Photogrammetry

Stonehenge Uncovered

June 1, 2013
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Sophisticated visualizations aren’t necessary for folks around the world to conjure images of Stonehenge. The name alone takes care of that.

But advanced visualization techniques have led to some startling discoveries about the nearly 5,000-year-old megalithic site near Wiltshire, England. Among them are Bronze Age carvings of axe heads and daggers created at least 1,000 years after the monument was erected.

“It was pretty amazing, to be honest, almost disbelief,” says Marcus Abbott, leader of a geomatics and visualization team at ArcHeritage, an archaeological and heritage services provider that documented the carvings. “If you think about a monument like Stonehenge, it’s probably the most studied monument in the country, and yet there’s still new information (being discovered).”

In 2011, English Heritage, an organization chartered with protecting the United Kingdom’s historic places, commissioned the most detailed 3D laser scan of the monument ever undertaken. Each stone was recorded with point cloud spacing of 1 millimeter, and four stones of particular interest received 0.5 millimeter scans.

The resulting data, 850 gigabytes worth, was turned over to ArcHeritage in 2012. It produced 3D meshed models and other maps from that data and took high-
resolution photogrammetric scans of certain stones, which enabled the discoveries.

 

Previous laser scans and research had identified 44 possible or certain carvings of axe heads. The latest surveys indicated 115 axe heads and three daggers in use by Bronze Age peoples. Those tools were in use from about 1750 to 1500 B.C. Fifty-nine illustrations were uncovered on one stone alone. Researchers say unidentified shapes and shadows in the datasets that do not appear to be natural may prove to be additional examples of artwork.

None of that would have been possible without 3D animation techniques and the use of advanced point cloud processing software. Specifically, the layering and plane-shading functions in Bentley Pointools were instrumental in creating high-quality renderings. ArcHeritage moved a grayscale band 7.5 centimeters wide through the data at 1 millimeter intervals and repeated that step 75 times on each image. Individual points were assigned a grayscale value that shed light on hidden features of the stones. Combining all those images through animation software revealed the prehistoric carvings and other archaeological evidence.

“We were seeking a piece of software that would create an image of the surface of the stone based purely on the data—no color information, no reflective data, purely based on the position—so that’s why we chose Pointools, because it has a really good imaging system in the range-shading aspect of the product,” Abbott says.

Faraz Ravi, Bentley Systems’ director of product management for point clouds, says English Heritage has been using the system now called Pointools since 2003, nearly a decade before Bentley acquired the United Kingdom-based software company that developed it. He says Pointools’ capability with large datasets and ArcHeritage’s “incredibly ambitious specifications in terms of the point spacing” made Pointools the ideal choice for the task.

“(These discoveries) explain, in a very graphical way, this new era of technology using laser scanning and point clouds,” Ravi says. “It doesn’t take much of an extension of the imagination to see (Pointools’ potential for) occupations elsewhere. On a wider context, we’re able to tell a story about the value of three-dimensional data, the value of capturing an existing structure as it is, in a context that people can understand.”

 

Beyond the Bronze Age carvings, laser scanning and photogrammetric analysis exposed individual tool marks showing hand-manipulation of the stones from the time Stonehenge was erected. Further research into the point-cloud data may provide more clues to help determine how the monument was built or answer other archaeological questions.

According to “Stonehenge Laser Scan: Archaeological Analysis Report,” a document written by Abbott and Hugo Anderson-Whymark of the University of Liverpool, the use of both laser scan and photogrammetric data allowed researchers to analyze the information in different ways. It also provided opportunities for comparison between the datasets and options for visualizing the stones.

Naturally, the stones have texture and surface variations that are not always evident to the naked eye. However, in a 3D environment, researchers stripped off the texture to create a uniformly colored digital surface. Then they experimented with new surface textures and different camera angles, and that uncovered hidden archaeological features on all the stones.

Researchers then examined the 0.5 millimeter dataset on specific stones, and rock art that was missed by previous investigations was discovered. Within the point cloud data, researchers used individual XYZ files and visualized them in Pointools.

In total, researchers identified and recorded more than 700 surface features on the stones, including graffiti and other damage from visitors. New evidence also indicates that some stones have been removed, suggesting that Stonehenge may have been completed when it was constructed–contrary to some previous theories.

“By far one of the most encouraging and exciting aspects to this project is that even though Stonehenge has been subject to decades of extensive study, the application of cutting-edge technology has brought about significant new discoveries,” Abbott and Anderson-Whymark wrote in the conclusion of their report. “This stands as a testament to the benefits technological advancements can bring to heritage projects when incorporated into a sound research strategy.”

The discoveries, enabled through the application of laser scanning, photogrammetry and 3D visualization software, have captured the public’s imagination. Richard Zambuni, Bentley’s global marketing director for geospatial and utilities in London, says the findings have brought notoriety to the company from around the world.

“I think it’s because Stonehenge is iconic,” he says. “Bentley (typically) deals with infrastructure more modern than Stonehenge, but it’s still built infrastructure, and we’re still using Bentley technology to help people like (Abbott) and his organization (and) English Heritage to understand and interpret that infrastructure.”

 

 Douglas D. Fisher is the managing editor of POB. He holds a bachelor’s degree in journalism and history and a master’s degree in history. His latest book, “Border Crossings: The Detroit River Region in the War of 1812,” was published by the Detroit Historical Society in 2012. He can be reached at fisherd@bnpmedia.com. For more information about Bentley Systems 3D visualization tools, visit www.bentleysystems.com. The ArcHeritage website is www.archeritage.co.uk

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