# Geotechnical Study of Raspadalica Cliff Rockfall, Croatia
## 3 Materials and Methods  
### Raspadalica Cliff 3D Model Establishment  
In recent years, unmanned aircraft vehicle (UAV) photogrammetry has been developed to collect spatial data. This very popular subject has found wider application in geomatics [16; 17; 18; 19; 20; 21], engineering geology [21; 22; 23; 24; 25; 26], and geotechnical engineering [27; 28; 29; 30; 31; 32; 33]. The products generated from UAV photogrammetry are usually datasets, such as point clouds, high-resolution digital surface models, high resolution digital orthophotos, and photorealistic 3D models and visualizations [16; 17]. Used in interdisciplinary studies, these datasets resemble high-resolution spatial data acquired by micro technology, IT systems and _structure from motion_ (SfM) techniques for photogrammetric data processing. In this study a custom-made vertical takeoff and landing (VTOL) rotary wing hexacopter UAV was used as an aerial vehicle platform. The system consisted of a carbon fiber tubular frame with a radial setup of engines powered with a high-capacity lithium polymer (LiPo) battery. The total weight of the system was approximately 3.6 kg. The flight time of the system in normal conditions was about 26 min. The platform was equipped with the Pixhawk flight controller (Version: 1.8.2.) with a single-frequency GNSS receiver and additional sensors, such as gyroscopes, accelerometers, a 3-axis magnetometer, and a barometer used for navigation support. The UAV was monitored and controlled by a ground station using dual commands over the RF link. An essential component for the aerial survey was the open-source software Mission Planner (version: 1.3.50.0, firmware: APM: Copter 3.4.4), which was used for planning purposes, control, and the real-time management of the UAV. Figure 5: Photo of the rockfall originating from the Raspadalica Cliff on 16 December 2013. The UAV was equipped with a Sony Alpha 7R digital camera with the 36.3-megapixel full-frame CMOS sensor (35.9 mm \(\times\) 24 mm) and a high-quality Sony FE 35 mm Carl Zeiss lens. The system used in this research has the ability to determine the 3D motion trajectory of the platform with PPK. The development of micro-technology and autonomous systems as well as the increasing demand for precise spatial data enabled the determination of the 3D trajectory using the on-board GNSS receiver and the PPK method. The raw data were stored on a memory integrated in the receiver of the moving platform. The data from the base station were able to be simultaneously stored in raw-data format or subsequently downloaded from a network service. Then, it was possible to post-process the data together with the rover data to obtain the 3D trajectory of flight and coordinates of events. In our example, the position was derived from a dual frequency Sepentrio AsteRx-m UAV GNSS receiver. The reason for using the multi-rotor VTOL was its ability to assign precise positions of images captured from different positions, its applicability for many specific tasks, its ability to take-off and land with a minimal required area, and flight stability and operation at lower altitudes. In addition, the above-mentioned custom-made multirotor UAV with onboard precise positioning GNSS was able to capture full-frame high resolution oblique images of steep cliffs in the Raspadalica Cliff area. The field survey with UAV was conducted during two flight missions. The first flight mission covered wider research area while the second flight mission was focused on a detailed survey of the cliff. During the field surveys, an area of 20 ha was covered with 273 oblique images with 80% longitudinal and lateral image overlapping. Post-processing of the 3D trajectory was performed with an image position accuracy of 3 cm planar and 5 cm in the vertical direction. The initial data processing in the office consisted of assigning the coordinates of the image center and setting up coordinate systems.