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 capturing surrounding furniture, surveying a metal pipe must pose the most challenging subjects for LiDAR as there are few clues in the data to assist with the registering and alignment of scans. We set about this by mounting a VZ-400 into a crawler provided by the Water Services Group and set up a process to take scans at 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 scanner via a fibre optical link, which via ethernet switching also allowed us to capture photographs and video at the same time.
Back on the office, the scans were realigned using data from the IMU and then brought into RiScan Pro for manual, fine stitching using the MTA tools. It was vital to prevent roll and pick tie points from small imperfections in the pipe revealed by the scanners. 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 just under two metres over 1.8km; that will do nicely.
The survey was able to show in detail four additional anomalies that were unknown to the client engineers. Had these not been identified, it could have rendered planned maintenance ineffective. It did mean that additional access had to be dug to address issues, but this remains hugely less expensive and time-consuming than the alternative.