The announcement earlier this year that the Environment Agency was to make its Lidar data freely available was welcomed by archaeologists, who are able to use the detailed terrain height data to add a new dimension to existing aerial photographs, and provide a new source of data for studying archaeological features. This is of particular importance to amateur archaeologists, who are unable to afford the cost of commercially acquired Lidar data. The access to this data, in addition to the maturity of open source computer applications, means that such archaeologists can begin to make a real and effective contribution to the study of landscape features.
Lidar data can now be downloaded from the Environment data.gov.uk portal. Simply click on the map; a square will appear on the map and the available downloads will appear in a list below. There are two types of Lidar data:
- Digital Terrain Model (DTM) – the bare earth, with buildings and vegetation removed
- Digital Surface Model (DSM) – the model includes buildings and vegetation
For archaeological purposes, the DTM is probably most useful, as trees and bushes are stripped away to reveal any earthworks below. The downloaded zip includes a series text files with a .asc extension; these are text files which contain an array of height values for a specific small tile. Many Geographical Information System (GIS) software packages can read this data directly; I used the open source and freely available QGIS package.
The most effective way of visualising Lidar data is with some sort of hillshade applied, which simulates the lighting of the landscape with sunlight from a particular direction and height. The problem with shading from just one direction is that linear features aligned with the direction of shading will not create a shadow. For archaeological applications, it is normal to create a composite of hillshades from a number of directions, using different colours to represent each hillshade. I used the excellent and freely available Relief Visualization Toolbox to create these hillshade composites. The picture above of Whitsbury Hillfort, near Fordingbridge in Hampshire, is an example of this type of rendering. A vertical exaggeration can be used to strengthen the depiction of subtle features.
Once all of the hillshade images were created, I used a command line tool called GDAL to create a mosaic of all the images, called a VRT file. GDAL tools are available from the menus within QGIS. A screenshot is shown below [click on the image to enlarge].
It is worth noting that there is not a full national coverage of EA Lidar data; most river valleys and surrounding areas are available. The black areas in the screenshot above are locations where no data is available.
So what have I seen so far during an initial browse of the data in the general region of the Avon Valley? I have picked out a few areas below.
The screenshot of Whitsbury hillfort above shows the potential for depicting existing earthworks which are normally shrouded in thick vegetation. The detail shown in the ramparts can be compared to the original Royal Commission earthwork plan, and reveals subtle details not previously recognised.
Bokerley Dyke and the end of the Great Dorset Cursus
[Click on the image to enlarge]. This is perhaps my favourite area, showing a palimpsest of landscape features of true temporal depth. The end of the Neolithic Cursus can be seen in the centre of the image as a large subtle rectangular feature, which heads off towards the south west corner of the image. The much clearer long thin linear feature which points at the end of the cursus is a long barrow, as is the similar long barrow to the south. The former long barrow has long been suspected of being two adjoined long barrows, and this is borne out by the Lidar, which shows two sections on slightly different alignments. There is another ploughed out long barrow to the north of the cursus, which shows as an oval mound.
The large bank and ditch which snakes from north west to south east is Bokerley Dyke, a linear boundary which probably had significance in the Bronze Age and Iron Age, and was most likely strengthened in the late Roman period. To the east of Bokerley Dyke, a series of probable Prehistoric field boundaries can be seen. Finally, the substantial rectangular earthwork in the top centre of the image brings us into the 20th Century, representing an army training range.
[Click on the image to enlarge]. This image shows an extensive field system on and around Rockbourne Down. Most of the field boundaries are under plough, but the crisp boundaries to the east of the image are still under pasture and much better preserved. There also happen to be three long barrows on this image. The one at the centre top is the huge and well preserved ‘Round Clump’, with the mound and ditches clearly depicted in the Lidar data. At the western side of the image is the ploughed out Rockbourne Down long barrow, pointing in a SSE direction. To the east of the image, crossed by the horizontal blue grid line is a much smaller long barrow mound.
In summary, now that the Environment Agency Lidar data is now freely available, amateur archaeologists have at their disposal a rich new source of data to study past landscapes and discover new sites. The images above show the potential for this resource, which can be used to illuminate even the most subtle earthworks with an unprecedented clarity.