LIDAR (Light Detection and Ranging), a laser mapping technique, has been used extensively for mapping and monitoring the environment and is now being applied by archaeologists to reveal previously unknown historical features, map existing sites and to monitor the effects of environmental change on archaeology.
Aerial LIDAR data is acquired using an aircraft fitted with a laser scanner, which fires tens of thousands of laser pulses every second at the ground. The signal that is reflected back can be measured and processed to produce a highly accurate three dimensional map of the Earth's surface. Complex filtering tools can then be applied to remove surface objects such as vegetation, cars, and buildings, leaving a 'bare earth' map or DTM (Digital Terrain Model). These DTM's can be extremely effective in archaeological research. In forested areas it can allow archaeologists to 'look under trees' and map features that would be difficult to see on site.
In June 2011, Time Team visited Dunwich on the Suffolk coast, now a sleepy village but once a thriving medieval port and even the capital of East Anglia. Suffolks coast, consisting of some of the UKs youngest and softest rock and battered by North Sea winds, is particularly vulnerable to erosion. Over the centuries, coastal erosion and the silting up of the harbour have had a detrimental effect on Dunwich's economy, leading to a dramatic decline in its fortunes.
Much of Dunwich's medieval past has been lost to the sea, however the remains of a 13th century Franciscan priory (Greyfriars) and the leper hospital of St James can still be seen today. It is this rich history and its status as a 'lost city', that attracted Channel 4's Time Team to explore its past.
To support the dig and provide context to the programme, Time Team were keen to utilise LIDAR survey data, to gain a better understanding of the site within its landscape setting and to demonstrate how the effects of erosion can impact sites of historical importance. Their focus was on the monastic areas of the village, with the primary aim to identify the location of a Maison Dieu hospital ('House of God'), thought to be located under a car park on the outskirts of the village. The LIDAR data provided a tool for being able to visualise the site and its position within the surrounding landscape, putting the archaeology into a wider context.
The Environment Agency's Geomatics Group, who have been acquiring high resolution LIDAR data for over 13 years, provided 2 metre resolution data from their archive for initial site investigation and analysis. Using their extensive experience in mapping coastal change, Geomatics Group undertook elevation change analysis using two LIDAR datasets, acquired in 1999 and 2008.
By undertaking this analysis it was possible to determine how much the coastline around Dunwich has altered just in the past decade. The results showed a variety of changes occurring along this stretch of coastline, including the migration of a coastal shingle ridge, cliff retreat and some areas where there was an increase in sediment height. As well as providing important contextual and historical information, these results might influence the way the archaeology is preserved and managed in the future.
LIDAR can also be particularly useful at detecting very slight earthwork remains that are not visible to the naked eye from the ground, allowing for very targeted onsite investigations. Time Team scanned the LIDAR data to see if it could provide any clues as to the location of the Maison Dieu. Unfortunately if there was archaeology present it provided no surface expression and so could not be clearly identified from the LIDAR data alone.
This technique of interrogating the LIDAR for signs of archaeology has however proved invaluable on other projects that Geomatics Group have been involved in. For example, LIDAR data was used to identify archaeological features across a large area of Dartmoor National Park. Not only were new remains discovered, but the existing database of known features was updated to correct positional errors of some marked features. LIDAR was also used on the Ancient Cwmbran and the Cistercians Project in Monmouthshire. Data gathered specifically for the project were used to map hundreds of meters of "pudding stone" walls hidden within extensive woodlands and previously unmapped.
For archaeologists, the key value of using LIDAR data is in providing three dimensional images of the earths surface, which can be an extremely useful tool when used alongside other data sources or to record features and inform further archaeological survey work. Analysis techniques and data resolutions continue to improve, which is likely to see the uses of LIDAR within archaeology expand in years to come.