Liss Archaeology was asked by James Brown, the National Trust's regional archaeologist, to assist them with a project on their Hinton Ampner estate. This project is being run in conjunction with other volunteer groups and managed by Geoff Buss, who is a member of both our group and of the NT's HART volunteers.
The National Trust owns a large amount of land on their estate near their Hinton Ampner House in the Meon Valley. They need to plant trees as a cash crop, to make revenue for the Trust, and two fields have been earmarked for this purpose. However, before tree planting can take place they have to undertake three types of survey to ensure that planting does not happen where there is significant archaeology: field-walking, metal detecting and geophysics.
Some of our members have been helping with all three surveys but we were particularly asked to help by carrying out a magnetometry geophys survey using our Emlid surveying equipment and our new Bartington gradiometer.
After a quick site recce in January, our first action was to position a ground marker spike as a TBM (temporary benchmark) in a position where we could site our Emlid base station. This point had to be in a reasonably secure location so that the Emlid unit could be left unattended during the day (because each unit costs £2000) and within line of sight radio range of both fields to be surveyed, i.e. within about 5km.
The Emlid base station needs to be placed over a known point and it then sends corrections by its built-in LoRa radio to its twin 'Rover' unit, so that using three Global Navigation Satellite System (GNSS) 'constellations' we know the position of the Rover to within a centimetre.
We chose a base location on a high point on the edge of an old chalk pit, 500m from one survey site and 1.5km from the other. We hammered in the ground marker (seen left). We then left the base unit (also seen left) logging its approximate position from about 35 satellites for over 90 minutes. Then we took the unit home and applied corrections recorded by the nearest Ordnance Survey station at Chilbolton over the same period. This gave us the base location to centimetre accuracy.
We use mapping software - technically a 'geographical information system' - called QGIS, so the next stage was to map the site. To do this we added layers of information:
Ordnance Survey mapping;
Satellite imagery from ESRI;
Details of known cropmarks, parchmarks and findspots, kindly provided by the County Archaeologist;
Our TBM by the pit; and
LIDAR data, downloaded from the DEFRA website.
The initial plot for the first field can be seen here on the left, but we knew we needed to add to this the positions of the power poles, the overhead high-voltage power lines and the newly-laid aviation fuel pipeline running through the field that is part of the London to Southampton network.
James had told us the priority part of the field to be surveyed so the next step was to use the Emlid to survey the positions of the power poles and the pipeline with its temporary boundary fence, to avoid with our grid. We therefore surveyed and plotted these in QGIS as seen on the left.
The yellow line shows the high-voltage power lines. The pink line is the fuel pipeline with its boundary fences. The green markers are where Geoff thought there might be buried features, based on cropmarks on satellite imagery.
The next step then was to use QGIS to plot where to put the marker canes for our geophysics grid, to cover as much of the survey area as possible.
For magnetometry we usually use 20-metre grid squares which have to be north-south aligned - preferably grid north rather than magnetic north.
With QGIS it is very easy to load in the co-ordinates of the corners of the grid squares as a CSV file from Excel and see what it will look like on the ground. With a bit of experimentation, partly to avoid a fallen tree not showing on the satellite imagery, we came up with this grid layout shown here.
The white crosses 20 metres apart show where we agreed the grid square corners were to go.
Meanwhile Keith and James asked if we could produce a detailed aerial photo of the field using a drone. We therefore pre-programmed a drone to fly over the whole field in a zigzag pattern at a height of 50 metres, with the camera pointing vertically downwards taking a photo every 5 seconds. The flight took 40 minutes and produced 550 photos. These were subsequently 'stitched together' using photogrammetry software called Metashape Pro to produce a single huge image. The image was incredibly detailed: 1 pixel =1.4 centimetres on the ground. It was also vast at 1.2 gigabytes. Unfortunately, and unsurprisingly in February, there were no cropmarks and this 'orthophoto' provided no useful new information about archaeology. Such images are usually best in May for cropmarks and August for parchmarks.
Grid set up
We uploaded the grid co-ordinates into the Emlid app on an iPhone and used the Emlid Rover to take us precisely to the spot on the ground where each grid cane needed to be placed. The Emlid app directs the operator which way to go to reach each point, with arrows on the display (as seen on the left), and in 'stake-out' mode you then get a 'bulls-eye' display which turns green when you are within a centimetre of the point. Setting out the grid for the day is incredibly easy and therefore takes about 45 minutes, depending on how quickly you can walk.
With the new Bartington gradiometer, which has two sensors, you can scan far faster than with the old machine we used to borrow from Historic England. With practice we were covering a 20x20 metre grid square in just 6 minutes, with the machine taking readings every 25cm. We were able to survey up to about 40 grid squares a day with one operator and two line movers. Our team was enlisted from those members who had previously expressed an interest in, or who had attended training in doing geophys.
At the end of each day the log file of readings was downloaded from the machine by Carl and processed through Snuffler software to de-spike, de-stripe and interpolate the readings and produce plots. These were then circulated by email to the participants and the archaeologists each evening.
Seen here are the cumulative results for the first field from 3 days of scanning. You may be able to make out some of the areas of past intense heat (white spots), some possible ploughed-out barrows and a probable east-west trackway.
In March we moved on to the second field about 1km away. Our process was exactly the same as the first field but there were some differences:
There was a public footpath running around the perimeter;
The site was steeply sloping;
There was a great deal of interference with the magnetometer from three sources:
electrical interference to a distance of 8 metres around a power pole,
ferrous interference from a buried anchor for a guy wire on one of the power poles, and
ferrous interference from two old Esso fuel pipelines running across the south-east corner of the field.
These can be seen in the results plot on the left.
Interestingly, like us you might have noticed a diagonal line of the above results plot running from the top-left to the bottom-right corner, that looked suspiciously like a trackway. There was no sign of this on the surface but we were curious and so looked at the historic 19th century Ordnance Survey maps (scale 25" to the mile) on the National Library of Scotland website. Sure enough, the relevant map (seen left) showed that there had once been a 'bridle road' in that position, providing a route between the lane to the south and a chalk pit at the top of the slope. Old maps are very useful!
In summary, it was a very good test of our new technology and it gave more people a chance to practise magnetometry. We learnt that all helpers need to leave their mobile phones and batteries well away form the magnetometer, it is so sensitive. And we had a wonderful time watching the amazing wildlife - we were treated to displays by buzzards, kites, lots of deer and even five hares chasing each other in circles one morning.
Our thanks to everyone who took part.