The walk began beside the A345 in Wiltshire, a mile or so outside Salisbury, in a field of pigs rummaging methodically around a cannon sunk deep into the ground. In front of us, a faux-antique standing stone proclaimed that “in 1794 a line from this site to Beacon Hill was measured by Capt W Mudge of the Ordnance Survey as a base for the triangulation of Great Britain”.
Today, the Ordnance Survey, Britain’s national mapping agency, has acquired the status of national treasure. I never embark on a hike without one of its folded maps lodged safely in my jacket pocket. But during this walk, the OS was to dictate my route more profoundly than usual.
The Ordnance Survey was founded in 1791, in the aftermath of the French Revolution. It was intended to produce a military map of Britain’s south coast, to help defend against a Gallic invasion. Just like today, the first OS maps were distinguished by the sophistication of the instruments and methods used in their construction. The early maps were based on a technique called triangulation. Surveyors identified thousands of elevated or prominent landmarks across the nation, such as hills or church spires, known as “trig points”. With fiendishly heavy, intricate instruments, poised at a trig point’s summit, a mapmaker was able to observe sightlines radiating towards other visible stations. He would note the angles between these sightlines and then use trigonometry to deduce the distance between the trig points.
Occasionally the mapmakers had to check their observations by measuring a straight distance on the ground itself – a “baseline” – and comparing that result with the one produced through trigonometry. In the summer of 1793, the OS’s director, William Mudge, started looking for a suitable baseline to check the triangles he’d already measured. Salisbury Plain, a chalk plateau covering 300 square miles of Wiltshire and Hampshire, was a perfect site.
“We found that a base line of nearly seven miles might be measured without much difficulty between Beacon Hill, near Amesbury, and the Castle of Old Sarum”, Mudge wrote. So between late June and mid-August 1794, a team of Ordnance Surveyors painstakingly laid measuring chains and rods end to end along that stretch. Two hundred and seventeen years later, I decided to retrace their steps. While researching my book on the OS’s history, Map of a Nation, I’d tried to visit as many meaningful sites as possible. But I hadn’t managed to make it to the Salisbury Plain baseline, and this was an omission I was keen to rectify.
The base, and my walk, began in the shadow of Old Sarum, the Iron Age hill fort that marks the earliest settlement of Salisbury. In the 1790s, the mapmakers buried iron cannon at each end of the base as permanent markers of the achievement. Today the southernmost cannon is in a pig farm, and from there, the baseline extends north-north-east through private agricultural land. The closest public route to this now-invisible line lies along the busy A345 road towards Amesbury. But rather than stomp beside lorries, my partner and I detoured to visit Old Sarum, and then followed a footpath north-west through fields of corn, poppies and woodland harbouring deer. After a couple of miles, the path disappeared into a thicket and re-emerged beside a tiny reservoir, shortly after which we began heading back east, towards the route of the baseline.
Reaching a busy crossroads, we took the Old Amesbury Road footpath, which diverges to the east of the modern Amesbury Road, the A345. In the 18th century, this path was the main road north – not the highway that now bustles alongside – and it runs very close, and almost parallel, to the OS’s baseline. Just over a mile along this well-trodden route, barbed-wire fences appeared to the east, outlining Boscombe Down Airfield. Salisbury Plain has been used for army training since 1898. Today the Ministry of Defence owns 150 square miles – around half of the old Plain – and around 39 square miles of the Plain are permanently closed to the public.
The restricted nature of Salisbury Plain is not ideal for walkers, but it has made the area remarkable for wildlife. Its grassland has not been lost to agricultural cultivation, and the Plain boasts the largest expanse of unimproved chalk downs in north-west Europe. It supports around 80 species of rare plants and invertebrates. Almost the whole course of the walk abounded with butterflies, meadow grasses, wild flowers and thriving hedgerows.
Briefly ducking into the town of Amesbury, the path surfaced beside four Bronze Age burial mounds, the Ratfyn Barrows: examples of the extraordinary remains of extreme antiquity that characterise the entire Plain. These made a profound impression upon a man who visited Salisbury Plain at the same time that the Ordnance Survey were first identifying their baseline: William Wordsworth. Returning from Portsmouth, where Britain’s naval fleets were preparing for battle with Revolutionary France, Wordsworth described himself as full of “melancholy forebodings”. Such seemingly incomprehensible remnants of history reinforced Wordsworth’s sense of the land’s secretive, indifferent nature, and his own temporary alienation from his nation.
Shortly after these tumuli, we emerged – with a jolt – on to the hurtling A303. The end of the baseline is located at the summit of Beacon Hill, on the outskirts of Bulford military camp, which we reached via an uncomfortable scuttle along the dual carriageway. The OS’s trig points have no functional use today, but they do usually indicate the most panoramic spots of the surrounding landscape. And Beacon Hill was no disappointment: we could make out Old Sarum to the south-west, Stonehenge to the west and even Highclere Castle – where the television period drama Downton Abbey is filmed – 25 miles away, to the north-east.
This stunning visual display made Mudge particularly fond of the Salisbury Plain measurement: he realised that this final spot provided “a commanding view of almost the whole” of the base. In one gaze, he was able to visualise all the territory that his surveyors had covered over the meticulous seven-week measurement. And, he was pleased to discover, the painstaking measurement from Old Sarum to Beacon Hill almost exactly corresponded with the distance produced through trigonometry: measuring chains revealed it extended over 36575.401 feet, or 6.927 miles. The Salisbury Plain baseline confirmed the superlative accuracy of the Ordnance Survey, and was a crucial landmark on the agency’s journey to becoming a national treasure.
Rachel Hewitt is author of ‘Map of a Nation: A Biography of the Ordnance Survey’ (Granta)