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.
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 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.
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.
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.
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.
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.
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.
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.
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……