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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.

Technical details

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.

A colour image showing a stylised ground surface with three circular enclosures set within a modern field system.
A lidar derived image of three of the Priddy Circles on Mendip looking approximately east. The lidar image is used courtesy of Mendip Hills AONB – Original source Unit for Landscape Modelling (ULM) Cambridge University © Historic England

History

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.

Colour image, primarily blue-green, showing a stylised ground surface with lots of linear banks within the field pattern
Environment Agency lidar image of fields south of Cricklade, Wiltshire used for flood management. © Environment Agency copyright 2008. All rights reserved.

Archaeological potential

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.

A colour image, primarily green, showing a stylised ground surface with a sub-rectangular enclosure in a field
An image of the Roman fort at Newton Kyme showing as a slight earthwork on lidar imagery supplied by the Environment Agency and used with their permission. © Environment Agency copyright 2008. All rights reserved

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.

A colour image showing a stylised ground surface. To the left is the tree canopy; to the right a sub-rectangular enclosure
Lidar imagery demonstrating canopy penetration in open woodland in Savernake Forest. The left-hand image shows the first return of the lidar pulse that effectively shows the tops of the trees similar to a traditional aerial photograph; the right-hand image shows the filtered data processed to remove the vegetation, which reveals the presence of an Iron Age enclosure. © Historic England; Source Cambridge University Unit for Landscape Modelling March 2006.

Guidance

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.

The Light Fantastic

The Light Fantastic

Published 1 April 2010

Using Lidar to Survey Archaeological sites

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