With a control network established, the team would rely heavily on a key component of the Mobile LiDAR system: Inertial Measurement Unit (IMU). The IMU accounts for movement of the sensor system as it travels through space, adjusting for the smallest deviations and correcting the position 100 times per second. “The difference with our team was confidence and a mindset.” Wygant explained, “Even though we knew exactly what the LiDAR equip - ment’s capabilities were, and how it was technically built and supposed to function, we had to compensate for the lack of GPS. We knew that the Inertial Measurement Unit (IMU) should perform as we anticipated and were initially confident yet concerned that when we reviewed the data it would not be looking upright with the track always down. We needed to see that it wouldn’t look like a corkscrew.” The IMU did perform as expected and was key to realizing their success. The software they used was also not built to manage such a wide vari- ance between actual and captured location. The positional error (hori- zontal and vertical drift as well as a longitudinal expansion and con- traction) was well beyond any pre-defined error index in the software. But that was expected. They trusted the integration and robust qualities of the technology, and their experience provided the confidence they needed to know that it could be accomplished, successfully. “Colliers’ approach to project management also enabled us to have the latitude to be inventive, creative and to take risks,” Wygant explained. “The existing geometry of the tracks was a known quantity derived from as-built plans which provided a relatively accurate guide as to how the track was constructed. The plans showed the geometry of the curves, horizontal and vertical position and superelevation of the align- ment. We used this valuable information to supplant the GPS positional information that we were lacking in the tunnel environment.” The team used several different criteria to adjust the Mobile LiDAR model to what they knew about the control that was measured by distances and defined a hierarchy of controlling data. This ranged from highest (traversed and direct measurement) to known objects seen in scan and measured by various survey methods (this included pull boxes to rail components) to ancillary information gleaned from records or as-build documents to common structures and features seen by both opposing track runs in dual-track tunnels. Through the multiple iterations of post-processing the datum, redefining and quality control, the team arrived at a solution they could stand behind.
After collecting the LiDAR mapping, the team routed the 3D posi - tion of the cable tray system based on the LiDAR mapping results and intended locations. They checked the dynamic car body to ensure clearance to the proposed cable tray locations. Accuracy At the conclusion of the project, their relative accuracy ended up being within hundredths of a foot making it extremely accurate for clearance measurements. Their absolute accuracy, which is their placement on planet Earth, was dialed-in to within one foot. This was necessary to make the factory cuts for the leaky coaxial cables fit the field condi - tions and was a huge accomplishment. “All things being equal, all Mobile LiDAR post-processed to project control should match the provided values at control. It’s what happens in between control that is in question,” Wygant said. “Our extensive review, ever expanding site knowledge and dedicated champions all contributed to the successful creation of base mapping data spanning the entire tunnel network.” Conclusion Once the complete railway and tunnels were scanned, it produced both an accurate condition survey as well as the most accurate as- built the rail authority has ever had. Perhaps the bigger achievement was getting this information into people’s hands. Getting it to the rail authority in an accurate, digital manner that would help guide their future was invaluable. The successful upgrade of this system mapping was accomplished through a combination of technology and human-power. As a result of the combined efforts and innovation on this project, Colliers Engineer - ing & Design has been awarded a US patent for Tunnel Mapping. The patent recognizes the utility of Mobile LiDAR in GPS-denied environ - ments combined with alternative means of geometric trajectory meth- odologies. This paves the way for other transit and rail tunnel systems to be mapped with high accuracy and repeatability. And they said it couldn’t be done.
MARALIESE BEVERIDGE is Senior Technical Writer at Colliers Engineering & Design.
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