ANYONE who has been up a hill will be familiar with trig or triangulation points.

In Britain, they are usually marked by trig pillars made from concrete or stone and about four feet in height. These much-loved structures are often found on hill tops acting as a beacon for ramblers.

The Ordnance Survey’s first trig point was erected on 18 April 1936 near Cold Ashby in Northamptonshire. This was the start of the process known as the ‘retriangulation’ of Great Britain. It eventually became part of a state-of-the-art network used to create the British National Grid and accurately map the country.

In low-lying or flat areas some trig points are only a few metres above sea level and one, in Low Ouse in Cambridgeshire, sits at one metre below sea level.

The highest in the UK is at the top of Ben Nevis in Scotland, at 1,345 metres.

The triangulation was largely the work of mathematician Martin Hotine, who also designed the trig points.

Maps and mapping data produced by Ordnance Survey today are still based upon the framework which he devised.

More than 6,500 trig pillars were built across the country and used for the retriangulation between 1936, and June 1962, when the trig pillar at Thorney Gale, Westmorland, was used for the final calculation.

When all the trig pillars were in place, it was possible, in clear weather, to see at least two others from any one trig pillar but subsequent vegetation growth means that this is not necessarily still the case.

Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country.

This year marks the 80th anniversary of trig pillar monoliths that helped Yorkshire.

It has been revealed by Yorkshire Water that there are 454 trig pillars still standing in the region, 80 years since the first one was built by ordnance surveyors to help map Great Britain.

One of Yorkshire’s most popular trig pillars is located on top of the 730m summit of Whernside in the Yorkshire Dales, making it the highest trig pillar in the region.

Trig points near Bradford include Stanbury point trig pillar on Stanbury Moor, Little Wolf Stones trig pillar on Keighley Moor, near the Top Withens ruin made famous by Emily Bronte's novel Wuthering Heights and Hollin Hill trig point on Oxenhope Moor which represents the highest point on the moor 451m above sea level.

At 402m above sea level Ilkley Moor trig point makes for a great focal point to a walk that also takes in the Cow & Calf rocks

Visiting these landmarks is an addictive hobby for some, with the phrase ‘trig bagging’, coined to describe those who aim to visit as many as possible. Dave Woffenden from Harrogate is one such trig pillar enthusiast who has succeeded in ‘bagging’ all 454 pillars in Yorkshire.

He explained: “I first got interested in OS triangulation pillars during the 1980s, having walked the Pennine Way with some colleagues. I realised they all had unique numbers on the plate near the base, so recording each number and grid reference became my hobby. By 2010 I had visited all 454 trig pillars in Yorkshire.”

He adds: “My family think I’m slightly mad, but the fresh air and exercise have been great. Hidden valleys and hills have been visited, pub lunches enjoyed, farmers, gamekeepers, water board officials and many other wonderful and curious people have been encountered.

“I am looking forward to extending my searches and wherever I go for holidays my OS maps, camera and GPS monitor come with me.”

Within Yorkshire, most trig pillars - more than 200 - are located in the north of the region at high points in the Yorkshire Dales and North Yorkshire Moors national parks.

While there are many trig-baggers out there, trig-bagger extraordinaire Rob Woodall from Peterborough, this summer completed a 13-year mission to bag all of Britain’s 6,190 trig pillars.

Modern map-making has meant trig pillars are no longer used for surveying, but they still act as a beacon for ramblers and are a symbol of the countryside. Though the Ordnance Survey no longer uses the trig pillars, maintaining them remains its responsibility.

Like an iceberg, there is more of the trig pillar below the surface than above it.

Trig pillars are seen as quintessentially British and even made it onto American novelist Bill Bryson’s list of favourite British items in his 2015 book The Road to Little Dribbling.

In Scotland, there are some pillars called Vanessas, which are taller, cylindrical concrete pillars.

Although 6,500-plus trig pillars were built, hundreds have been lost to housing developments, farming, coastal erosion and other causes.

Many countries overseas, including Japan, Australia and New Zealand have their own network of trig points.

Geoff Lomas, Recreation Manager at Yorkshire Water, said: “We thought it would be interesting to find out how may trig pillars are still left in Yorkshire to mark their 80th anniversary.

“On our land there are 33 trig pillars but across the region as a whole there are re-assuringly still hundreds of these surveying relics still standing. For ramblers looking for walking routes, we think they offer a great focal point and photo opportunity for a memorable walk.”

To commemorate the 80th anniversary, Yorkshire Water - as the second largest landowner in the region - is encouraging people to post a photo on its Facebook page of them posing next to one of these historic monoliths for the chance to win a £100 outdoor gift voucher.

*facebook.com/yorkshirewater