Lidar (Light Detection and Ranging)
Airborne lidar (light detection and ranging) measures the height of the ground surface and other features in large areas of landscape with a very high resolution and accuracy. Such information was previously unavailable, except through labour-intensive field survey or photogrammetry.
It provides 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.
Airborne lidar operates by using a pulsed laser beam fired from a plane. The beam is most commonly scanned from side to side as the aircraft flies over the survey area. It measures between 20,000 to 100,000 points per second to build an accurate, high resolution model of the ground and the features upon it.
The speed of capture has been consistently growing over the last decade and will no doubt increase further. For further details of the technology see 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. This has been particularly true for utilities companies, highway agencies and other developers.
The possibilities of lidar for archaeological recording in the UK were first recognised at a NATO sponsored workshop held in Leszno, Poland in November 2000. This had been organised to discuss future practices in aerial archaeology, and how they might be applied across Europe.
An example was shown, where a survey covering the River Wharfe in Yorkshire revealed evidence for the earthwork survival of a Roman fort previously thought to have been completely levelled by ploughing.
English Heritage (now Historic England) recognised the potential for lidar to record very slight earthwork remains and contracted the Environment Agency Geomatics Group to fly a survey of the Stonehenge World Heritage Site.
Since then we have worked with a number of lidar providers looking at different areas of the country with varying levels of monument survival.
Survey in woodland
English Heritage (now Historic England) was also closely involved in working with the former Unit for Landscape Modelling (ULM) at Cambridge 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.
Historic England is looking at the best ways to utilise the lidar data for archaeological purposes. A set of guidelines for the use of airborne lidar for archaeological survey has been published. It is available both as a booklet and as a downloadable PDF.
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