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