Retrieving cross-sectional contours from tunnel point clouds
For the tunnel point clouds acquired by 3-d laser scanner, a method of automatically retrieving cross-sectional contours of tunnels is proposed. By the use of abundant normal vectors embedded in point clouds, this method can accurately identify the tunnel axis, and build a coaxial cylinder to project 3-d points to the cylindrical coordinate system, which takes a 2-d form of coordinates. An improved 2-d Delaunay triangulation algorithm is employed to construct the triangulated mesh by the spatial distribution of these projection points in their 2-d space. Following this mapping correspondence, the 3-d triangulated mesh of tunnel is finally constructed in accordance with the 2-d triangulation result. By intersecting the tunnel mesh with cutting planes always perpendicular to the tunnel axis, a series of contours are generated. In every position to generate contours, a fine tuning operation on the cutting plane is conducted to make its normal direction always keep consistent with the ever-changing direction of curved tunnels. At the last part, this paper demonstrates effectiveness and high efficiency of this method by the use of the point clouds of a real curved tunnel.