The Stourhead LiDAR

Whatever next?

When Archaeology takes advantage of new techniques, whole new landscapes of information emerge.

One of the most exciting developments in recent years has been Light Detection and Ranging or LiDAR for short. Using a drone or an aircraft, pulsed light signals are sent using a laser. When linked to a scanner and a global position system (GPS), It can create an ultra-fine 3D record of the ground surface over wide areas.

The boundary of the National Trust’s Stourhead Estate in South Wiltshire. Whitesheet Hill is on the edge top right. Stourhead Park and mansion, garden, lake and Stourton village are lower centre. Park Hill Camp hillfort lies in woodland centre left.

In large surveys, millions of light points are plotted and tied to existing mapping with the GPS. Each point has its unique XYZ position… latitude, longitude and height above the datum level.

The total Stourhead survey area tilted slightly to show the contours covering roughly the same area as the map above. Whitesheet Hill on the right The hillfort faintly visible on the lower right hand edge. To the left, two valleys separate a ridge which has Park Hill Camp hillfort near the middle.

The Environment Agency has been using this technology for years and have made their data freely available. A quick visual link can be seen here https://houseprices.io/lab/lidar/map. This survey data was collected mainly to predict levels of flooding and consequently it tends to be concentrated along valleys and coasts. It has given good results but the detail tends to be at 1.0m resolution or in the better areas 0.5m. The best quality is 0.25m density of coverage.

There are still large gaps in the land area currently covered by LiDAR and therefore the National Trust is commissioning its own surveys at 0.25m.

In the South West, there are new surveys for the Bristol and North Somerset properties, the Bath and Dyrham properties and most recently the data has arrived for the Stourhead Estate in South Wiltshire. Bluesky collects the data and it is analysed by ArcHeritage who provide the baseline digital imagery in various forms as well as the core GPS files.Their report picks up many new sites which have now been uploaded onto the NT Historic Buildings Sites and Monuments Record. This is not the end: new archaeological sites can still be discovered by further manipulation of the data combined with other information sources.

The LiDAR data can be uploaded into the digital mapping system and then it can be overlaid as a layer on digitised historic maps, onto geophysical surveys and onto aerial photographs. It is so easy these days to zoom in an out of maps and also to fade one layer of information and then see another in direct relation to it.

Detail of Whitesheet Hill causewayed enclosure. Four round barrows can be seen along its bottom edge all with little dimples in the top where the owner of Stourhead Estate, Richard Colt Hoare, excavated them in he early 19th century. One at the lower left hand edge of the plot was cut by a chalk quarry in the 19th century. Close examination of the plot shows phases of trackways and faint embankments.

A great ability of LiDAR is to fell forests and woods (virtually) to see the ground surface beneath. Something impossible with air photography.

Imagine the light pulses from the aircraft like rain falling on the ground. Some will bounce off the tree tops (the first returns) but many will hit the ground below the tree canopy (the second returns). There are systems to filter out the first returns so that only the ground can be seen. It is why I always ask for surveys to be done in the winter when the leaves have fallen from the trees and the ground surface can be most clearly surveyed.

Park Hill Camp from the air surrounded by conifer plantations. In the last 10 years the National Trust has gradually removed the trees from the scheduled monument. The LiDAR defines the earthworks in a better way than can be seen by aerial photography when trees interrupt the view (see below)

Stourhead’s Park Hill Camp Iron Age hillfort has been covered in trees for many years making it difficult to see. Over a number of years, gradually, the National Trust has been clearing the woodland and bringing it back to grass. The LiDAR survey has enabled the ramparts and ditches to be clearly seen as well as showing its strategic position on the ridge top unimpeded by the conifer plantations that surround it.

The ramparts and ditches of Park Hill Camp Iron Age hillfort revealed by the LiDAR survey.

Another great thing: the LiDAR light point cloud is three dimensional and this enables a digital terrain model to be created. This can be viewed on its own or it is possible to drape aerial photographs and/or historic maps across it…as though the map or photograph has become a gigantic cloth thrown over the contours of the landscape. There is now the ability to screen- fly through the Stourhead landscape switching on or off other layers of information while weaving up the valleys or skimming over the hillfort ramparts.

Stourhead Lake (bottom) and the position of Park Hill Camp Iron Age hillfort clearly revealed on the ridge top between two valleys. This area is planted with trees and this vantage point of the fortification would not normally be appreciated.

During a bright winter day, low sunlight will traverse the landscape bringing different shadows in sharp relief and revealing new details. LiDAR analysis can introduce its own light source and the survey plot can be re-generated.. with the light source at any angle and direction. This shows up very faint archaeological earthworks when the light source is beamed from a particular direction.

The LiDAR survey shows the quality of surviving archaeology and reveals where conservation should be concentrated across the Stourhead Estate.

This image looks down to the arable land from the hillfort and causewayed enclosure on Whitesheet Hill. On the lower land, the earthworks have been almost levelled by modern ploughing but old quarries can be seen clustered, where stone outcrops on a low hill, and faint traces of prehistoric ‘celtic fields’ can be seen.

The Stourhead farmland, ploughed for many 100s of years, has lost much of its archaeology but the survey still shows traces of medieval and prehistoric agriculture and traces of buried enclosures suggesting settlement remains below the ploughsoil….(though much worn down buried pits and ditches will survive).

However, there is fine earthwork survival in Stouhead Park and on Whitesheet HIll.

The prehistoric earthworks on Whitesheet Hill show up very clearly: the Iron Age hillfort to the south, the Neolithic Causewayed enclosure in the middle and the other enclosure (also probably Neolithic) to the north and in between Late Bronze Age cross ridge dykes, Early Bronze Age round barrows and medieval pillow mounds all crossed by banks, trackways and quarries of various periods.

Whitesheet Hill: LiDAR shows the ramparts and ditches of the Iron Age hillfort (c.300BC) at the bottom. The three rectangles are modern reservoirs just east of the National Trust boundary. A cross-ridge dyke (c.1000 BC) divides the narrow downland ridge separating the hillfort from the Neolithic causewayed enclosure (c.3600BC) which has a Bronze Age round barrow (c.2000BC) built over its southern edge and across its north side runs the old cattle drove road from the the west towards Salisbury and then on to London. Further along the down to the north (the upper edge of the plot), is another faint enclosure (c.3000 BC) of similar size to the causewayed enclosure (this site has been ploughed in the 20th century but can be seen clearly on the LiDAR).

The parkland is a very precious survival. The ridge and furrow of medieval open field furlongs was fossilised when the park for the mansion house was created. This must have happened before 1722 which is the date of our earliest map of the park.

The 1722 Stourhead Estate map (Wiltshire and Swindon Record Office). The newly built Stourhead House is centre right. The north-east corner of the park is Spencers Mead. The strip fields and curving enclosure ditch shown on the LiDAR lie within this field and Slade Mead below. Buildings in red between these fields were demolished in the late 18th century, the building sites lie where there are earthworks shown on the LiDAR image below.

Near the Stourhead House and near the landscaped garden obelisk are two turbulent areas of earthworks, outside the areas of agriculture and therefore places already occupied ….before the open fields were created it seems.

One of these, east of the House, is likely to be the site of the medieval Stourton Castle..demolished when the present mansion was created in the early 18th century. The other area near the obelisk is a mystery… the LiDAR raises many new archaeological questions…. wonderful.

Next year the Cotswolds and Hidcote NT properties will have LiDAR We await the results with anticipation…what new Roman sites lies beneath Chedworth woods……

The site of the present Stourhead House is lower centre. The buildings show as triangles. Top right is the NE corner of the present park where a large oval enclosure (prehistoric?) underlies the regular furlong blocks of strip fields divided by trackways (this is just grass not visible on the ground). Centre right and NE of Stourhead House is an area of earthworks likely to be the site of the medieval Stourton Castle. At the left edge of the picture is another grouping of mixed earthworks, perhaps an early settlement to the right of the mound with the 18th century obelisk monument on it.