Texas-based Laser Geomatics, a division of civil engineering firm Bohannan Huston Inc. (BHI), is a pioneer of laser scanner use. The firm was one of the first companies in the world to use laser scanners when it purchased the 20th Cyrax laser scanner from Cyra Technologies in 2000. In October 2005, it applied its expertise with this technology when it was contracted to map twin 3.5-mile tunnels located on the northeast side of downtown Dallas, part of the 45-mile Dallas Area Rapid Transit (DART) Light Rail Transit (LRT) system that serves Dallas and 12 surrounding cities.
Tasked with providing an “as is” model of the shotcrete-lined surface of the tunnel and mapping the “areas of concern” where water was seeping through the liner, Laser Geomatics crews had a challenging project ahead of them. The tunnel liner consists of 2- to 4-inch-thick polypropylene-fiber-reinforced shotcrete pneumatically placed against the tunnel’s formation. Crews knew that measuring the irregular liner surface and the water seepage areas to meet the 10,000-points-per-square-meter requirement with traditional survey methods would have been impossible. So, Laser Geomatics turned to its laser scanning talent to survey the liner. It was an important job; the surface will be used as a baseline against future data sets to determine any possible movement in the liner due to the buildup of moisture behind it. It was a project that required resilience and creative vision.
The Scope and the SiteThe project’s requirements and environment presented unique challenges for Laser Geomatics crews. The entire project required 346 individual scans, more than 1,300 control points and more than 38 GB of data. Due to the extremely tight accuracy constraint (+/-0.25 inch) and the density requirement (10,000 points per square meter), Laser Geomatics chose to deploy the Leica Geosystems (www.leica-geosystems.us) HDS4500 phase-based laser scanner. A portable rail cart was designed and built in-house that included a mount for the scanner, a platform for moving related equipment and a passive braking system, which locked the cart into position at each scan location.
Shutting down the rail system to acquire the data was not an option, so the field crews were only able to occupy the work area between 1:00 a.m. and 4:00 a.m. Poor lighting conditions made visibility difficult. “This was a unique experience,” says Survey Crew Chief Roy Hendershot. “Getting accurate survey data while working in an environment that had bad lighting and no electricity to power lights was extremely difficult.”
Input and OutputEach night, after permission from DART and coordination with DART flaggers, the Laser Geomatics team proceeded into the DART right of way to the beginning location. Each 360ox310oscan contained approximately 12 million points, so to save time during the scanning process, BHI technicians adjusted the scanner setting to filter the point cloud data so the accompanying Cyclone software processed fewer than one million points for each scan. The entire collection of 12 million points was saved into a proprietary Leica HDS4500 .ZFS file, which office technicians later used to extract the points needed.
“The ability to only display user-defined amounts of points in the Cyclone .IMP file is a helpful tool when collecting 12 million points in four minutes, says Jim Flint, PE, project manager. “The time it takes in the field to display all 12 million points is significantly more than a few hundred thousand points. But to be able to store all 12 million points instantly for later use is invaluable.”
Six targets placed over premarked surveyed control points in each scan were used to register multiple point clouds and position the data into DART’s control network.
A Complex MissionAt the project’s end, the client received a highly accurate as-built model that met expectations on time and under budget. “DART’s surveyors estimated six months of conventional surveying. Bohannan Huston did the work in six weeks,” says Subhasis “Suku” Banerjee, PE, DART project manager.
With the baseline completed, DART plans to have an additional as-built performed in the next three to five years that will allow engineers to analyze the data and compare the two surfaces to see if the shotcrete liner is being compromised by the water seepage.
“It was exciting to work on this project because of the challenges associated with creating a surface on a tunnel where a single coordinate point can have multiple elevations,” says Sonja Ellefson, scanning technician.
The diligent work performed by BHI and Laser Geomatics on the DART tunnels was honored as the winner of the Technology Innovation category of the first annual MAPPS Geospatial Products and Services Excellence Award hosted by the Management Association for Private Photogrammetric Surveyors. For more on this awards program, look for a feature article about its specifics in a summer issue ofPOB.
BHI at the Box OfficeIn addition to providing valuable data on tunnels, Bohannan Huston has been a Hollywood hit in some respects. The firm provided high-tech special effects for the November 2006 Touchstone Pictures box office hit “Déjà Vu.”
“Déjà Vu” was directed by Tony Scott, produced by Jerry Bruckheimer, and stars Denzel Washington, Val Kilmer and Jim Caviezel. Washington portrays a federal agent who falls in love with a complete stranger while racing against time to track down her killer.
Touchstone Pictures contracted BHI to provide laser scanning services to be used in the visual effect of passing through a time portal. BHI employee Steve Snyder spent several weeks on set in New Orleans and in sound stages in Los Angeles working with the visual effects company, AsylumFX. He scanned several locations in New Orleans, including restaurants, street scenes, river boats, vehicles and many sets on the sound stage. He also scanned several of the actors.
The scanned data was processed in BHI’s Dallas office and then sent to AsylumFX. BHI processed the scanned data by decimating the point clouds to varying degrees. AsylumFX then took that data and imported it into its rendering software for coloring and animation and inserted it into the appropriate scenes to create the amazing visual effects seen in the movie.