Tunnel Vision

February 25, 2008
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This single raw point cloud captures approximately 12 million points of a 100-foot section of a Dallas Area Rapid Transit tunnel


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 Site

The 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.”



Crew members get to work setting up targets, discussing points to be checked and preparing to scan each of the 346 points in the tunnel.

The crews’ first task was to establish XYZ coordinates on approximately 40 existing monuments the original contractor had placed in the tunnel but had not yet assigned coordinates to. Using established control outside the tunnel on each end, Laser Geomatics survey crews ran independent 3.5-mile-long traverse lines through each portal. Fire control corridors connected the northbound and southbound portals and gave the survey crews the ability to cross-check their accuracies to each linear traverse every 800-1,000 feet. The poor lighting conditions caused many setups to be duplicated to achieve the desired closure results. Assigning XYZ coordinates to the monuments in the tunnel gave a basis for all laser scanning activities to be tied to DART’s specific coordinate system. The crews tied the 1,300-plus scan control points to the newly assigned coordinates using the existing monuments.



Near the DART CART, so coined by Laser Geomatics crew members, leader Brian Thompson directs crew member Curtis Deckard where to put a target on the complex tunnel project.

The crews’ first task was to establish XYZ coordinates on approximately 40 existing monuments the original contractor had placed in the tunnel but had not yet assigned coordinates to. Using established control outside the tunnel on each end, Laser Geomatics survey crews ran independent 3.5-mile-long traverse lines through each portal. Fire control corridors connected the northbound and southbound portals and gave the survey crews the ability to cross-check their accuracies to each linear traverse every 800-1,000 feet. The poor lighting conditions caused many setups to be duplicated to achieve the desired closure results. Assigning XYZ coordinates to the monuments in the tunnel gave a basis for all laser scanning activities to be tied to DART’s specific coordinate system. The crews tied the 1,300-plus scan control points to the newly assigned coordinates using the existing monuments.



A typical 26-foot section containing the “cleaned” points used to create the 3D surface; the centerline alignment is in white, and the InRail template is in red.

Coordination between Laser Geomatics and DART was critical during the entire project. BHI worked very closely with the DART Train Control Center, the operations hub for the entire light rail transit system. Weekly progress meetings were held to avoid conflicts with other maintenance programs. Daily conversations with control center managers were needed to avoid conflicts with track usage. Since the two tracks in the tunnel--one each for northbound and southbound traffic--are the only way into and out of the downtown Dallas area on this line, all trains, equipment and maintenance vehicles had to use the tunnels despite the scanning process. For safety purposes, the track’s high voltage catenary lines were turned off while scanning activities took place. This meant all other traffic had to use one single track in and out of the tunnel. The Control Center occasionally interrupted scanning efforts to allow maintenance vehicles to use the track being scanned. Laser Geomatics crews had to then relocate to the fire control corridors located approximately every half-mile in the tunnel while the catenary lines were powered on, the maintenance vehicle made its way through the tunnel and the catenary lines powered off again. This process sometimes took nearly one hour. The control center, however, kept these interruptions to a minimum.



BHI staff and a DART flagger prepare to enter the tunnel to begin their scan work.

Input and Output

Each 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 360ox310o scan 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 screen capture of an “unrolled” InRail surface color-coded by elevation.

Post processing of the data began by cleaning out all obstructions to the liner, including pipes, hand rails, rail tracks, lights and the tunnel floor, in Leica’s Cyclone software. The resulting scans represented only the liner surface and a small amount of the concrete floor. The data set was then clipped into 26-foot sections, with the beginning and ending one-half foot overlapping data to the previous and next section. A typical design template was then created for each section. The 26-foot sections were a correction from the original workflow in which 100-foot sections were created. The final surface files of 100-foot sections, collected in GEOPAK by Bentley Systems Incorporated (www.bentley.com), were too large to open. Therefore, the modification to 26-foot sections produced file sizes in a usable range. Even with 26-foot sections, the average file sizes were nearly 25 megabytes. “This project was as much a data management project as it was a scanning project,” Flint says.



A typical cross section with the X, Y and Z coordinates at 10-degree intervals.

Three-dimensional vectors and supporting digital photography defined the limits of all “areas of concern,” relating back to the design tunnel alignment for location and monitoring. The typical section and scan points were then exported as MicroStation .DGN files and XYZ ASCII files, respectively. Each ASCII file was then imported into a surface in Bentley InRail and “unrolled” using the corresponding template. The unrolled InRail surfaces were triangulated, and BHI technicians created 1-foot cross sections from the unrolled surfaces. InRail then automatically rolled the cross sections back to their original positions using the typical section template. In order to place XYZ coordinates at 10-degree increments on the cross sections, BHI created a new MicroStation .MDL routine to read the cross sections and relate them back to their original coordinate system, automatically placing XYZ coordinates on the cross section in the process. The unrolled InRail surfaces were then translated into GEOPAK .DAT format using propriety BHI software created specifically for this purpose.



Crew leader Brian Thompson (seated) enters data into a laptop, while crew member Curtis Deckard looks on. In the background, instrument operator Duane Dozier and a DART flagger inspect an “area of concern.”

Deliverables to the client included the 3D vector lines for the “areas of concern,” more than 32,000 cross sections on 1-foot intervals with coordinates at 10-degree intervals, GEOPAK surface files of the “unrolled” surface and ASCII files of points that defined the surfaces. All data was delivered on external hard drives to avoid having DART upgrade hardware to store the data.



It was promoted all around the state through the Texas Education Agency as a continuing education offering. This year it is being held in Tyler, Texas, hosted by Tyler Junior College through their Surveying program.

A Complex Mission

At 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 of POB.

BHI at the Box Office

In 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.

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