Archaeological prospecting in woodland using LiDAR - summary of 2011 surveys

Light Detection And Ranging (LiDAR) surveys are one of our Woodland Heritage Services

Valley of Visions (Medway)

Part of the Medway catchment (Kent) was surveyed for the ‘Valley of Visions’ project. The area is dominated by high ground to both the east and west of the river and its floodplain. One interesting image from the lidar survey showed an unknown linear feature crossing the river (see image). This feature (shown by a sudden change in colour which represents elevation) was artificially created by the survey, as the area in question was captured in two halves and on different days. The tidal nature of the Medway at this point resulted in two different heights being recorded for the river during the two surveys. When the survey data are merged and processed, this change in elevation for the river remains evident. 

  • LiDAR model of sudden changes in river elevation due to tidal influences in the Valley of Visions project in Medway catchment (Kent)
    Sudden changes in river elevation due to tidal influences

The quality of the lidar data is evident within Shorne Wood Country Park in the northern part of the survey area. The park itself is predominantly located within the remains of a clay extraction site, which is clearly seen as a scalloped area within the image. To the north of the clay workings is the site of Randall’s Manor. The photograph of an on-site interpretation board shows a plan created from a previous earthwork survey. The same area is visible in the lidar image and the similarities are evident. The detail is also shown by images from Chobham Hall, just to the south of Shorne Wood. Within Chobham park, there is a mausoleum, a photograph of which can be seen. An image of the lidar surface model for the same area shows how the laser interprets such a feature. In addition to the building and the ditch surrounding it, some laser reflections are also evident from the enclosing fence. The terrain model for the surrounding area also shows many short, linear features which resemble pins or nails lying across the landscape and roughly in the same orientation. Fieldwork showed these to be the decaying stems of fallen trees, presumably the result of a significant storm event.

  • LiDAR terrain model of Shorne Wood Country Park
    Terrain model of Shorne Wood Country Park
    Enlarge (image size 105K)
  • Interpretative map of earthworks<br />at Randall Manor
    Interpretative map of earthworks
    at Randall Manor
  • Chobham Park mausoleum
    Chobham Park mausoleum
  • Lidar interpretation of Chobham Park mausoleum
    Lidar interpretation of mausoleum

During fieldwork in the southwest of the survey area, a substantial and sinuous ditch system was discovered beneath a woodland. However, it wasn’t until the lidar data was analysed, that the full extent of the feature became evident. Running for over a kilometre and roughly following the line of the hill, this ditch (more accurately a trench) is assumed to form part of a previously unmapped defensive line designed to facilitate training and help repel any invading forces.

  • Surface model of Valley of Visions ditch system showing the canopy
    Surface model of Valley of Visions canopy
  • LiDAR terrain model of Valley of Visions ditch system showing the defensive trench
    Terrain model of Valley of Visions with defensive trench

Whitwell Wood

The landscape beneath the canopy of Whitwell suggests a very diverse former land-use, with the lidar indicating a wide variety of features. For example, at least three irregular shaped enclosures and possible settlement sites are visible. An earlier field system is more evident under the western half of the wood, whilst a wide variety of cultivation patterns (some very fine and intricate) are visible throughout. Pits of various sizes occur throughout the wood, but are slightly more common to the north. Several significant linear features are evident, perhaps marking former boundaries. In the Eastern half of the woodland, the lidar revealed what resembles the footprint of a former building, but may relate to previous excavations of a tunnel system which remains open in that part of the wood.

  • Aerial photograph of Whitwell Wood canopy
    Aerial photograph of Whitwell Wood canopy
  • LiDAR image of terrain beneath Whitwell Wood canopy
    Show terrain beneath Whitwell Wood canopy

New Forest Crown lands

The Crown Lands survey captured much of the heart of the forest, with over 370 square Kms flown and incorporating large areas of managed forest and open heathland. The New Forest is well known for its large number of archaeological features and many were mapped by the survey. Examples include burial mounds, enclosure banks, hunting lodge sites, park pales and hill-top settlements. However, whilst the lidar has undoubtedly identified other, previously unknown features, especially in woodland, it has also added to information about those already known. For example, in the image of the surface model below, a part of the river Avon, close to Woodgreen, is coloured to show changes in elevation and reveals the complex patters of active and historic channels and drainage systems. The 3-dimensional nature of the data allows identification of which areas would be prone to flooding should water levels rise, perhaps allowing prioritisation of any necessary flood defences or site protection.

  • LiDAR surface model showing many periods of water management in New Forest Crown lands
    Surface model showing many periods of water management
    Enlarge (image size 104K)

In other open parts of the forest, the remains of military airfields are common, some of which are now partially hidden beneath woodland. Where they occur on open heathland, the predominantly flat nature of an airfield can mean that there is little for a lidar system to detect from changes in elevation alone. However, the changes in the intensity of the reflected lidar signal for the different types of surface struck, allow other detail to be seen. Similarly, within the New Forest, controlled burning is sometimes used on such sites to reduce the encroachment of plants such as gorse, the intensity data can also be used to map the areas of scorched earth where the burning has occurred. This could also be useful for further vegetation management, especially where other sources of current information such as Aerial Photographs are not available.

  • LiDAR intensity image of Hatchet Moor airfield in New Forest Crown lands
    Intensity image of Hatchet Moor airfield

Hidden under the forest canopy, the lidar survey has revealed previously unrecorded earthworks such as burial mounds, possible saltpetre workings, mineral extraction areas, settlement sites and extensive networks of drainage systems.  The two examples below show circular features (believed to be the location of previous ornamental plantings) and some of the wide variety of drainage systems in use across the forest, presumably dating from different periods.

  • LiDAR surface model of canopy over plantings rings in New Forest Crown lands
    Surface model of canopy
  • LiDAR terrain model of plantings in New Forest Crown lands
    Terrain model with planting rings
  • LiDAR surface model of canopy over drainage system in New Forest Crown lands
    Surface model of canopy
  • LiDAR terrain model of drainage system in New Forest Crown lands
    Terrain model with drainage systems