## 2 Methods  
### Data Introduction and Method Flow  
A typical sea area in Longkou City, Shandong Province, China, was selected as the experimental object. The data size is 165 MB with a total of 5,922,266 points. The lower left coordinate is x = 516,728, y = 3,372,488.2, z = 89.639999; the upper right coordinateis x = 510,528.21, y = 3,370,108.7, z = 72.589996. These coordinates are coordinates under the space rectangular coordinate system. The coordinate origin of the space rectangular coordinate system is located at the center of the reference ellipsoid, the Z-axis points to the north pole of the reference ellipsoid, the X-axis points to the intersection of the starting meridian plane and the equator, and the Y-axis is located on the equator plane and forms a 90\({}^{\circ}\) on the X-axis with the right hand system. Each point contains three-dimensional coordinate information, echo intensity, and echo times information. The original image is shown in Figure 1a, in which the red box shows the coastal zone area composed of a larger cliff area. The size of this cliff area data file is 11.196 MB, with a total of 405,650 points. The point cloud image is shown in Figure 1b. Three concepts are defined here. (1) Connection points. The outer line of the point cloud in the sea area, which includes the cliff section, is the process of passing through the outer line of the normal ground, then passing through the transition point to the outer line of the cliff section, and then passing through the transition point on the other side to return to the outer line of the normal ground. These two transition points, the two points connecting the outer line of the ordinary ground and the outer line of the cliff section, are called \"connection points\". (2) Depth points. From the outer line of the cliff section, the ground of the cliff section will be lowered from the water body side to the land side (in this case, the ground of the cliff section will be lowered from the land side to the water body side, which is almost non-existent in practice). When the ground elevation is lowered to approximately the same height as the water body, the ground at this time is considered as the last position of the cliff section. This position can be divided by a line, which is imaginary in the data. However, the ground of the cliff section will decrease from the water body side to the land side; that is, the elevation of the ground will decrease. The ground is lowered to approximately the same height as the water body; that is, the elevation of the ground is close to the elevation of the two connection points. Therefore, the point with the smallest difference between the two connection points in the x, y plane direction and the z elevation direction can be found in the cliff section area data, and these two points are called \"depth points\". (3) Side points. The point cloud of the cliff segment is segmented at fixed intervals according to the x-coordinate (x-coordinate when the main coastline is east-west or west-east, and y-coordinate when the main coastline is north-south or south-north). Each segment is called a point-cloud block, and the centroid coordinates of each point-cloud block are calculated. Each block can be imagined as a rectangle. Take the coastline whose main trend is along the x-axis as an example; the short side of the rectangle (that is, the block spacing) is parallel to the x-axis, and the long side is parallel to the y-axis. Only the x and y coordinates of the point cloud in the block are considered. It can be imagined that the plane coordinates of these points are all within the rectangle. Then, the parallel line to the short side of the rectangle is made through the plane coordinates of the centroid point of the block. Then, the line is parallel to the short side and also parallel to  
Figure 1: (**a**) The image of point cloud in original sea area; (**b**) the image of point cloud in cliff area. the x-axis. The parallel line will have two intersection points with the two long sides of the rectangle.