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31 Oct 2003
Technology commonly used for computer-aided engineering is revolutionizing archaeology.
Thanks to a high-resolution 3D laser scanner, a UK team has discovered a set of “invisible” axe-head carvings at Stonehenge, the famous 4300-year old British stone circle.
Two ancient carvings of axes were found on one of the circle’s sarsen stones and are so faint that they can not be seen by the naked eye. One measures approximately 15 cm square and the second 10x8 cm, both are badly eroded.
It took a £30,000 laser scanner which can record the 3D shape of a surface with sub-millimeter resolution, to reveal the presence of the carving. The purpose of the newly-found carvings is a mystery.
The investigation was carried out by a team of local archaeologists from Wessex Archaeology, and Archaeoptics, a Scottish consultancy that specializes in laser scanning ancient artefacts.
For the survey, Archaeoptics used a Minolta VI-900 scanner which is capable of capturing 300,000 3D data-points in 3 seconds. The scanner determines the shape of the surface by the principle of triangulation.
A thin-stripe of 690 nm-laser light is projected onto the target surface and imaged by a CCD camera. The process is repeated across the surface to give a set of data-points which a computer can turn into a 3D model.
“The scanner has a resolution of one-and-a-half times the width of a human hair,” explained Alistair Carty of Archaeoptics, “This means that it can take 25 measurements in a square measuring 1 mm by 1 mm. If you scan a piece of sandstone, you can actually pick out the granularity of the rock.”
Although the team only needed 30 minutes to acquire 9 million 3D data-points from the surface of the stones, it took 2 days with a powerful computer to create highly-accurate 3D maps from the data.
The team scanned only part of three of the circle’s sarsen stones and believe that a full scan of all 83 stones would reveal more ancient carvings.
“We have used 3D scanning previously to enhance badly weather carvings on monuments but never on details as fine as the Stonehenge axeheads,” said Carty. “The possibility that other unknown carvings exist on the other stones is very exciting and may hopefully lead to a more complete interpretation of Stonehenge.”
Archaeoptics next project is to scan structures at the top of St Pauls Cathedral in London. Who knows what they may find?
Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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