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Geophysical Survey Results

Data from the geophysical survey was downloaded onto a computer during our lunch break and at the end of the day. This allowed us to begin to ‘see’ the site as it unfolded very quickly, as well as allowing us to identify any problems with our survey technique, and in the case of any technical difficulties would could limit the amount of data loss. I for one, couldn’t wait to download the data! This blog post will briefly describe the process we undertook after the data was downloaded.

Raw Data


Field B showing grid (raw data)

Survey data downloaded from the magnetometers is considered ‘raw’. The data for each grid is stored separately and has to be compiled using software. Computer programmes do not know how to put each grid together, so you have to keep accurate notes when surveying!

We used GEOPLOT 3.0 to work with our data. Because we ended up grid-ing out each of the fields separately, we created compiled images for each individual field. That way, we could take the fences into account.

Data Processing

Once the grids for each field were compiled, we then had to ‘process’ the data. Processing corrects for the slight (but unavoidable) inaccuracies caused by the field slope, different gaits from different surveyors, and even very slight inaccuracies in the grid layout. It also helps to smooth out some of the data. Then the files are exported from GEOPLOT 3.0 and imported into GIS software (I used QGIS for this project). You can see a comparison of the raw data and the processed data below.

SWALL14 raw data

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Raw Data)


SWALL14 processed data

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Processed Data)


Based on the fully processed data and our understanding of the site from visits and the 1920s excavation report, we then began to interpret the survey.

Field A

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Field Names)

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Field Names)

Despite the visible earthwork in Field A, which is only faintly visible in the geophysical survey results (marked in blue), no other features of archaeological interest were visible. Dr Clay described this ring earthwork as an amphitheatre or ‘circus’ and considered it to be part of the settlement enclosure visible in Field B. It is unfortunate that the road that separates the two sites cuts through anything that may have indicated the relationship between the two features. However, given the other known dated evidence for both prehistoric and historic features in the immediate vicinity, there is no reason for this earthwork to be directly related to the Iron Age activity.

Field B, C, Ca

The linear features forming the ditch enclosure (marked in green) is particularly strong to the west (were it is visible on the surface), but also on the eastern side. The eastern portion of the ditch visible in field Ca may continue into the south-eastern corner of field C. It is possible that originally both ditches were connected along the southern side, but the road later cut through the site. Another, disconnected, linear feature far to the west of the enclosure ditch is also faintly visible on the survey results.

A number of roughly circular negative features (marked in red) are visible in the area that appears to be the inside of the enclosure. Many of these are likely to be the pits that Clay previously excavated, but it is not possible to exactly identify each pit.

There also seem to be several possible curvilinear features that may indicate round-structures (marked in purple). Clay was unaware that Iron Age people lived in roundhouses at the time of his excavations and instead believed that they lived in pits, so he didn’t know to look for evidence of roundhouses.

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Interpretation)

2014 Swallowcliffe Down, Wiltshire Geophysical Survey (Interpretation: Green=Visible Ditches, Blue=Visible Banks, Red=Negative Features, Purple=Ring?)

A full report with technical details about the survey is available here.