Preserving China's Past

November 1, 2008
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Laser scanning opens new market opportunities in historical preservation.

The Iron Lion of Cangzhou in Hebei Province, China.


Laser scanning technology has created new work for land surveyors, mainly through its ability to gather accurate 3D information rapidly. But in addition to speed, scanning is also noninvasive--the surface being scanned is not touched by human hands, prism poles or any other object, which makes it ideal for sensitive sites. As the technology becomes better known and more widely available, this “light touch” characteristic is proving to be as much of a selling point as its speed and precision.

One company that has benefited from the noninvasive nature of laser scanning is Dynasty Group. The Chicago-based engineering and surveying firm first tapped the technology in 2001 to survey a historically important stone facade slated to be torn down and rebuilt as part of a reconstruction of Wacker Drive, a two-level street in Chicago that was originally constructed in 1926. Historical documentation using laser scanning has since become a growing segment of Dynasty Group’s business. Investing in this new technology has allowed the company to expand into new markets−including some culturally significant preservation work in China.

 A computer-generated model of the lion based on 3D laser scan data.

The Iron Lion of Cangzhou

At 19 feet high, 21 feet long and about 11 feet wide, the 1,055-year-old Iron Lion of Cangzhou in Hebei Province is China’s largest and oldest surviving cast-iron sculpture. But time hasn’t been kind to this important cultural icon: The lion’s tail disappeared in 1603, the entire sculpture was toppled by a storm in 1886, and restoration attempts in 1984 appear to have caused cracking. Today, the hollow sculpture is supported by bulky, obtrusive pipe scaffolding.

In 2007, officials at the China National Institute of Cultural Property decided to find a way to stabilize the sculpture so that the scaffolding could be removed. Historic preservation architects and engineers at the institute determined that modeling the sculpture through finite element analysis (FEA), a simulation technique commonly used in structural design, would be the best way to identify weak points and determine alternative reinforcement methods. However, comprehensive engineering drawings or 3D scans of the sculpture would be required to create the FEA model, and the sheer scale of the work was daunting. To obtain the required data, the institute turned to Dynasty Group, which had done some work for the institute previously.

When Zhong Chen, PE, PLS, Dynasty Group’s president, was approached for the work on the Iron Lion in August 2007, he knew that laser scanning’s light touch would be ideal for the project. To gather the required high-resolution, high-accuracy scans with a minimum amount of field work, Dynasty Group used a FARO LS 880 laser scanner, which is designed to create photorealistic 3D images for dimensionally accurate CAD models. “We promised a high-resolution model suitable for FEA, and the LS 880 is a phase-based scanner so it’s fast and accurate,” Chen says. “And because we could get close to the lion, we didn’t need the longer range of a time-of-flight scanner.” Chen achieved 2-millimeter accuracy by using 25 scanner setups and taking shots about a millimeter apart. Each scan took approximately seven minutes to complete.

 A computer-generated wire-frame model based on 3D laser scan data.

To avoid setting targets on the iron surface, the four-person survey team set control points around the outside of the hollow sculpture as well as inside it through the opening underneath. “We validated the data by setting targets and then acquiring them with a Leica TPS703 total station,” Chen explains. “If I have at least four points in space accurately located independently, I can line them up in 3D and compare to the same scanned points. If they fit within tolerance, they validate each other.” Scanning under the lion allowed surveyors to see both the interior and exterior control targets simultaneously, which enabled them to register 12 scans in the lion’s tight interior without touching the iron.

To scan the top of the sculpture, the team mounted the scanner on a specially constructed 5-by-5-foot platform stabilized by ropes and anchors. This scaffold also allowed the team to see additional control targets inside the lion through an opening in the top of the sculpture. “Scanning on a scaffold is always a challenge, and there was some trial and error involved,” Chen explains. “Sometimes we had to do a couple of scans and pick the best one. But without the scaffold, we couldn’t have shot the top of the lion.”

Within just three days onsite, about half a gigabyte of data was gathered from each scan setup for a total of 13 gigabytes. These data were sent to the office, where FARO’s software suite was used to register point clouds before exporting the data to InnovMetric’s PolyWorks software for further processing, including building the digital renderings that would be required to develop the FEA models.

According to Chen, the scan results are providing a valuable tool for the institute. “Using scanned data for FEA isn’t being done very much yet,” he says, “but they’ve already worked out the bugs for pieces of the lion, and they’re getting very close to analyzing the whole model.” Once this step is complete, the institute will use the data to replace the current bulky, obtrusive props with an effective and minimal stabilization method. If results are positive, missing pieces might even be fabricated and used in a restoration effort.

While the LS 880 can be set up with a camera at the same focal point as the scanner, Dynasty Group hasn’t used the technique on this project--yet. “The goal of this contract was structural analysis,” Chen says. “But once the client saw the results, they liked the product so much that we’re now negotiating for a next phase, which will be documentation of surface corrosion.”

A computer-rendered model of a statue in one of the Mogao Grottoes with color and texture added.

The Mogao Grottoes

Not all historical survey work has been handed to Chen quite so easily. When he learned in early 2007 that conservation collaborator Dunhuang Academy wanted to document the Mogao Grottoes, Chen had to take a different approach to win the project and build the academy’s confidence in laser scanning capabilities.

Commonly known as the “Caves of the Thousand Buddhas,” the grottoes comprise more than 700 caves that were carved out of the hillsides of Gansu province beginning in 366 A.D. The caves, which were designated as a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in 1987, document 1,000 years and several major periods of Chinese Buddhism through intricate art, sculpture and other artifacts. Today, 492 caves remain to shelter approximately 2,100 colored statues and almost 484,400 square feet (45,000 square meters) of murals. The tallest cave is more than 131 feet (40 meters) high.

Many cave features are made of a molded, stuccolike mud and are extraordinarily delicate. And the murals, though still surprisingly bright, can’t be exposed to strong light for fear of fading, which prohibits the use of flash photography by the millions of tourists that visit the caves (although a limited amount of flash photography is permitted for documentation and preservation work). Traditional documentation of these grottoes has been painstaking and involves the use of a custom-built wire grid system along with a total station to take measurements on a coordinate system. However, this method is slow, costly and not especially accurate--documenting just one cave could take a three-person crew up to a year.

“We were sure we had a better way to quickly document the caves in support of archaeological reports,” Chen says. But Dunhuang Academy was somewhat difficult to convince. “Laser scanning had been attempted by another firm when the technology was quite new, and it didn’t go well,” Chen explains. Besides the academy’s uncertainty about the quality of the scans, there was some concern initially about the effect of the laser light on the murals. However, Chen notes that the light from the modern laser scanners used by Dynasty Group is “eye safe” and less intense than that used in flash photography. “It took a couple of presentations and some good timing before the academy agreed to try again.”

Point cloud data and line work on the statue.

As a trial, the academy agreed to scans of two of the oldest caves, each about 1,600 years old. The work began in April 2007 with the FARO LS 880 as Chen’s tool of choice due to its ability to capture the cave carvings in extreme detail. Shot spacing and resolution were similar to the Iron Lion project, and the control setup for the project was relatively straightforward. “We set out a baseline and moved along it,” Chen says, “and since we couldn’t touch walls or hang targets, we used the artwork itself to register scans by identifying corners of paintings.” But since there were hundreds of painting corners, it was easy to get confused. To get around this challenge, Chen and his team used a Leica TPS703 total station laser dot to pick out target corners, and they took just enough digital photos in dim light to keep track of registration points. Scaffolds were only used once, but since so much of the wall art is bas-relief, most sections of wall were shot from several angles.

Not many people have been inside the grottoes in recent years, and Chen says it was an interesting experience. “The caves are absolutely packed with sculpture and detail on the walls and ceilings--lots of carving. And it was fascinating to see the different religious ‘flavors’ of Buddhism from different eras and the different styles of artwork.”

Using the laser scanning technology, crews were able to document one cave per day. “Not only did this cut costs,” Chen says, “it helped preserve the caves. Since humans are always breathing out carbon dioxide, which contributes to cave degradation, just being in the caves is doing damage.” Reducing the amount of time surveyors spent in the caves for the documentation work minimized this impact.

Data collection and scan registration was handled by FARO software, and Bentley CloudWorx was used to pull the 3D point cloud data into the MicroStation software so that detailed renderings could be developed. Dynasty Group staff then spent two months working with curators showing them how to import models into MicroStation and work with them to develop the renderings. Chen says his employees were also learning. “While our scans were highly accurate from a technical standpoint, they lacked the depth that is needed for this kind of artistic documentation work,” he says. “The curators at the academy helped us understand the artistic aspects and touch up the models to make them look more alive.”

After seeing the results from the first two caves, the academy invited Chen and his team to scan 11 additional caves. The work has since expanded to include a total of 19 caves. The academy has grand ambitions for the scanned data. In addition to compiling simple reports, curators eventually hope to create a virtual-reality experience of the caves that can be displayed in a theater environment and perhaps shown in other countries. In this way, today’s cutting-edge technology will help preserve and share the sacred spaces of an ancient era.

A composite image created from computer-generated line work and a digital photo of the statue.

A Technology and Marketing Advantage

Simply buying a scanner didn’t get Dynasty Group into a new market sector. It took substantial investments in hardware, software and training. And it took dedicated marketing efforts. In 2004, Dynasty Group set up an office in China to better serve clients in that region, and Chen has spent a great deal of time there working with existing clients and developing new leads. “I traveled to China nine times in 2007,” Chen says. “And I’ve already been five times this year.” The firm has also hired employees with specialized degrees to cultivate contacts in the Chinese history and archaeology community. For example, Vivan Wang, Dynasty Group’s office manager in China, has a fine arts degree, and another employee in China has a degree in sculpture, both from China Central Academy of Fine Arts.

This groundwork is already paying off through additional projects for the China National Institute of Cultural Property and other historic preservation groups. By combining the latest surveying techniques and expert employees with persistent, skillful marketing, Dynasty Group expects to continue to serve new markets with new capabilities.

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