C+S July 2021 Vol. 7 Issue 7 (web)

Dynamic rail car-body and LiDAR analysis

carriers. Aside from being extremely expensive, the length of each cable was critical and had to be measured with laser-accuracy as it had to be segmented in predetermined lengths before installation. Each cable run also had to accommodate an installation path that wove from the inside of one curve to the outside of another across the tunnel while circumventing other appurtenances inside the dynamic envelope. So, every measurement counted and the only way to achieve that kind of accuracy was by using the 3D solution that LiDAR brings to the table. Hence the Challenge Rail maintenance personnel work in an unforgiving environment. The number one rule of trains is that the schedule never stops. So, to get the job done, portions of track were only shut down for short periods of time overnight during non-revenue hours to allow multiple survey teams to get their work done. In order for a hi-rail truck carrying the LiDAR instrumentation to physi - cally enter the track zone, it has to be mounted on the rails at a rail/road- way crossing or in a rail yard. The closest crossing might be miles from the job site, so that factor alone can eat up precious track time. So how was it done? To accomplish the mobile mapping of the 104-mile tunnel system, the project team established survey controls from the surface station traversing down into underground stations. Then, scan targets were placed and surveyed along the tunnel corridors. This method continued to the next station where the team would traverse back up to ground surface checking into their GPS control. There were challenges presented by various coordinate systems his- torically used. Combining, translating, matching, and checking was an essential task in the making of this continuous mapping network. The authority developed their own low-distortion projection coordi- nate system designed to minimize mapping distortion associated with curved-to-flat mapping surfaces however all as-built data had not been re-projected at the time of survey. The approach was to traverse the entire tunnel system with the idea of capturing multiple survey datums and bring them into one. That way, they could use this datum through all the jurisdictions and serve-up multiple data sets and convert it on the fly.

Hi-Rail truck mounted with Mobile LiDAR unit

great deal of discussion the authority was convinced they had a viable approach and qualified team, so they championed the innovation. “No one had ever performed mobile mapping underground for more than a few miles,” said Paul DiGiacobbe, Director of Geospatial Ser - vices at Colliers Engineering & Design. “We knew it would be dif - ficult, given the challenging geometry and sheer length of the tunnels.” The project team, consisting of three full-time survey crews, the Mo - bile LiDAR team, and key members from the authority, performed a 10-mile pilot study in which the initial results were validated and further substantiated the team’s confidence in the project’s success. “The pilot study was key because it gave us the opportunity to learn how to deal with the performance of the Mobile LiDAR system with - out GPS,” explained Clay Wygant, Mobile LiDAR Manager at Colliers Engineering & Design. “We captured enough data to test and confirm our tunnel mapping theory. This set us up for success for the remainder of the tunnel system.” Easier Said than Done The design and installation of cell service had to be accomplished by fitting pre-fabricated radiant coaxial cable onto the tunnel walls us - ing a cable tray throughout the entire 104-mile tunnel system. It was also required to have a safety clearance verification performed which ensured that the new cabling system was far enough away from the passing train cars. Radiant, or leaky, coaxial cable is a type of cable designed to both transmit and receive wireless signals and can be utilized by multiple Working on the railway is hard. Using LiDAR underground is really hard— but not impossible.


july 2021


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