Abstract: A new method based on a special accessory made for 3D laser scanner and 7-parameter coordinate transformation algorithm is introduced to directly locate every 3D tunnel scan station with the closest tunnel control point and transform the point cloud from the relative scanner coordinate system into the uniformed tunnel control coordinate system without registration of multiple scans, which is called "APM" method(absolute positioning method). Error propagation analysis and corresponding numerical calculation are also used to evaluate the accuracy of APM results and the key error source. In the end, the tunnel scan results acquired with the APM method under certain preconditions are proved to meet the accuracy requirements of tunnel as-built survey based on the Code of Urban Rail Transit Engineering Surveying.

Key words: tunnel scanning, absolute positioning method of point cloud, coordinates accuracy of point cloud, coordinate transformation of point cloud