3D scanning assists in Hoover Dam bypass project.

A DEI surveying crew member utilizes a Topcon total station to establish scan control points.
Over 14,000 cars and trucks travel the tiny two-lane crossing of the massive 726 ft high, 1,244 ft long Hoover Dam in Arizona each day, double the volume of 15 years ago. After conclusive research, a massive project of building a four-lane bridge spanning the Colorado River is now in the works. Planners hope to increase the traffic safety around the Dam area, as this section of highway is narrow, winding and steep, inadequate and unsafe for the current volume of traffic. Other proponents of the project desire to unclog a valuable North American Free Trade Agreement (NAFTA) trade artery and spur additional local commercial use, all within the confines of reducing the overall environmental impact as much as possible.

In late 2001, AMEC, an engineering services and consulting company of Phoenix, contracted with DEI Professional Services, an engineering and surveying consulting firm in Phoenix, for a monumental 3D geological mapping of the Hoover Dam area for the massive $198 million bypass project. The new crossing will feature a four-lane divided highway and a 1,900 ft bridge that will bypass the narrow and heavily congested U.S. Highway 93 crossing at the Hoover Dam. AMEC is also charged with developing approach bridges and roadways in Nevada and Arizona, a tunnel in Nevada, other interchanges, and several wildlife underpasses and overpasses. DEI Professional Services provided detailed digital terrain models of the rough terrain at the point where a proposed bridge will cross the Colorado River.

Faced with the potential hazards of 100 degree F-plus weather, treacherous terrain and heightened security, DEI had to ensure the safety of its field personnel. Management decided to use 3D laser scanning technology to meet the demands of the project that at some points placed employees on cliffs over 800 feet above the Colorado River.

DEI’s arsenal of equipment included Riegl USA LMS-Z210 3D laser scanners (Riegl USA, Orlando, Fla.), Trimble 4800 GPS systems (Sunnyvale, Calif.) and Topcon GTS 312 total stations (Pleasanton, Calif.). Each contributed to the successful final digital terrain model AMEC used. The survey work was conducted in September 2001. A team of four DEI field personnel worked on the project for three consecutive days. Final completion of the project is anticipated for 2007.

Due to the climate and terrain surrounding the Hoover Dam, Mother Nature was the biggest obstacle for DEI to overcome.

“We spent three days in the extreme climate where the average temperature was close to 105 degrees Fahrenheit,” says Jason Kack, RLS, DEI vice president and survey division leader. “I had grave concerns about our personnel safety as well as the proper functioning of the equipment in such a harsh environment.”

A DEI scanning team stand atop the Nevada Needle valve building to complete scans of the Arizona side of the Dam.

Danger: Curves Ahead

The DEI survey team was charged with developing a detailed 5 ft Digital Terrain Model (DTM) of the area where the new bridge would be constructed. This target area was 1,500 ft downstream from the Hoover Dam and 840 ft above the Colorado River. The actual scanned area encompassed a 500 ft wide x 2,400 ft long area spanning from the Nevada to the Arizona side, with a steep 900 ft elevation change that proved to be perilous and unforgiving.

A view from the Arizona side, looking west across the Dam. Intake towers are visible.
The team used the Riegl LMS-Z210 3D laser scanner to capture 3D point clouds with a high degree of accuracy. The project entailed strenuous climbing using rappelling techniques on Black Canyon’s vertical walls. The laser scanner allowed DEI to work with little or no light, which was helpful since the steep canyon created a false twilight.

The Z210 scanned a beam along two axes over any field of view. Using a high speed-rotating mirror to direct the laser beam over a precise pattern allows accurate three-dimensional scanning of the surface. In 30 seconds, up to 240,000 points were mapped. Generating nearly 10,000 measurements per second, the result was a very densely populated point cloud of spatial measurements yielding an immediate and accurate 3D representation of the scene. The large 300¿ x 80¿ field of view reduced the total number of scans needed as well as the post-processing time needed to mesh final scans.

DEI captured 40 scans from 11 locations over the three-day period. The resulting millions of individual measurement points comprise a 3D representation of the site. The data was then exported to PolyWorks software (Innovmetric Software, Sainte-Foy, Quebec, Canada) for post processing.

The top of an overflow outlet gate structure looking north upstream was scanned for the Dam project.
The rough terrain facade of the river terrain also included precarious undercuts or cave-like openings that were dangerous obstacles for crew members. The Topcon GTS 312 and Trimble 4800 equipment were used to establish master horizontal and vertical control that were used for topography mapping. The crew also used the Topcon GTS 312 to establish coordinate values for 36" reflectors used to measure the enormous distance across the canyon. By using such reflectors, longer measurement ranges were possible and lessened the danger of excessive climbing by the crew.

Riegl’s LMS-Z210 includes a True Color Channel enhancement that dramatically improves texturing of 3D models by gathering additional color data that realistically portrays acquired targets. The RGB color values were correlated exactly to each measurement pixel. This allowed display of complete red/green/blue (RGB) target representations in modeling software packages. It also provided effective modeling calibration for high-resolution photography.

“We needed to get a true laser mapping of the bridge span area,” AMEC’s David Peterson says. “Our goal here was to verify distances and elevations for our pre-construction phase.”

A DEI scanning technician sets up his equipment on the roof of a Nevada valve house.

Key To Success

As with any project, the Hoover Dam project had its challenges. DEI worked closely with Riegl USA, who optimized the LMS-Z210 and the field software package for the project. Riegl included its 3D-RiSCAN software, a companion package to the LMS-Z210 scanner, which allowed the operator of the 3D imaging sensor to perform a large number of tasks including sensor configuration, data acquisition, data visualization, data manipulation and data archiving. The 3D-RiSCAN was then used to export data to the post-processing software: Innovmetric’s PolyWorks, Boss International’s QuickSURF (Madison, Wis.), Autodesk’s AutoCAD (San Rafael, Calif.) and Trimble’s Terramodel.

“Using PolyWorks to align the scans allowed us to place the scanner in any orientation necessary for scan coverage,” Kack says. “Conventional methods would have taken weeks to perform with only a small net percentage of usable data.”

Post-processing data also proved to be challenging. DEI had to mesh the final scan DTM into an aerial DTM of both sides of the canyon walls. This process took approximately 100 hours to complete because of vegetation editing and modeling. DEI completed a series of curvature maps in PolyWorks. These maps identify areas of steep curvature on polygonal surfaces, used as an aid in identifying geologic structure and fault lines. DEI was able to present a number of physical and electronic deliverables, accurately map 1 ft contours of the canyon walls and provide data on fault structures that could affect the bridge project.

“It would have taken weeks for us to complete this project conventionally,” Kack says. “The Riegl technology dramatically brings more valuable information to our clients.”