## 1 Introduction  
Rock discontinuities (e.g., structure plane, fracture or fault) play a key role in the strength, in the permeability of rock masses and in the stability of rock slopes and underground excavations [1; 2; 3; 4; 5]. Thus, a thorough understanding of the properties of rock mass discontinuities, including their orientation (i.e., dip and dip direction) and dominant grouping, is crucial for the rock engineering [6; 7; 8]. For the measurement of rock mass discontinuities information in high and steep rock slopes, the contact measurement methods in the field have disadvantages, such as low efficiency, poor safety, and the inability to collect data at high slope surfaces [9; 10; 11]. Moreover, contact measurement methods are limited by the complex terrain of high and steep rock slopes and cannot obtain global field measurements. However, field contact measurement has the advantage of high reliability and obtaining comprehensive survey information for the local slope surfaces. Today, non-contact measurement technologies (such as global positioning system (GPS), light detection and ranging (LiDAR), and differential SAR interferometry (DInSAR)) have gradually replaced traditional contact measurement methodsin high and steep rock slopes due to their ability to quickly and accurately obtain 3D data acquisition, and automatically characterize the rock mass discontinuities [12; 13; 14; 15]. For typical non-contact measurement methods, the 3D point cloud data of the scanned slope surface can be obtained from 3D laser scanners. For the application of 3D laser scanners in the field, [PERSON] et al. [16] evaluated the orientation of discontinuities in a red granite quarry in Missouri, USA, based on the obtained point cloud and optical images. [PERSON] [17] studied the rapid extraction of rock discontinuity information, proposing a process and preprocessing method for obtaining point cloud data. The intelligent recognition of the rock discontinuities was achieved using the obtained 3D point clouds. [PERSON] and [PERSON] [18] analyzed the reliability of joint orientation measurements in rock slopes using a 3D laser scanner and the program Split-FX (v.2.4), which is point cloud data analysis software, and 495 piece discontinuities were automatically extracted. Terrestrial laser scanning was used to achieve dense 3D point cloud data sampling for slope geometries by [PERSON] et al. [19], and real scenarios were simulated in a water flume facility using a wave attack experiment, comprising a storm of 3000 waves. Previous studies show that 3D laser scanning is an effective and feasible method for the rock mass discontinuity analysis of rock slopes. For a large-scale and long-distance rock mass slope, 3D point cloud data are usually obtained by ground-based fixed 3D laser scanners, but the setup position requirements are strict, and scanning blind spots are easily produced for the complex terrain of slope surfaces. Given the complexity and uncertainty of on-site geological environments, unmanned aerial vehicle (UAV) technology has become a popular tool for the remote collection of slope information and for stability assessments [20; 21; 22]. UAV photogrammetry can quickly obtain point cloud data of high and steep rock slopes, overcoming the defect problem that manual measurement cannot. Compared to traditional methods (i.e., geological compass, ordinary camera), UAV photogrammetry also has the ability to capture high-resolution data, reduce fieldwork time, and enhance safety. The high-precision aerial images obtained by UAV photogrammetry technology can not only construct realistic 3D slope calculation models [23; 24; 25] but also achieve the recognition and extraction of rock mass discontinuities based on computer algorithms [26; 27]. A rapid rock mass identification method based on UAV photogrammetry was proposed by [PERSON] et al. [23] to evaluate and segment slope point cloud models through core density distribution, and then the clustering calculation was performed based on point cloud density. [PERSON] et al.