Laser Scanning Aids Lady Liberty
When The National Park Service re-opened the crown of the Statue of Liberty to visitors on July 4, 2009, after an eight-year closure for safety and security reasons following the 9/11 attacks, there were some restrictions. The age of the structure and lack of modern safety features meant that groups were limited to 10 visitors at a time. Secretary of the Interior Ken Salazar pledged to continue upgrading the interior to make it safer and more accessible.
Today, that promise is becoming a reality through a year-long, $27.25 million renovation project that began in October 2011. “We are taking a major step in bringing a 19th century icon into the 21st century,” Salazar said.
Led by Joseph A. Natoli Construction Corp. of Pine Brook, N.J., the project includes installing code-compliant stairways within the monument; updating mechanical, electrical and fire suppression systems; replacing the elevators; and rehabilitating restrooms. Some of the existing structures must be demolished and carefully reconstructed to incorporate the necessary safety features while preserving the historical details of the beloved icon. Before beginning the demolition and construction work, Natoli turned to Matrix New World Engineering Inc., headquartered in Florham Park, N.J., to capture crucial data about the existing structures. “There was a need to verify existing vertical dimensions between the various landing points (known as P levels) through the pedestal,” says James Sens, PLS, PP, director of HD surveying and mapping for Matrix. “In addition, there were several points of critical clearance that needed to be verified. Another goal of the project was to check the vertical alignment of the shaft through the pedestal."
Matrix had a limited window of opportunity to gather the required data; Natoli wanted to have the deliverables in hand within just two weeks of the authorization to proceed. Sens, who is president of the New Jersey Society of Professional Land Surveyors and has a 25-year history in land surveying, carefully considered all of the tools available to him in light of the client’s needs. After visiting the monument in person, Sens decided that laser scanning with the firm’s Leica ScanStation C10 would be the best way to complete the project.
“I decided to use the scanner because I could capture both the vertical and alignment data while creating a time savings,” he explains. “I also believed the scanner could help me reduce some of the error that may have been induced by the multitude of setups required using traditional methods. In addition, I believed that using the scanner would provide flexibility in measuring and correlating the alignment checks.”
The scanner’s ability to capture an abundance of data was also a major consideration. “As we scanned through the shaft, the scanner would capture virtually all the existing conditions through the shaft, not just the specific points of concern, all in less time than a traditional survey.”
Access restrictions meant that Sens would have to work quickly. He arrived at the Statue of Liberty after hours on Oct. 11 and began setting fixed-height targets and paper targets for control. Registering the scans together would provide control for both the horizontal and vertical component.
Using the Leica ScanStaion C10, Sens took a total of 19 scans; 16 were for the primary vertical survey, while the remaining three supplemental scans acquired some of the structural elements that were voids in the primary scans. The field work took about six hours over two evenings.
Back in the office, Sens used Leica Cyclone software to process the scans, which comprised approximately 240 million data points. To verify the vertical dimensions between the P levels, Sens first used the Leica software to create a best fit surface at each level. He also created points on a grid that he averaged and manually ran statistics on to verify his choices. Sens says he used about 20 to 30 points per level on average in his manual analysis. Once he was satisfied with his “best fit” of the P levels, he then subtracted out the elevations for the vertical dimensions.
To measure the vertical alignment, Sens established a baseline at 1P by manually selecting points in the cloud from the 25- by 25-foot shaft. “I picked a series of points along each of the four walls—near the corners and approximately at the midpoints of the wall,” he explains. “This was done at each of the P levels and at a number of intermediate points through the shaft.”
Sens then exported these points to AutoCAD Civil 3D to create line drawings. A least squares best-fit baseline was created from the points along each of the four walls at 1P that served as the baseline and the point of analysis. The points going up through the shaft were then compared to the baseline to determine whether there was any deviation. “The masons that built the pedestal did an amazing job—there is little deviation in the alignment running through the shaft,” Sens says. “No single point measured varied more than 1-1/8 inch (0.9 feet) from the baseline at 1P.”
In less than two weeks, the deliverables were ready for Natoli. Sens delivered the project on Oct. 28, the 125th anniversary of the Statue of Liberty’s dedication. “It was pure coincidence that we delivered the scans on that date,” Sens says. “I was so focused on getting the work finished for them that it never really dawned on me until after the fact.”
For Sens, who has eight years of experience in laser scanning, the Statue of Liberty project was a highlight of his career. “One of the best things about this project was just being on Liberty Island after hours, with no people and virtually no lights except for those shining on the monument itself and the lights from the Manhattan skyline. The view is so awesome it takes your breath away,” he says. “Also, it was so thrilling to be able to integrate a new technology into a more traditional construction workflow.”
Although this was a high-profile project, Sens believes the experience carries broader implications. “Surveyors need to understand that laser scanning is a reliable solution not just for the elegant or extraordinary projects, but also for projects that might be considered more mundane,” he says. “The increase in safety and the time savings are major benefits. And the amount of data we can collect is just phenomenal. We now have the ability to provide information that the client might eventually need but isn’t even anticipating now—that’s the really exciting part of it.”
Dennis Petrocelli, PG, senior vice president of Matrix, is equally convinced of the value of scanning technology. “Matrix has always been driven by our mission of resolving complexity and enabling progress,” he says. “We are committed to providing our clients with excellence and innovation. Acquiring and deploying the latest 3D scanning and survey technology to support our clients is critically important to us. I am honored that Natoli Construction and the U.S. National Park Service selected Matrix as a team partner to survey the interiors of the Statue of Liberty for the life and safety upgrades project.”