Airborne lidar (light detection and ranging) measures the height of the ground surface and other features in large areas of landscape with a resolution and accuracy hitherto unavailable, except through labour-intensive field survey or photogrammetry.
It provides, for the first time, highly detailed and accurate models of the land surface at metre and sub-metre resolution. This provides archaeologists with the capability to recognise and record otherwise hard to detect features.
Lidar operates by using a pulsed laser beam which is scanned from side to side as the aircraft flies over the survey area, measuring between 20,000 to 100,000 points per second to build an accurate, high resolution model of the ground and the features upon it. For further details of the technology see the Unit for Landscape Modelling - Cambridge University (ULM) or the Environment Agency Geomatics Group.
Airborne lidar was conceived in the 1960s (for submarine detection), and early models were used successfully in the early 1970s in the US, Canada and Australia. In the United Kingdom the Environment Agency Geomatics Group has used lidar for over a decade for the production of cost-effective terrain maps suitable for assessing flood risk.
They therefore have data available for large areas of the country. In more recent years many other bodies have acquired the capability to carry out lidar surveys particularly for use by utilities companies, highway agencies and developers.
The possibilities of lidar for archaeological recording in the UK were first recognised at a NATO sponsored workshop to discuss future practices in aerial archaeology, held in Leszno, Poland in November 2000, where a survey covering the River Wharfe in Yorkshire revealed evidence for the earthwork survival of a Roman fort that had previously been thought to have been completely levelled by ploughing.
Following on from this and recognising the potential for lidar to record very slight earthwork remains English Heritage contracted the Environment Agency Geomatics Group to fly a survey of the Stonehenge World Heritage Site and has since worked with both the Environment Agency Geomatics Group and the Unit for Landscape Modelling (ULM) looking at different areas of the country with varying levels of monument survival.
Survey in woodland
English Heritage has also been closely involved in working with ULM and the Forestry Commission looking at the potential for lidar to penetrate wooded terrain. Because lidar uses light beams it has the potential to penetrate gaps in the woodland canopy and so record the ground surface under the trees. This can reveal features that would not otherwise be seen.
English Heritage is looking at the best ways to utilise the lidar data for archaeological purposes. Working with other users of lidar and other laser scanned data in a forum called Heritage3d guidance on best practice for data recording and storage has been produced. A further set of English Heritage guidelines for the use of airborne lidar for archaeological survey has recently been published. It is available both as a booklet and as a downloadable PDF.
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For further information on a project or any other aspect of the work of the Aerial Survey team please contact us at: AerialSurvey@english-heritage.org.uk.