches without GCPs involve position and orientation measurements being obtained during camera capturing, and each aerial image is geolocated without any need for GCPs [24; 25]. The position (\(X_{0}\), \(Y_{0}\) and \(Z_{0}\)) is measured by the GNSS system, and the camera orientation angles (w, \(\varphi\) and k) by an Inertial Measurement Unit (IMU). The measurements form six parameters, called Exterior parameters (EO), which are used in the collinearity equation for georeferencing [26]. A majority of UAVs use GNSS systems that do not provide high spatial accuracy (i.e., meter-level accuracy) [27; 28] of the images, meaning that the support of GCPs is crucial for increasing accuracy.  
Due to its highest accuracy, indirect georeferencing is preferably used in the SfM photogrammetry. While the preparation and execution of a UAV flight, in the case of indirect georeferencing, can be performed from a safe location, the placement of the GCPs needed in this situation must be performed directly within the area of interest. Surveying of rockfalls can be an extremely challenging and dangerous field operation [27; 29], due to their environmental and geomorphological conditions (e.g., steep slopes, rock walls, scree slopes, etc.), making them hard to access. To achieve the highest possible accuracy, GCPs need to be placed homogeneously throughout the area of interest, covering the edges of the area and having similar distances between them [30; 31]. In the case of rockfalls, this means placing them within rockfall deposit areas and also above the rock cliffs, which could result in potentially more field workers in the field and separate surveys of the area. The field workers are exposed to the dangers of new rockfall events, displacements of already deposited rocks in the rockfall transit area, and other steep terrain-related dangers (e.g., falls, damages to the surveying equipment, etc.). The process of establishing the GCP network and measuring them with, for example, a total station or GNSS with RTK (Real-Time Kinematic) mode to achieve horizontal accuracies of 0.010-0.020 m and vertical accuracies of 0.020-0.030 m [32], is therefore not only dangerous but also time- and labor-intensive [29; 33].  
Due to the time-consuming procedure of indirect georeferencing with GCPs, UAV platforms that allow for georeferencing of BBA without GCPs have been introduced (see, e.g., References [34; 35]). They use the so-called RTK, NRTK (Network Real-TimeKinematics), and PPK (Post-Processing Kinematics) solutions, based on a multi-frequency multi-constellation of GNSS receivers. By using GNSS RTK, positional data are acquired from satellites and a base station providing real-time high-accuracy positions, such that corrections in real time are applied to the data onsite. Although the PPK method gathers positional data by using virtual reference station (VRS) or continuously operating reference stations (CORS) in a similar manner, the corrections are applied in the post-processing stage. With the accuracies of those methods being comparable to that of indirect georeferencing, the GCPs would not be needed, thus decreasing the time required for the acquisition and processing of the data [29; 36; 37].