Using Lidar to calculate long barrow mound volume

Neolithic long barrows come in an amazing array of shapes and sizes, which is demonstrated by this graphic, where long barrows have been rotated to be East-West, and are shown at the same scale, sorted by length:

Long barrow sizes are often analysed by comparing their length, width or height (eg Roberts et al 2018, Figure 12), but it is a combination of all these factors, plus their 3D profile, that determines their volume and hence an approximation of the amount of effort expended in their original construction. Volume can be roughly estimated from the dimensions of the mound, but often involves an estimate using a somewhat abstract shape (for example, Ashbee 1966, 32). Wouldn’t it be great if we could more effectively estimate the volume of a long barrow mound? Now Lidar elevation data has been released by the Environment Agency, this is a possibility. Conceptually it should be easy to calculate the volume from a Digital Terrain Model (DTM), but the fact that long barrows don’t always sit on flat ground complicates the calculation. The steps involved in this calculation are described below. The computed volumes are sensitive to the degree of erosion / destruction of the mound through, for example, ploughing or quarrying, but, even so, the results provide interesting comparative data that could be further refined by categorising the degree of erosion.

The Lidar DTM data used for this analysis stores, for every square metre, a value for the elevation above the Ordnance Datum. In order to calculate the volume of a long barrow, it is necessary to find the difference between that elevation value, and the estimated elevation of the base of the long barrow at that position. If long barrows were all constructed on flat ground, a constant base value could be used and simply subtracted from each value in the terrain model. Unfortunately, long barrows often sit on sloping ground, meaning another method is needed to estimate the base of the barrow mound.

The workflow involved in calculating mound volume involves the use of Geographical Information Systems (GIS) software that is able to interrogate and calculate statistics on the elevation data. A summary of the workflow is included below:

  • Digitise the outlines of the bases of the long barrow mounds. This is actually quite a subjective task for the following reasons:
    1. for mounds with flanking ditches, the mound has often slumped across the gap, or berm, between the mound and ditch, so that it is difficult to determine where the mound ends and the ditch begins
    2. where a barrow is subject to ploughing, it can be difficult to define exactly where the spread mound ends
  • Overlay the digitised outline of the barrow onto the Lidar elevation data, assigning an elevation value to points on the outline.
  • Join the 3D points on the outline using triangles (ie create a triangulation). The heighted triangle faces approximate the base of the mound.
  • Convert the triangles to a base digital elevation model at the same resolution as the original Lidar data
  • Subtract the base elevation model from the Lidar elevation model. Each pixel now contains a value representing the height of the mound at that point above the estimated base of the long barrow.
  • Calculate the volume at each pixel location: pixel_height * pixel_width * height
  • Sum all the pixel values covered by the mound

The above steps represent a simplified version of the entire process, and other optimisations were employed such as resampling the Lidar to a higher resolution. The entire process was automated so that it can take as input the Lidar data and a set of mound outlines, and the output is a set of volumes in cubic metres.

The above process was run for 122 long barrows in Hampshire, Dorset and Wiltshire for which Lidar data is available. The top 50 long barrows in terms of calculated volume are shown in the graph below:

The graph should be viewed with the following caveats:

  • not all long barrows have been analysed, just those with Lidar data available. So, for example, the very large mound of Old Ditch, near Tilshead, has not been analysed.
  • as mentioned above, the definition of the mound edge is subjective
  • some long barrow mounds have been subject to denudation due to quarrying or ploughing
  • a mound may have been built on ridged or peaked ground, in which case the calculation will over-estimate the volume of the mound material

Keeping these caveats in mind, it is interesting that East Kennet stands out as a particularly voluminous mound, and that four of the top seven mounds are located in the general Avebury area. Two of the other very large mounds, Pimperne and Telegraph Clump, sit close to each other on Cranborne Chase, near to Blandford Camp. Here is a Lidar visualisation of the top seven mounds for which Lidar is available, all shown at the same scale:

The graph below shows the volumes for all 122 long barrows that were analysed. Note that only a subset of labels are shown on this graph:

This analysis has obvious limitations, due to the caveats described, but provides a useful approximation of relative potential effort involved in construction of the mounds, in particular those that are relatively well preserved. Potential further work in this area includes:

  • the categorisation of the preservation of the mounds, allowing mounds within different categories to be compared
  • the inclusion of more long barrows when the Lidar data becomes available. The Environment Agency have a programme of Lidar data collection that will culminate in the release of a full National Lidar dataset for England

References
Ashbee, P., 1966. The Fussell’s Lodge Long Barrow Excavations 1957. Archaeologia 100, 1–80

Roberts, D., Valdez-Tullett, A., Marshall, P., Last, J., Oswald, A., Barclay, A., Bishop, B., Dunbar, E., Forward, A., Law, M., Linford, N., Linford, P., López-Dóriga, I., Manning, A., Payne, A., Pelling, R., Powell, A., Reimer, P., Russell, M., Small, F., Soutar, S., Vallender, J., Winter, E. and Worley, F. 2018 Recent Investigations at Two Long Barrows and Reflections on their Context in the Stonehenge World Heritage Site and Environs, Internet Archaeology 47. https://doi.org/10.11141/ia.47.7

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