The Trig Pillar that stands in place of the northern Cannon
For anyone who has ever consulted an Ordnance Survey map, the precision and detail are immediately apparent. But few might realise that the foundations of this remarkable mapping endeavour lie in meticulously measured "baselines," and one of the most historically significant of these is the Salisbury Plain Baseline.
Established in 1794 by Captain William Mudge of the Royal Artillery, under the nascent Board of Ordnance (the precursor to today's Ordnance Survey), the Salisbury Plain Baseline was a crucial component of the "Principal Triangulation of Great Britain." This ambitious project aimed to accurately map the entire British Isles, driven in part by military considerations during a period of threatened French invasion.
The baseline on Salisbury Plain stretched for approximately 11.2 km between a point near Old Sarum Castle and Beacon Hill, east of Amesbury. The measurement was an incredibly painstaking process for the time. Surveyors used meticulously calibrated 100-foot steel chains, taking into account factors like temperature changes and the Earth's curvature. To mark the precise ends of this vital line, cannons were buried muzzle-up at both locations one of which can still be found today near Old Sarum, a testament to the enduring legacy of this work.
The significance of the Salisbury Plain Baseline cannot be overstated. By accurately measuring this fundamental distance, Mudge and his team could then use a large, precise theodolite (an instrument for measuring angles) to establish a network of triangles across the country. Through trigonometry, the precise positions of hundreds of landmarks could be determined, forming the bedrock for all subsequent, more detailed local surveys. The accuracy achieved by these early surveyors was remarkable; it's estimated that Mudge's overall survey across Britain was no more than 20 metres out compared to modern GPS measurements. The Salisbury Plain Baseline, therefore, stands as a tangible monument to the dawn of systematic, scientific mapping in Britain, a pivotal step in creating the detailed maps we rely on today.
Further Reading: Principal Triangulation of Great Britain
The Baseline route
I discovered the Hounslow Baseline in 2018 and cycled between the 2 cannons in Kingston and Heathrow. On discovering that there was another Baseline in Salisbury I was keen to cycle that too. I changed my plans when I discovered that you can’t cycle very close to the line itself. The route, which I drove instead, takes you along the busy A345, through Amesbury then onto the even busier A335 dual carriageway. Driving is much safer and there’s free parking spaces a short way from each cannon.
Salisbury Plain Baseline Cannon South
To access the southern cannon there’s a layby suitable for 5 cars opposite the Harvester Old Sarum Castle on Castle Road. If you’re visiting the Old Sarum English Heritage site you can also use their large car park although there is a charge.
Cross over the road to the eastern side and walk along the pavement in a northerly direction for approximately 500 metres until you reach the welcome to Salisbury sign. You’ll see the Mapping Monument Stone on the right of the pavement. Look behind the Monument Stone and you should be able to see the top of the Cannon poking up through the earth in the maize field. Some reports on Trigpointing say that they couldn’t see the Cannon due to hedge overgrowth or crops in the field but it was clearly visible on the day in July that I went.
Trigpointing.uk record: TP7570 - Salisbury Plain Baseline South
Salisbury Plain Baseline Cannon North
After the southern cannon drive north along the A345, then onto the A335 heading east after Amesbury. Shortly after the A3028 joins the road, pull into the small parking space at the side of the road. Be very careful getting out of the car here as it’s a very fast road.
Take the path on the left just east of the parking space and follow it up to the top of the hill. After ~650m you’ll reach the Trig Pillar, just off the main path. The Trig Pillar stands in place of the original cannon. From the pillar you can get great views across to Salisbury to the south-west.
Trigpointing.uk record: TP8928 - Salisbury Plain Baseline North
Trigpointing.uk record: TP0742 - Beacon Hill
Peakbagger record: Beacon Hill, England
