By 2012 the technology of handheld GPS instruments was very advanced with an accuracy good enough to make them a suitable tool for field research, so I decided to surf the internet for an instrument with the most suitable specification. I settled on the Garmin GPSMAP 62 and carried out some field tests to check the accuracy. Results varied from nearly perfect to eight metres of error in the worst case compared with known Ordnance Survey (OS) coordinates established by professional surveying instruments. Because of these variations, it was necessary to revisit critical sites to re-record the coordinates and take an average of several readings.
Once I had visited a few sites with the GPS and plotted the results onto the database, it became clear that there was a discrepancy between the WGS84 grid used by the Garmin instrument and the OS grid titled OSGB36. There is a lot of information on the OS website about how this grid originates and how to use conversion programs to compute exact coordinates. Although the grid used by Garmin is called OS grid in the format selection, it did not appear to conform to the OS map grid. Therefore I decided to check the difference between the two grids. Check readings would be taken on-site from known points. First chosen was the OS trig pillar at Jacobswell just south of Whitmoor Barrow. Trigonometrical pillars are usually concrete structures, standing around four to five feet high, and constructed on prominent hilltops affording views over long distances and are part of the network of triangulation stations upon which Britain’s OS is based. The precise coordinate values of these pillars can be found online. Also, points were taken on the corners of other sites such as the church and churchyard walls on St Martha’s Hill. When these were plotted into the database, it was seen that there was indeed a discrepancy. Due to the inherent inaccuracies of Ordnance Survey detail, it is not possible to attain spot-on fitting of data. After meaning out the various results and taking the trig pillar coordinates as the main data point, all GPS data needed to be moved 8 metres south and one and a half metres west. It would have been possible to calculate an exact difference between the two groups, but the handheld GPS’s accuracy is no better than about 7 feet or two metres; therefore, it would seem that refining the difference between them would be excessive and a waste of time. I have since confirmed these conversion factors by taking readings at other pillars in West Surrey.
The first exercise with the new instrument was a visit to Whitmoor Barrow. I walked around the ditch, taking readings at about five-metre intervals. Each reading was taken after holding the GPS at eye level, pointing in several directions until the readout settled down and became constant. These readings were stored as waypoints in the instrument and transposed onto my AutoCAD base plan in the office. The Ordnance Survey extract of the barrow was already on this plan, and it was seen that the adjusted GPS coordinates and the map coordinates of the barrow were an excellent match. This confirmed my intention of using the Whitmoor Barrow as the base point for the overall pattern of rays.
The screenshot above of the barrow shows the Ordnance survey in white overlaid with the GPS results in pale blue.
I then carried out a similar exercise at Culverswell Barrow and the twin Crooksbury Barrows a short distance away. The coordinates of the Culverswell Barrow confirmed my original theodolite survey of some years ago, and the coordinates of the Crooksbury Barrows again proved an excellent fit.
Encouraged by these satisfying results, I was keen to use these newfound techniques to discover other lost sites. The first I looked at was the Mount Pleasant Barrow on Whitmoor Common. This is marked on The OS as the site of an Ancient Monument no longer existing. The coordinates on the OS were noted, as was the coords of the point on my alignments. On reaching the Ancient Monument’s supposed site, nothing could be made out amidst the rough tussocky grass. Still, on navigating to the predicted point on the pattern, some seventeen metres to the north-west, a circular bank some seven metres in diameter could just be made out beneath the scrubby birch trees (see photos at header MOUNT PLEASANT LINE).
This page needs the addition of later work, such as the use of LIDAR.