# Photogrammetry-Based 3D Textured Point Cloud Models Building and Rock Structure Estimation
## 2 The Point Cloud Model Generating for the Study Site  
The study site is located in Dalian City, Liaoning Province, northeast of China. The city has plenty of rain, sunshine, and lush vegetation. The risk of rockfalls is the primary potential geological disaster influencing the stability of the study site. A rock slope at the site is adjacent to the Fengcai road and faces the Longwangtang reservoir (Figure 1), a section of which was selected as the case study to illustrate the processes of the FMBPM and to verify the applicability of the FMBPM to rock structure estimation. The studied area of the slope covers a rectangular region with dimensions of approximately 29 m in length and 7 m in height. The joints in the slope are well-developed, and the rock structure of the slope is generally blocky.  
A NIKON D7100 digital single-lens reflex camera with a 35 mm lens and a South NTS-362R total station were employed to capture images and to measure the coordinates of feature points. The maximum pixel value of the NIKON D7100 is 24,000,000 (4000 in height and 6000 in width). The precision of the distance measurement of the total station is \(\pm(3+2\times 10-6D)\) mm. \(D\) is the distance between the total station and a measuring point, and it is not greater than 30 m in this case. Therefore, the precision of the distance is less than 3.6 mm in theory. The accuracy of angle measurement is 2 s, which means the error of the coordinates of a measuring point is theoretically less than 0.291 mm when \(D\) is less than 30 m.  
Digital photogrammetry calculates the spatial coordinates of a point based on its planar geometric positions in different photographs. Hence, the same feature point of the slope should be presented in at least two images. A total of 194 images of the slope were taken from different camera positions. The textured 3D point cloud model of the slope was generated by Meshroom, which is a free, open-source 3D reconstruction software ([[https://alicevision.org/#meshroom](https://alicevision.org/#meshroom)]([https://alicevision.org/#meshroom](https://alicevision.org/#meshroom)), accessed on 20 December 2021). The pipeline of photogrammetry of Meshroom was used. The processes of generating textured 3D point cloud models of study subjects from images in the pipeline are highly automatic. The steps related to image processing, such as feature extracting, image matching, feature matching, meshing, and texturing, were all done automatically. Users can modify the degree of fineness of a 3D point cloud model; the finer the point cloud model, the longer it takes to generate. The textured 3D point cloud model of the study site includes 3,566,492 vertices and 7,127,652 faces, and it took about 15 h to create the model. The CPU model of the computer is an Intel(R) Core(TM) i7-9700 with 3.00 GHz, and the graphics card model is Nvidia Quadro P1000 (4 GB). It is worth noting that the surface of the rock slope where it is not directly covered by vegetation can be modeled by adding camera positions (Figure 2).  
Figure 1: Location of the study site.  
The covered part of the surface must be exposed in images taken from the added camera positions. Figure 3 is the 2D and 3D comparison image of rock structure identification.  
Figure 3: Rock structure identification 2D and 3D comparison image.  
Figure 2: Positions of the camera and total station. (Note that the coordinate origin and coordinate system of the study area can be determined after setting up the total station).