Laser-scan and visualisation tools help in revealing extraordinary Bronze Age art at UK’s Stonehenge World Heritage Site
While the mysteries of Stonehenge, constructed in England between 3000 BC and 2000 BC may never be truly revealed or understood, a recent examination of the historic monument using cutting- edge visualisation tools has unearthed some fascinating carvings that date to the Bronze Age. It is not yet known what questions about Stonehenge these carvings will answer, but the project did uncover the potential for using laser-scan and visualisation technology on the world’s antiquities.
The plan to examine the stone structure more closely began in November 2011 when English Heritage, the UK government’s statutory adviser on the historic environment, commissioned the most detailed laser scan survey of Stonehenge ever undertaken. During the project, each stone was recorded in unparalleled detail with point spacing of 0.5 mm by the Greenhatch Group survey company. This huge data resource, comprising more than 850 gigabytes, would lead to new discoveries about the monument.
The enormous task of examining the data was awarded to ArcHeritage, part of the York Archaeological Trust, in April 2012, whose Geomatics and Visualisation team examined the laser scan survey. One challenge was to visualise a large amount of information and identify and isolate very subtle features. Preliminary examination of meshed models showed promising signs of useful information in the dataset. For example, individual tool marks more than 5,000 years old could be seen and identified, but there were also tantalising hints that the data contained prehistoric artwork carved onto the surfaces of the stones.
Detailed examination of data
Examining the meshes alone was not sufficient to draw out these ancient carvings from the data and a different method needed to be employed. The team decided to visualise the original point-cloud data and created a workflow using Bentley Pointools, which enabled large datasets to be loaded, which facilitated the examination of the full 0.5 mm resolution data. In addition, the shading functions proved instrumental in visualising the most subtle features. The tool created a greyscale band 7.5 cm wide, which was moved at 1 mm intervals through the data to make a high-quality rendering of the plane shaded image. The process was repeated 75 times to complete a full colour change for every point in the data. Depending on the position in relation to a preset camera plane, each point would be assigned a greyscale value, which creates the potential to see very subtle features hidden in the data.
The team combined all 75 images into an animation, which proved astounding; as the greyscale band was moved through the data, prehistoric carvings could be seen fading in and out of view.
With the help of visualisation capabilities it was possible to see the eroded prehistoric artwork for the first time in possibly thousands of years. Once the team identified the extent of the carvings, they used measuring and point-location tools to accurately plot the carvings to the Ordnance Survey grid.
Technology enables major discoveries
When the examination was completed, the team had made some major discoveries. For a start, 72 previously unknown prehistoric carvings had been uncovered — almost double the number of known carvings at Stonehenge. The carvings of Bronze Age axes are estimated to be made Marcus Abbott, a member of the ArcHeritage Geomatics and Visualisation team that worked on this project says, “English Heritage presented us with over 800 gigabytes of from 1750 BC to 1500 BC, roughly a thousand years after Stonehenge was constructed.
It is hoped the carvings will help archaeologists understand the type of civilisation that existed there more than 3,000 years ago. Moreover, these discoveries, exciting in their own right, illustrate how project teams can use laser scan data to make ground-breaking discoveries on the world’s ancient wonders. In addition, using laser scan visualisation techniques on other sites has the potential to greatly impact how archaeologists perceive and utilise technology on future heritage projects.
Marcus Abbott, a member of the ArcHeritage Geomatics and Visualisation team that worked on this project says, “English Heritage presented us with over 800 gigabytes of data; we needed a software solution that could handle and visualise vast quantities of survey data. Bentley Pointools is capable of loading both 3D mesh data and point-cloud data; furthermore Bentley Pointools has a full suite of measuring tools and unique visualisation tools.” This functionality was crucial to the success of the Stonehenge project, and the discovery of unrecorded prehistoric rock art on the stones was first realised in Bentley Pointools.