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.

Early magnetometer survey trace plot, copyright Historic England

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.

Plot chosen for reverse engineering (copyright Historic England)

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

Plan of survey squares (copyright Historic England)

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).

Intersect points between vertical lines and trace lines

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:

Greyscale plot of reverse engineered readings from 1970s trace plot

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.

1970s survey area chosen for reverse engineering shown as red box on Bournemouth University survey (copyright Bournemouth University)

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.

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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

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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

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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:

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):



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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.




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AVAS Newsletter

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.

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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

Continue reading

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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.

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A lost long barrow rediscovered?

In a previous post, I described an initial survey which aimed to look for a lost long barrow near Sopley.  The barrow was apparently bulldozed by American troops in WW2 to make way for Winkton airfield.  The first survey took in two 20m squares, and although inconclusive, gave a tantalising hint that there may be a large feature which potentially continued outside the survey area.

Yesterday, members of AVAS and The Christchurch Antiquarians returned to the site to extend the survey area.  Conditions were challenging, with numerous trees and bushes to navigate around.  However, the results were amazing, as shown below

The image above shows a large high resistance feature, shown in white and light grey, which appears to have a curved end to the south.  This feature is about 65m in length.  The shape of this feature suggests it is man made Continue reading

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Searching for a lost long barrow

On a web page recording memories of Winkton airfield, near Sopley, there is the following reference to the blatant destruction of a barrow to support the war effort:

“Apparently the site of the airfield contained an old burial mound, a barrow, claimed to be the site of the burial of warriors killed in a local battle between the Danes and King Alfred’s forces. The barrow was flattened during the preparation work for the aerodrome and Canon Kirkham expressed some concern at this to the commanding officer of the unit concerned. The officer was apologetic and offered to put it back again !!!”

At that time it was popular to attribute such barrows to the Danes or Saxons, but it is more likely that this barrow was Neolithic in date.  Sue Newman has uncovered further documentary evidence, including the following extract from the Gentleman’s Magazine 1861 describing activities of the Christchurch Archaeological Society:

“A barrow of seventy yards in length, and twenty yards in breadth, in the neighbourhood of Dane-rout, or Danat Lane, in the Clock-field on his property, will be opened by him in the course of the spring.”

The old OS maps of the area show an interesting long thin enclosure, of similar dimensions, in a wooded area called Clockhouse Copse.  This copse was truncated on two sides when Winkton airfield was built, and the area of the enclosure is in the part of the copse that was destroyed.  This can be seen in the maps below.

The trig point within the enclosure is also of interest, as these were usually located on raised ground.  Sue Newman has also discovered that on the tithe map for the area, the field surrounding this enclosure was called ‘Burrow Close’, possibly a derivation of ‘Barrow Close’.

Given this evidence, AVAS recently undertook a joint survey with The Christchurch Antiquarians to Continue reading

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