FIG. 3, namely how to establish a topological structure for the point cloud. [0038] In step S110, the topological structure establishing module 21 first identifies an outermost point on each surface edge of the point cloud by analyzing coordinates values of the points in the point cloud, and determines a cubical figure that can confine the point cloud. Secondly, the topological structure establishing module 21 retrieves a length, a width and a height of the cubical figure. Finally, the topological structure establishing module 21 computes an axes quantity in surfaces of the cubical figure based on the length, the width, the height of the cubical figure and an axes interval of the grid preconfigured by the user, and then divides the cubical figure into a plurality of cubical grids according to the axes quantity of each surface of the cubical figure. Thus, the points in the point cloud may be separately confined in different cubical grids. According to the axes interval of the grid and the density of the point cloud, each cubical grid may confine one or more points. [0040] In step S112, the topological structure establishing module 21 relates one of the cubical grids with twenty-six cubical grids that are adjacent to the cubical grid in three-dimensional space by saving the serial numbers of the twenty-seven cubical grids into a table. [0041] In step S113, the judging module 25 judges whether all cubical grids have been related. If no, the procedure returns to step S112 depicted above. Otherwise,