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Looking for the High Places

One of my aims for lockdown walking was to find a Triangulation stations, commonly called a Trig Point, marked on the OS maps by a blue triangle. It is rather ironic to use a map to search for a Trig point when the maps themselves were drawn using the Trig points we were looking for.

The Trig Point near Lyndhurst, The New Forest.

So who built them and why?

It all started many years ago, in 1784, when William Roy was commissioned by the Royal Society to determine the exact positions of the Royal Observatories of London and Paris. He did this by surveying a network of accurately measured triangles between the two points. He used a huge theodolite and a baseline of just over 5 miles (Which he measured on flat ground on Houndslow heath. It now lies under Heathrow airport).

The success of this operation led to the Board of Ordnance deciding to create a triangulated survey of the country. The date was 21 June 1791 and is celebrated as the Birth of the Ordnance Survey although the name was not used until 1801.

In the early days of the triangulated survey the drive was to map the coastal areas of the country. Fearing invasion from Europe the British military needed to know where they could conceal their armies and what the terrain was like. As the area was surveyed the surveyors would mark the corners of the triangles, where they placed their theodolites, with small piles of stones. The survey continued after peace broke out but the rate slowed and it was 1870 before maps for the entire country were available.

In 1935 the process was repeated, this time the corners of the triangles were marked permanently by ‘trig points’ made of cast concrete or stones concreted together. On the top of the pillar there was a brass fitting for the theodolite. The task was massive involving building materials being hauled to the top of mountains regardless of the weather. About 6500 trig points were built and these were what we were looking for.

Near Bruton, some trig points are:

On the footpath to Redlynch

Lamyatt Hill

Lodge Hill, Castle Cary

Windmill Hill, Wincanton

Unless you are an employee of the Ordnance Survey you do not have a right to walk to a Trig point. Try to find those on a right of way or ask permission of the landowner.

Today Trig points are no longer in use or maintained by the OS, they have been superseded by GPS technology. Some have been adopted by groups and individuals. There is face book page ‘Finding all the Trig points’ and a website ‘ Trig Pointing UK’ ( which will give you a list of all the trig points, and help you find those nearby.

Not all Trig points are on high points, there is one near Longleat, Wiltshire in a valley. They just need to have good line of sight to at least two other trig points, so wherever they are you are guaranteed a good view.

The Beacon, Corton Denham, Somerset.

Not all Mountains have trig points at the top.

Trig points on Mountains are not necessarily the highest point on the mountain.

The first concrete trig point built by the Ordnance Survey was near Cold Ashby in Northamptonshire in 1936

Near Little Ouse in Cambridgeshire the trig point height is 1m below sea level


The Ordnance Survey Puzzle book

Triangulation Station Wikipedia

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