High-definition surveying provides centimeter-accuracy data for a highway widening project
Project: Dallas North Tollway
Civil engineer: PBS&J
Project application: Laser scanning produces highly accurate digital terrain models and as-built drawings of a busy Dallas highway without closing lanes or endangering surveyors.
Often described as "the main street of the Metroplex," the 22-mile-long Dallas North Tollway (DNT) is the primary corridor between the downtown Dallas business districts and growing cities north of Dallas. This stretch of six-lane highway supports as many as 140,000 vehicles and more than 450,000 toll transactions each day. So when the North Texas Tollway Authority (NTTA) was ready to embark on a tollway widening project, they looked to do so while maintaining safety for motorists, as well as for engineers, planners, and land surveyors who would need to collect the necessary data.
Time estimates for the project indicated that gathering the data using conventional survey techniques would take at least a month, given the required accuracy.
Surveyors would have to take standard survey cross sections every 100 to 200 feet, as well as collect data at top of curb, center of roadway, and edge of road positions. To achieve these requirements, at least one lane of this busy corridor would have to be shut down during the day for at least a month.
Mark Bouma, P.E., director of engineering for the NTTA, explained, "Our first responsibility is to the customer. There is no good time for lane closures or other distractions that would be required with conventional survey methods." The NTTA looked to their engineering consultant PBS&J, a Miami-based engineering firm specializing in a wide range of infrastructure planning and engineering services, for an alternative. PBS&J readily agreed that conventional methods would require lane closures and intense safety procedures to protect motorists and surveyors. Instead, they recommended one of the industry’s latest technology advancements—high-definition surveying, a nonintrusive means of rapidly collecting detailed and accurate as-built data. PBS&J is one of the first consulting engineering firms to apply the high-definition surveying techniques to help transportation departments gather as-built data to support a wide range of projects.
During the following month, the NTTA and PBS&J evaluated a 1-mile stretch of tollway planned for widening. This segment included a 14-lane toll plaza facility and exit ramps to major corporate business and residential areas. In the end, PBS&J delivered centimeter-accuracy, digital terrain maps and as-built drawings to the NTTA. And they did it without stepping foot onto the busy tollway corridor.
Throughout the high-definition scanning process, the corridor remained open at all times to tollway customers. There was no need for traffic maintenance and, most importantly, survey crews were able to do their work completely off the busy roadway, in a safe environment and within the original budget established for a traditional survey approach.
Acquiring field data
Laser scanning, or high-definition surveying, requires the use of a laser scanner that includes a laser beam, scanning mirrors, and a camera. PBS&J relies on the Leica HDS2500 3D laser scanner from Leica Geosystems HDS LLC. The HDS2500 scanning mirrors direct the laser to the object(s), and then the laser beam automatically detects the object’s 3-D coordinates and measures thousands of points in a single second. The system’s internal camera projects a picture of the 40-degree by 40-degree field of view seen by the scanner onto the operator’s laptop computer.
According to the manufacturer, the HDS2500 3D laser scanner has a singlepoint range accuracy of +/-4 millimeters, angular accuracies of +/-60 micro-radians, and a beam spot size of only 6 millimeters from 0- to 50-meter range.
Prior to beginning the data gathering study, the PBS&J team prepared a scan plan to determine where the laser scanner could be situated best to acquire full coverage of the objects to be scanned along the 1-mile route. A quick survey of the surrounding buildings around the tollway provided many opportunities, the best being the roof of one of the adjacent office buildings. Using a 12-foot tripod outside the right-of-way fence, the surveyors positioned the scanner to acquire the most coverage, encompassing the entire roadway, retaining walls, and the toll plaza.
Once in position, the surveyors simply oriented the scanner toward the designated location, selected the desired measurement area and scanning density, and then pressed auto-scan. In just minutes, the complete surface geometry of all exposed surfaces within the field of view was captured remotely in the form of dense, accurate, 3-D point clouds.
In three days, the survey team completed about 20 scans spanning the entire 1-mile segment, capturing more than 45 million points of data with an average of 0.5-inch spacing. Once the scan was complete, the surveyors used GPS/RTK techniques to validate the scan data, and to establish horizontal and vertical control of the point clouds. The resulting surface model recorded accuracies of less than 0.1 foot over the mile-long stretch of linear roadway.
From data to drawings
Back at the office, the PBS&J team uploaded the laser scanner’s data points into Leica’s CloudWorx 3.0 software.
CloudWorx 3.0 is designed to manage large data sets, such as those created by laser scanning, within the traditional CAD desktop environment, without scan data conversion.
Once uploaded, the software allowed the PBS&J team to create digital terrain models and final as-built drawings quickly.
The complete as-built map of this portion of the Dallas North Tollway was delivered to the NTTA one month after project go-ahead. Better yet, the laserscanned data continues to provide ongoing benefits to the NTTA staff. Already, management has plans to use this information to prepare detailed inventory management systems and improved corridor maintenance strategies.
"We’re extremely pleased with the final product," said NTTA’s Bouma. "The accurate as-built data created from this project will have long-term benefits to the agency, our design consultants, and especially to our customers who depend on us to maintain a safe and efficient travel environment."
Dave Stewart is division manager for PBS&J in Houston. He can be contacted at DRStewart@pbsj.com.