For this month’s Science Notes, we will be exploring a technology that is mentioned frequently in the pages of CA, and which, in a recent survey of the Isle of Arran, off the west coast of Scotland, has allowed hundreds of previously unknown sites of archaeological interest to be discovered.
LiDAR has been used by archaeologists for many years to survey areas of potential significance, but as the technology develops and becomes more widely available, it is offering more opportunities to discover sites at a pace that was previously unimaginable. The Arran survey, which covered the whole island (about 430km2), is the first large-scale project of this type in Scotland. It is significant for the huge amount of data it was able to collect, due to the LiDAR technology’s ability to support rapid coverage of a large area.
LiDAR stands for ‘Light Detection And Ranging’. The technique involves the use of either ground-based or airborne laser systems to record 3D data points. The LiDAR data from the Isle of Arran were collected using an airborne laser-scanner, which was mounted in a fixed-wing aircraft. A laser beam was transmitted in pulses from the aircraft, and the time taken for the light to reach the ground and return to the sensor on the aircraft recorded. Using this information, it was possible to calculate the location and height of points on the ground, and to use this data to create a digital terrain model that showed the topography of the area.
In just a couple of days, the survey was able to cover the entire island, providing the data that subsequently allowed almost a thousand sites of interest to be detected. LiDAR records topography, so the detections were focused on earthworks – features that survive with some kind of topographic expression on the ground surface, which are identified through the highly accurate 3D measurements collected by the airborne LiDAR array.
The LiDAR data, which were gathered as a cloud of data points, were then processed to produce digital surface models. These models were visualised through techniques such as hillshading, which illuminates the topography using an oblique light source or multiple sources to create highlights and shadows. Hillshade maps provide a 3D representation of the surface, making it easier to identify relative slopes, ridges, and other features. The visualisations of the data were then examined in a Geographical Information System (GIS). A GIS is a computer system that can be used to view and analyse maps and data related to the position of things on the earth’s surface, making LiDAR easy to read and interpret.
The visualisations were also looked at alongside other sources of information about the area, such as aerial photographs and 19th-century maps. Possible site identifications were given a level of confidence from one to three, and this ranking was followed by six weeks of fieldwork to see if ground observation could help to clarify any identifications that were less confident (levels two and three).
The results were illuminating: the survey showed that there are more than double the number of ancient sites and monuments than had been previously recorded on the Isle of Arran. Among these, the project identified 55 prehistoric settlements (adding to the 90 previously known examples) and 150 medieval and post-medieval shieling huts (small buildings used by shepherds during the summer months) – before the investigation, just 44 had been documented. This supports previous assumptions about the way that medieval and post-medieval populations were using upland grazing, and provides important new information about the locations where they stayed.
Another important feature identified was a Neolithic cursus monument, measuring just over 1km in length and about 35m across, defined by low banks around 0.4m high running along a slight ridge. It is situated in a landscape that is covered in later Bronze Age and Iron Age roundhouses, suggesting that this was an important area over long periods of time. The cursus lies just under 3km south-west of the Machrie Moor Stone Circles, but they are not inter-visible and there does not appear to be any immediate association between these monuments.
Several prehistoric enclosed settlements were also found, including two examples of an unusual form of roundhouse-within-enclosure. The survey provided a wealth of new information and extended the distribution of prehistoric settlement well beyond previously known extents.
The Historic Environment Scotland team has returned to the island to carry out further, more detailed investigations of some of the discoveries. It is hoped that future surveys can be undertaken using more LiDAR data; Dave Cowley, the project’s leader, predicts that there are tens of thousands of sites throughout Scotland awaiting discovery.