## 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

## The route of a Roman Road in Wiltshire rediscovered?

We have recently been doing some survey work on long barrows near Chettle in Dorset. In this area, the Lidar data shows clear evidence of the Roman road from Badbury Rings to Kingston Deverill, and on to Bath. Whilst reviewing the Lidar a few weeks ago for another purpose, I was distracted by this Roman road and started following it north. This was fairly easy to do until it reached Donhead St Mary, where the trail appeared to go cold. Checking in the Roman Road bible (Margery 1973), plus some other later sources, including the Wiltshire HER, it was clear that the route (named Roman Road 46 – RR46) is unconfirmed all the way the way from Donhead St Mary to Kingston Deverill, with the only confirmed section in the extension to Bath being just south of Bath. How could a road which was so clear in the landscape to the south suddenly disappear at Donhead St Mary? The challenge was on to see if the Lidar gave any clues.

I initially used some of the suggested points along the general route where it is thought traces of the road have been found. This included a useful discussion in a newsletter by the Bath and Counties Archaeological Society. I also thought the road builders might have aimed for a clear gap in the ridge adjacent to the east of East Knoyle, which the modern A350 takes advantage of, but all the lines I extrapolated drew a blank.

Then, by chance, when looking in the area south of East Knoyle, I noticed faint traces of a linear earthwork crossing fields at an oblique angle. This was clearest either side of the A350. To the north west a clear alignment could be seen crossing the woodland labelled on the OS map as Park Coppice, and rising up onto ridge near Wise Lane.

Although these traces in the Lidar were looking promising, this was only a short stretch and might not have Roman origins. It would be necessary to trace the alignment over a much longer distance in order to lend weight to an argument for a Roman road. Amazingly, when extrapolating the alignment back towards Donhead St Mary, there are traces at a number of points along an exact straight line for a distance of 5.5km.

At Semley, there are very faint traces of a possible agger either side of the railway which curves across the plot below, with a clearer traces towards the north of the plot. Click on the image for a full resolution image.

Finally, north west of Donhead St Mary, there are further traces on the same alignment, shown below:

The following plot shows the complete 5.5km alignment suggested by the Lidar analysis.

Below is a plot showing how this 5.5km alignment fits with the known section of RR46 from Badbury Rings to Donhead St Mary.

The existence of this alignment shown in the Lidar does not prove this is the route of the Roman Road. I have been mislead in the past by the routes of pipelines and other infrastructure projects. However, the signs are positive for the following reasons: the straight route, the fact the route is cut by established settlements and roads, suggesting its antiquity and the fact the route assumes the general expected alignment of the Roman road given known sections of road.

The power of the Lidar data to allow such analysis is also clear. Prior to the availability of Lidar data, aerial photos could provide some clues when conditions were right, but in order to look for subtle earthwork traces, researchers would be forced to climb over fences and walls looking for a ploughed out agger crossing a field, with access to private land being a clear problem. With Lidar data, analysis over wide ranging landscapes can be undertaken in a short time, allowing targeted field visits to corroborate clues seen in the Lidar.

The next steps for this research will be to trawl in more detail the available documentary records and maps, and to get out into the field in an attempt to understand the features seen in the Lidar. Also, what is the route of the road north west of East Knoyle, towards Kingston Deverill and Cold Kitchen Hill?

The technique can also be applied to possible Roman roads more local to the Avon Valley. It has been postulated that a Roman road might have connected Old Sarum with the New Forest potteries, perhaps crossing the Avon near Fordingbridge. Also, it is puzzling that the last stretch of Roman Road 45, from the Mendips to Old Sarum, is lost between Grovely Wood and Old Sarum. Could Lidar provide any clues?

Posted in Uncategorized | 4 Comments

## Reverse engineering a 1970s geophysics survey of Lake Farm Roman Fort, Wimborne, Dorset

During the 1970s and 1980s, the Ancient Monuments Laboratory undertook geophysics surveys with pioneering fluxgate magnetometer equipment. This was before there was access to personal computers, and results were plotted directly onto line trace plots. For the first time, buried archaeological remains could be surveyed in a non-invasive way, using a technique that was to revolutionise archaeological survey. However, the early trace plots were in some cases difficult to interpret, with broad patterns being clear but more subtle detail hidden in the mass of lines. Early attempts at interpretation consisted of annotation of the trace plot, as seen below.

A couple of weeks ago, a number of these trace plots were shown by Paul Cheetham of Bournemouth University during an excellent talk on Lake Farm Roman Fort, located near Wimborne, Dorset. This site has been subject to a recent magnetometer survey, and Paul showed a side by side view of an annotated 1970s plot, and the modern survey results. He commented how the 1970s interpretation was surprisingly accurate given the limitations of presentation of the results. It was however clear that the modern computer generated greyscale plot contained a huge amount of extra detail and subtle features. To me, this prompted a number of questions: how did the equipment used in the 1970s compare to the modern Bartington gradiometers? What would the results have looked like if there had been access to modern computers? What if we could reverse engineer the original readings from the trace plots, display them in greyscale, and for the first time directly compare the results?

I had tried the technique of reverse engineering trace plot results previously, for a long barrow survey near Old Sarum, with excellent results. For that site, however, there had been no modern survey, and so no comparison with modern techniques was possible.

The first step was to choose a suitable plot for Lake Farm. All the reports for the 1970s surveys are available on the Historic England website:

From those surveys, I chose a relatively small block of squares covering part of the southern section of the fort. I chose this plot because of the quality of the scan, with the lines mainly showing as clear black lines.

This area is shown by the eastern block of survey squares on the overview plan of the fort:

A process was then followed to reverse engineer the original readings. This involved vectorising the lines from the image into line geometries, represented by thousands of XY coordinates. These line geometries needed some manual editing to connect broken lines, and to join lines where trace lines crossed. This process was particularly challenging where there was significant magnetic disturbance, as shown in the centre of the plot below. This plot shows the vectorised lines after manual editing, each plot line being represented with a random colour:

By using GIS software, it was then possible to treat each plot line as its own line graph, and read off XY values at intersection points with a set of vertical lines, the readings varying as the line meandered up and down (see below).

The result was a set of readings for a set of row and column positions, matching the format of data produced by modern gradiometer surveys. As a result, this data could be loaded into Snuffler, a free software package for analysing geophysics data. This data could be displayed in a greyscale image and hence directly compared with the modern survey results. The result is shown below:

The quality of the greyscale plot is astonishing, and shows an incredible amount of detail that was held in the original line plot. Comparison can be made with the modern survey. The image below shows the reversed engineered plot alongside the original line plots (with and without annotation), and the same approximate area from the recent Bournemouth University survey:

The results are amazing. The vast majority of the features in the modern survey are visible on the greyscale plot of the reverse engineered 1970s data, including details of the internal rectangular linear features and significant anomalies. This underlines just how sensitive the 1970s equipment was. In addition, the 1970s survey fills in some narrow gaps that could not be surveyed due to the presence of modern fence lines.

It is clear that the 1970s and 1980s line plots encode an incredible amount of detail, this detail not being visible or capable of precise interpretation by the human eye, but recoverable through computer processing. This technique could be particularly valuable where previously surveyed sites are no longer accessible for survey, perhaps through destruction, contamination, or landowner permission.

Posted in Technical | Tagged , , , | 3 Comments

## Survey of a long barrow on Cranborne Chase

We have just completed the report on a small geophysics survey we did of a Neolithic long barrow, Gussage St Michael 10, East Dorset.  It is now a low ploughed out mound situated on the same ridge as a number of other long barrows, as well as the south-west terminal of the Dorset Cursus, an enigmatic Neolithic monument 10km in length.

Location of the Gussage St Michael 10 long barrow. Contains public sector information licensed under the Open Government Licence v3.0

The barrow had not, to our knowledge, been investigated before.  We obtained some fantastic results using a trusty Geoscan FM36 Gradiometer, supplied as part of the LoCATE project led by Bournemouth University and the New Forest National Park.  In the resultant plot, the irregular U-shaped ditch of the long barrow is clearly defined:

Plot of processed gradiometer results

The U-shape of the ditch is important, as it confirms this is a long barrow of the ‘Cranborne Chase’ type, a form of long barrow which is almost exclusively found in this part of Dorset.  It has a striking similarity with Gussage St Michael 12, another long barrow just over 1km away on the same ridge, excavated in the 1930s, and referenced B in the diagram below:

Comparative plans of Cranborne Chase Type long barrows. A: Gussage St Michael 10, B: Gussage St Michael 12 (after Drew and Piggott 1936, Plate XVI), C: Pentridge 26 crop marks, D: Wor Barrow (after Pitt-Rivers 1898, Plate 249)

The results from the survey can also be draped on a 3D model of the Lidar elevation data to show how the ditches are positioned in relation to the mound:

Although a small survey, it has made a significant contribution by confirming another ‘Cranborne Chase’ type long barrow in what is an important Neolithic landscape.

The full report can be downloaded from this link:  AVAS Geophysics Survey Gussage St Michael 10 Long Barrow v1_0.pdf

## Report on the survey of possible long barrow at Sopley

We have just completed the report on the survey we did at Sopley, near Bransgore, in September 2017, where we were looking for the lost site of a long mound, supposed to be a long barrow (burial mound).  The report contains details of the superb detective work involved in locating the site. The mound was originally described and partially excavated by Victorian antiquaries, then levelled to make way for Winkton airfield during WW2.  Our geophysics results have now relocated this mound, but it is yet to be proven whether it was actually a Neolithic long barrow.

We would like to thank Bournemouth University and the New Forest National Park Authority, who made the geophysics equipment available via the LoCATE project.

Here are a few screenshots of pages, see the end of this post for a link to the full report.

Link to the report:  AVAS Geophysics Survey Possible Long Barrow Sopley v1_3

Posted in Field Work | Tagged , , | Leave a comment

## Interactive 3D Lidar models in Sketchfab

Getting 3D models of Lidar data from QGIS onto a model sharing site such as Sketchfab used to be a hassle, with a number of steps as described here:  https://github.com/mike-gill/qgis2sketchfab

All this has changed with the 3.x release of QGIS.  The Qgis2threejs plugin has been revamped, with a new viewer, and the option to export 3D models to the glTF format.  Luckily supports .glTF files, and so the files exported from QGIS can be directly uploaded to Sketchfab without any intermediate steps.  Here are some basic instructions (in this case making use of QGIS 3.2).

• In QGIS, make sure the Qgis2threejs plugin is loaded
• Add a surface model dataset created from Lidar data to the map [eg a Digital Surface Model (DSM) or a Digital Terrain Model (DTM)]
• Add any other datasets (eg a hillshade) to the map.
• Click on the Qgis2threejs button
• In the dialogue that opens, in the DEM section, tick the surface model.  A 3D view of the map should appear, similar to that shown below:

• Use Scene > World settings to change the vertical exaggeration.  There are other settings that can be played with.
• When you are happy with the scene, choose File > Save Scene As > glTF (.gltf, .glb).  Save the file.
• The upload button in Sketchfab can be used to directly load the .gltf file.
• Here is a screenshot of the model in Sketchfab
• And here is the finished model (with a few annotations added in Sketchfab):

Posted in Technical | 1 Comment

## AVAS BBQ 2018

It was perfect weather for the annual AVAS BBQ last night, and we had a great turnout.  Thanks to Vanessa and Rachel and family for preparing the fantastic spread of food, and to Mark for providing the great riverside location.  Here are a few photos, there are more on the AVAS Flickr photostream.

A selection of past issues of the AVAS newsletter are now available on the main AVAS website.  They can be accessed on the AVAS Newsletter page.

## Re-visualising a long barrow near Old Sarum

A few weeks ago I was walking around the ramparts of Old Sarum hillfort when I spotted the cropmarks of a long barrow, and adjacent ring ditches, in one of the fields to the north of Old Sarum.  These cropmarks are shown in the photo below:

Long barrow and ring ditch crop marks, Old Sarum

I knew the long barrow had been surveyed back in 1978, and I found the original geophysics report on the Historic England website .  Unfortunately, the plot of the survey results was missing from the PDF report, and so I was unable to compare the crop marks with the geophysics results.  I contacted Historic England, and the Geophysics Manager Paul Linford kindly arranged for the original paper plot to be scanned and added to the PDF report.  Paul commented that the 1970s technology wasn’t up to displaying the results as well as modern computer visualisations, but that the ditch anomalies could still be seen on the plot.  Here is a copy of the plot as scanned:

Original scanned plot of magnetometer results. © Geophysics Team, Historic England

I found it difficult to draw my own conclusions from the plot due to the pencil annotation, so I spent a short time in a graphics package digitally rubbing out the pencil annotation, as shown below:

Scanned plot with pencil annotation removed. © Geophysics Team, Historic England

Now the annotation was removed, it was easier to draw my own conclusions from the results, but also it was obvious that some of the anomalies relating to the long barrow were far from clear.  In particular, the long barrow ditches were barely discernible.  I thought it was a pity we didn’t have the original survey readings, from which we could have generated modern day visualisations.  That gave me an idea; what if we could reverse engineer the readings from the scanned XY trace plot?  It was worth a try.

So I set out experimenting with the latest GIS software to see if this was possible.  Here is a high level summary of the steps I went through:

• Extracted the trace plot as an image from the PDF report.
• Converted the image to a 1 bit (black and white) image.
• Georectified the plot so the survey squares were perfectly square.  This was required because the original paper scan was slightly warped.
• Thinned all line work on the image to single pixel lines.
• Converted the single pixel lines to line geometries (ie collections of XY points).
• Tagged the lines with their y index in the plot.
• Batch created lots of vertical lines at known locations along the x axis.
• Intersected the vertical lines with the trace lines to get a ‘reading’ from the trace.
• Standardised the readings for each trace line by transforming to a common axis.
• Wrote out the XY values to an XYZ file.
• Imported the XYZ file into Snuffler (a geophysics data software package) and used the visualisation tools to produce a greyscale plot.

When I opened the final file in Snuffler, I was amazed to see that the process had been successful.  I had managed to reverse engineer the relative readings from the trace plot, to produce this:

Reverse engineered greyscale plot of magnetometer results

Posted in Technical | | 1 Comment

## Members Evening – Flint!

We are hoping for a good attendance at our members evening on Wednesday 3rd January 2018.  Steve and Chrissie have kindly offered to host an evening of flint!  They are going to bring along examples of flint tools, allow us to handle them and teach us how to recognise different varieties of worked flint.  It should be a really interesting evening.  Below is a poster for the evening, and we hope to see you there.