Combining mobile terrestrial laser scanning geometric and radiometric data to eliminate accessories in circular metro tunnels

Terrestrial laser scanning (TLS) is a noninvasive technique to monitor surface conditions and morphological characteristics of structures and has been successfully introduced to the regular inspection and maintenance of metro tunnels. To accurately analyze the deformation and structural conditions of a metro tunnel, nonliner points (e.g., outliers and accessories) should be detected and eliminated. Nevertheless, the accessories are attached very closely to the liner and cannot be thoroughly eliminated by three-dimensional (3D) geometric information. This study proposes to separate the liner and accessories by combining TLS geometric and radiometric information. A refitted mobile Faro Focus3D X330 system is used for data collection of a new-built metro tunnel in Hangzhou, China. The results show that the corrected intensity data are an effective physical criterion and a complementary data source to remove accessories that cannot be eliminated by geometric data. After the removal of accessories by geometric and radiometric data, the remaining liner points can accurately reflect the actual structural and deformation conditions of metro tunnels.