A challenging scanning task of an underground water pipe in North London, stretching to close to 1km, with no GNSS assistance
It was with some trepidation that Commendium accepted the challenge of surveying an 800m long, 0.9m wide water-pipe in North London. Being underground, without the aid provided by GNSS and the capture of surrounding furniture, surveying a metal pipe must pose the most challenging of subjects for LiDAR, with few clues in the data to assist with the registration and alignment of scans. We set about this by mounting a RIEGL VZ-400 onto a crawler provided by the Water Services Group, and set up a process to take scans every 3m through the tunnel. We supplemented the displacement measurement by using an IMU to measure orientation and displacement between scans as precisely as possible. Scans were taken manually by connecting the scanner to the computer via fibre optical link, which via ethernet switching also allowed us to capture photographs and video at the same time.
Back in the office, the scans were realigned using data from the IMU and then brought into RiSCAN PRO for manual, fine stitching using the MSA (Multi Station Adjustment) tools. It was vital to disable roll before picking tie points from small imperfections in the pipe revealed by the scanners. For this particular project, there was no way around this slow manual process, but the results were excellent, a tribute to the quality of the core RIEGL LiDAR technology. We ‘closed the survey loop’ by surveying over the surface to the other end of the pipe, this time enabling auto-registration as GNSS and plenty of street ‘furniture’ was available to stitch scans. In the end the loop closure was under two metres over 1.8km, data that met all requirements of the end customer and provided a highly satisfactory conclusion for a difficult project.
This project was completed with the VZ-400 displaying the robust reliability of RIEGL terrestrial scanners. The current VZ-400i model incorporates IMU as well as additional integrated sensors to assist initial positioning and registration. For more information about the current models of terrestrial laser scanners, please click here.