## 3 Establishment of the Slope Model Using UAV Photogrammetry Technology
### Reconstruction of the 3D Rock Slope Point Cloud Model  
UAVs can perform aerial photography along a pre-set trajectory or be manually controlled to achieve image acquisition, obtaining clear geological information such as structural planes and fissures on the rock slope's surface. To ensure that reliable rock mass discontinuity information was obtained, image acquisition was carried out at three different flight heights (i.e., high, medium, and low positions above the scanned slope surface, as shown in Figure 4). The blue areas in Figure 4 represent the UAV's photo shooting positions corresponding to each collected image. The distance between the UAV's photo shooting position and the slope surface was kept within 5 m, and the flight overlap rate was set to 80%. More than 60 images were collected using the above method, and the whole selected slope surface was covered. The scanned slope is the exposed excavation surface shown in Figure 2.  
The high-resolution images collected by the UAV were added to the Agisoft PhotoScan software (v.1.4), and all the photos were aligned and combined with each other according to the overlapping areas of the photos. Agisoft PhotoScan extracts point cloud data with coordinated information from the original photos and quickly reconstructs a 3D model of the target, including rock mass discontinuity lines, surfaces, positions, and other data, and reproduces real morphological characteristics of the scanned slopes. After the 3D model was established, the material was given to the 3D model. Agisoft PhotoScan can be adjusted according to different requirements in the open settings dialog box to achieve the display effect required. The 3D point cloud data model with the included material (Figure 5a) can be compared with the photos obtained by UAV in order to verify the accuracy of the established 3D slope model. The slope region in Figure 5a is derived from the exposed excavation surface shown in Figure 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Category** & **Value** \\ \hline Weight & 1388 g \\ Battery & 6000 mAh, 15.2 V \\ Horizontal velocity & 72 km/h \\ Vertical velocity & 6 m/s \\ Camera & 20 MP COMS \\ Image format & JPEG + DNG \\ Working temperature & 0–40 \({}^{\circ}\)C \\ Photo size & 5472 \(\times\) 3648 pixels \\ Satellite position module & GPS/GLONASS \\ Incline angle & \(-90^{\circ}\) to +30\({}^{\circ}\) \\ Working frequency & 2.4 G ISM, 5.8 G ISM \\ Working delay & 160–180 ms \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications of the DJI Phantom 4 Pro.  
Figure 3: The UAV used in this study.  
The obtained point cloud data cannot be directly used for rock mass discontinuity recognition and dominant grouping calculation due to the large number of useless data points (e.g., ground, overburden, and noise points) included. To retain the original point cloud data of the rock slope as much as possible, we compared the data to the original picture of the slope, and the areas that did not need to be analyzed in the software (for example, cover soil, vegetation, etc.) were identified and then deleted. The Cloud Compare software (v.2.13) was used to remove noise points and rock debris cover on the slope surface in accordance with the actual calculation area. The obtained calculation model with the accurate 3D point cloud data of the studied slope surface is shown in Figure 5b. The established calculation model (Figure 5b) can truly reflect the actual information of rock mass discontinuities on the rock slope surface.