## 1 Introduction  
The deformation of highway high slope usually causes the slope instability, which will lead to the occurrence of landslide and great harm to human society. Thousands of landslides have been reported annually, all over the world, and the number has extremely increased in recent years [1, 2]. At present, highway high slopes are widely distributed, and the instability of highway slope brings great loss to human society [3]. Therefore, it has attracted the attention of governments, as well as experts and researchers in related fields worldwide. Common slope stability monitoring methods, in project application, include laser scanner measurement, RTK positioning technology and UAV point clouds measurement [4, 5, 6]. At present, the more conventional slope deformation measurement method is the topographic survey carried out by Total Station and GNSS devices, as data acquisition sources of slope displacement. The accuracy of these methods, meet the accuracy requirements of slope measurement. However, there are many problems in the process of slope measurement, such as large amounts of tasks and vulnerable monitoring equipment a potential danger to the life of the surveyors during the steep slope operation. In recent years, experts and researchers in related fields began to turn to image detection analysis for slope instability in pursuit of efficient and safe analysis methods. The image detection of slope damages such as surface crack, local collapse has been applied to slope instability analysis by UAV On-Site Inspection that is an image detection method [7, 8]. However, it only identifies local slope damages. Thus, remote sensing-based techniques were proposed, including laser scanning point clouds, UAV photogrammetric point clouds, which can intuitive express the deformation of whole slope. Laser scanning is a widely used technology in a variety of fields especially in engineering. It mainly consists of three common types of laser scanning: ALS, TLS and MLS. ALS is mainly installed on the fixed wing aircraft or helicopter and can currently be fixed on a UAV, which has high scanning efficiency and flexible operation, but its measurement accuracy is lower. TLS is fixed scanning with high accuracy and long scanning range, but its scanning efficiency is lower. MLS is installed on the vehicle and has the characteristics of mobile scanning and high efficiency, but its measurement accuracy is lower. According to their own characteristics, ALS is more frequently used to estimate forest resources distribution survey [9], while TLS and MLS are usually used for engineering displacement analysis and deformation monitoring. In current research, the movement on surface and the displacement of landslide are studied [10] to prevent occurrences of hazard and risk by using TLS. Rock slope has also attracted extensive attention. The rockslide has been studied and some addressed approaches of using laser scanning were proposed to analyze the structure of steep scarp of rockslide and monitoring the failure position of large displacement of rock mass [11]. In a nutshell, TLS is usually used as a common monitoring and surveying approaches that has applied to a lot of research projects. However, in the actual field application, the data acquired by TLS often omits some level parts of slope surface because some level parts of slope surface cannot be captured by TLS. For instance, when the position of instrument is 25 m far from the toe of a 50 m high slope (consists of 4 horizontal platform), the TLS instrument cannot reach lowest platform to generate point clouds on it. Targeting the disadvantages of TLS, ALS was used to provide missing point clouds from the overlook view and register with TLS. ALS usually generate 1-8 points per square meter and more points can be found in overlapping area [12]. However, it requires a high skill level of operators and the point densities of ALS are too sparse to fit displacement monitoring [13, 14].