Trigs and maps

Before satellites, drones and electronic gadgets, maps were created by people walking across the countryside with chains and tapes to measure distances. Each feature on a map has to be surveyed somehow in order to establish its location in space – vertical (height) and also horizontal (eastings and northings co-ordinates). Significantly for The Ridgeway, the ‘Ridgeway Baseline’ and a second baseline in Scotland were used by Ordnance Survey to map the entire UK. Numerous trig points along the Trail are reminders of the people and stories behind the maps we use today to enjoy the countryside. Read on below….

Banner image credit: Dave Cavanagh

Struggling with winds and steep slopes, surveyors carried their heavy equipment across the hills with tapes just 24metres long to plot land between Liddington Hill trig point in Wiltshire and White Horse Hill trig point in Oxfordshire. Hedges, road signs and a pile of manure were removed to allow the surveyors to follow an 11km straight line between the two trig points. ‘Bilby towers’ were erected across a ravine (see image gallery above) and a hole was made in either side of a barn at Odstone to allow a tape to be passed through! Working long hours in exposed conditions, the surveyors had to endure wet clothing, frozen fingers and eye strain.

Surveying the ‘Ridgeway Baseline’ in 1937 and 1951 and also a second baseline in Scotland in 1951 produced on-the-ground accurate measurements from which to map out the entire country. Surveys expanded across the country with the installation of around 6,500 cast concrete ‘trig pillars’ in prominent locations. Trig pillars provided a permanent, fixed network of surveying stations from which to take measurements. Standing at a trig point on a clear day, a surveyor can see at least two other trig points and the combination of three trig point measurements enables ‘triangulation’. Positions of field boundaries, roads, railways, bridges and more could be accurately plotted against known reference points.

Numerous trig pillars can be seen along The Ridgeway, with the metal ‘spider’ on the top being the place where the surveyor fixed a theodolite (see image gallery above). A theodolite is an optical instrument for measuring angles between other points. Today, these trig points are redundant due to Ordnance Survey’s network of 110 Global Navigation Satellite System (GNSS) receivers. What once took a team of men many hours or even days can now be completed in minutes and to a far greater degree of accuracy, although the surveyors of the past were off by a few millimetres at the most!

So next time you spot a trig point along the Trail, look around you and imagine being a map-maker….

Notes: More information about The Ridgeway’s high points and viewpoints is available in other Top 50 entries (not all Top 50 entries will be available until end of December 2023).

Return to Ridgeway Top 50 homepage.