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Savernake Forest National Mapping Programme project

Savernake Forest was chosen to test the use of lidar remote sensing alongside traditional aerial photography in the survey of wooded environments to identify and map archaeological sites obscured by the trees.

Colour images showing stylised representation of the ground with (left) and without (right) trees
Lidar-derived images showing the effect of processing to remove the forest canopy. The image on the left is derived from the first return and shows the tops of the trees; that on the right shows the archaeology beneath the canopy. © The image is © Historic England; the lidar data is © Forestry Commission - Source Cambridge University ULM (May 2006)

The project

Previous work with lidar in co-operation with the Forestry Commission and the Unit for Landscape Modelling – Cambridge University (ULM) in the Forest of Dean had shown the potential for lidar to penetrate canopies in mixed woodland and reveal features on the forest floor. It was hoped that the same would be possible at Savernake and this proved to be the case.

The lidar survey was commissioned by the Forestry Commission and flown by ULM in spring 2006 covering the core of the wooded area together with a slight overlap into the surrounding farmland.

Although primarily desk-based, the Savernake Forest NMP project also included input from English Heritage's Archaeological Survey and Investigation team who carried out some detailed ground survey.

The forest

Savernake Forest lies to the south of Marlborough in Wiltshire and is known to contain numerous archaeological sites, in particular the courses to two Roman roads from the site of a Roman town at Marlborough, Roman pottery and kiln sites. Savernake is a former a medieval hunting forest with evidence of medieval and later woodland management and industry. The woods also contain the remains of an extensive ammunition dump which was in use throughout the Second World War.

The Forest, as its name might suggest, is a largely wooded expanse forest now managed by the Forestry Commission, and as such might be considered an unpromising site for a survey based on aerial photographs alone. The Savernake Forest NMP project was the first to use lidar (Light Detection & Ranging) as a primary source to complement aerial photographs for the examination of an area largely covered in trees. 

Colour aerial photograph showing extensive woodland with some small clearings
General view over the central part of Savernake Forest taken on 10-AUG-2001 © Historic England

Seeing through the trees

The data was provided to the Forestry Commission as processed Digital Terrain Models (DTMs) that represent the ground surface after all vegetation has been removed, so as to reveal features hidden beneath the forest canopy. These files were then further processed by staff within the Forestry Commission who provided image files to the Aerial Survey & Investigation team at English Heritage [now Historic England] to interpret alongside the standard aerial photographs. Part of the aim of the project was to assess which features were visible just on the lidar, which were only visible on photographs, and which could be seen on both; to this end features were plotted from both sources and were compared at the end of the project before being combined into a single seamless record of the archaeology of the Forest.

Colour image showing a sytlised ground surface in various shades of brown with OS map background
Lidar-derived DTM image overlain on the Ordnance Survey base map showing the relationship between features recorded on each. The image is © Historic England; the lidar data is © Forestry Commission - Source Cambridge University ULM (May 2006). OS background map © Crown Copyright and database right 2015. All rights reserved. Historic England 100019088.

A number of sites have been recorded which were previously unknown, together with significant additional detail being added to those few sites previously thought to exist within the Forest.

Beyond the trees

The interpretation of aerial photos in the areas outside the woodland also revealed a number of sites visible as cropmarks that helped to place similar features within the wood in their context. Of particular interest were the remains of a villa which had been previously recorded, but aerial photographs revealed significant details of its layout.

Colour close up aerial photo showing an arable field with certain lines in yellow against a generally darker green background
A Roman villa near Savernake Forest photographed on 16-JUL-1990. On this photograph it is possible not only to see the main walls of the villa, but also some internal detail such as the bases of pillars towards the righthand edge of the image (NMR 4621/16). © Crown copyright. HE

Second World War

Savernake Forest was used as an ammunition dump by both the British and US Armies during the Second World War and it was hoped that evidence of this activity might be visible on aerial photographs from the time. This was the case and sorties flown in the spring of 1944 proved particularly helpful as the leafless trees revealed evidence of the bunkers and storage areas beneath them.

Black and white aerial photograph showing parkland in winter so you can see through the trees to the structures below
Second World War ammunition storage in Savernake Forest photographed by USAAF on 08-MAR-1944 (US/7PH/GP/LOC/209 5010). © Historic England USAAF photography

Combining techniques to answer questions

The lidar survey recorded some very unusual irregular banked enclosures that were difficult to interpret and unlike anything else in the forest. Examination on the ground revealed these to be very irregular in form and without evidence for any ditch. Comparison with the historic aerial photographs taken during and immediately after the Second World War showed that apparently these had a connection with the ammunition dumps described above.

They are interpreted as areas of clearance, possibly by bulldozers levelling the ground prior to the construction of shelters. Unfortunately they do not appear on the photographs showing the shelters so an alternative explanation is that they are the result of levelling ground after the shelters were removed, possibly filling in any trenches that may have been dug at that time.

Colour image showing archaeological features in red and purple against a greyscale map background
NMP map extract showing the collocation of the banked features (red) with ammunition storage dumps (purple) recorded from wartime aerial photographs. Air photo mapping © Historic England. NMR. © Crown Copyright and database right 2009. All rights reserved. Ordnance Survey Licence number 100019088. 2009

The key findings from the project are published in Savernake Forest: A Report for the National Mapping Programme available through the Research Reports database.

The images used on this page are copyright Historic England unless specified otherwise. For further details of any photographs or other images and for copies of these, or the plans and reports related to the project, please contact the Historic England Archive.

For further information on a project or any other aspect of the work of the Remote Sensing Team please contact us via email using the link below.

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