In November 2011 English Heritage commissioned the most detailed laser scan survey of Stonehenge ever undertaken. Each stone was recorded in unparalleled detail with a point spacing of 0.5mm. This work was carried out by the Greenhatch Group. The resulting huge data resource of 850GB had the potential to hold the key to new discoveries about the monument; no one had ever recorded the stones in such detail.
In April 2012 the task of examining the data was awarded to ArcHeritage, part of the York Archaeological Trust, and its Geomatics and Visualisation team who set about the examination of the laser scan survey. One of the biggest challenges was how to visualise such a huge quantity of information, whilst still being able to identify and isolate very subtle features. Preliminary examination of meshed models of the data showed promising signs of useful information in the data set. Individual tool marks over 5000 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.
Examining the meshes alone was not sufficient to draw out these ancient carvings from the data. A different method needed to be employed. The team decided to visualise the original point cloud data and created a workflow using Pointools. The power of Pointools enabled large datasets to be loaded which facilitated the examination of the full 0.5mm resolution data. In addition to this the Pointools shading functions were to prove instrumental in the visualisation of the most subtle of features.
The Plane Shading function of Pointools was utilised to create a greyscale band 7.5cm in width, this band was moved at 1mm intervals through the data. As it did so, a high quality render was made of the plane shaded image. The team did this process 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, this creates the potential to see very subtle features hidden in the data. The team combined all of the 75 images into an animation, and played back the results. The animation was astounding; as this greyscale band was moved through the data, prehistoric carvings could be seen fading in and out of view. Onsite these carvings are invisible to the eye and had never been seen before, at least not for thousands of years. It was only with the visualisation capabilities of Pointools that these eroded prehistoric artworks could be discovered. Once the team had identified the extent of the carvings, the measuring and point location tools were utilised to accurately plot the carvings to the Ordnance Survey grid.
After the team had finished this examination it was clear that some major discoveries had been made. 72 previously unknown prehistoric carvings had been discovered. This was 3 times the number known at Stonehenge before the data was examined, and double the number known in Britain. The carvings are of Bronze Age axes and were carved a thousand years after Stonehenge was constructed in 1750-1500 BC. These discoveries are of great importance to the understanding of Stonehenge and provide a glimpse into a culture that existed over three thousand years ago.
Whilst the discoveries are exciting in their own right, one of the most promising aspects of this project is that by using laser scan data visualised in Pointools to analyze one of the most well-studied monuments in the country, the team were able to make ground breaking discoveries; the possibilities for using these techniques on other sites is quite astounding and has the potential to have a wide ranging impact on the way archaeologists perceive and utilise technology on future heritage projects. The question has to be asked: What other discoveries on other monuments can be made using these cutting-edge technologies and techniques?
Marcus Abbott, a member of the ArcHeritage Geomatics and Visualisation team that worked on this project said, English Heritage presented us with over 800GB of data. We needed a software solution that would handle and visualise vast quantities of survey data. Pointools is capable of loading both 3D mesh data and point cloud data; furthermore 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 Pointools.
Here is a more general discussion of the project.