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What is trig bagging – and why is it popular among hikers and runners?

Trig pillar on Ben Nevis
A trig pillar on the UK's tallest mountain of Ben Nevis (Image credit: Getty Images)

Next month marks the 85th anniversary of a quintessential British landmark, the  triangulation pillar, or "trig". In celebration of this vital piece of British mapping history, we reveal the outdoors activity of “trig bagging”.

It was in April 18, 1936 that the first trig pillar was installed by mapping agency Ordnance Survey (OS). It launched what was known as the “Retriangulation of Great Britain”.

Such was the extent and detail of the mapping that some 6500 trig pillars were used to measure the shape of the land and to rewrite the map of Britain.

Trig pillars are now a common part of the British scenery – and although no longer used for mapping, they offer a rewarding focus for a growing number of trig baggers.

trig bagging in the campsie fells

Trig bagging and hill running in the Campsie Fells, East Dunbartonshire (Image credit: Fiona Outdoors)

What is triangulation?


The triangulation system is a mathematical process that makes accurate map-making possible. An OS spokesperson explained: “Triangulation works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline.

“For OS maps, the known points were the thousands of trig pillars erected across the country.

“A theodolite was secured to the top mounting plate of a pillar so that angles could then be measured from the pillar to other surrounding points.”

It took many angle measurements to gain the most accurate primary points in the retriangulation and observations often took several hours at each pillar.

In total, the retriangulation task took 26 years, including break for World War II.

The OS maps that many hikers and runners use are based on this Retriangulation process, although today any new measurements are done by GPS.

Trig Point on Win Hill, Peak District, England

A trig pillar on Win Hill, Peak District, England (Image credit: Getty Images)

What is trig bagging?


There are thought to around 6000 remaining pillars spread across the Britain landscape and most can be "bagged" by using a map and compass.  

Trig pillar locations are marked by a small blue triangle on OS maps. 

You can choose to walk, run or, in some cases, cycle, to a trig. While some offer an easy going stroll others form the basis of a more challenging mountain hike. 

Many people enjoy "bagging" or "ticking off" a list of trigs. For example, you could hike, run or cycle to all the trig pillars in your country or region, or aim to reach all the trig pillars on one OS Landranger map.

Trig bagging is also a great activity for families, especially if you look for pillars that are located on smaller hills. Let your children show you the way by map and compass.

Some people set themselves the challenge of reaching a set number of trigs in a day, such as all the trigs in your local hills.

There are some useful resources for discovering the location of trigs, such as: 

trig point on Stanage Edge, Peak District national park

Stanage Edge summit trig in the Peak District National Park (Image credit: Getty Images)

Britain’s top trig bagger

It took one extraordinary trig bagger 14 years to bag all the existing trig pillars in Britain. On April 16, 2016, Rob Woodall walked to his final trig point on Benarty Hill in Fife. 

Rob, of Peterborough, completed a list of 6,190 trigs and celebrated his achievement with 29 friends and fellow trig baggers. 

He said: “It’s been a great adventure to walk to – and to try to discover – all the remaining trig pillars. I have visited some very remote places in Britain but also reached trigs that are located in very easy places to walk to.”

An OS spokesperson described Rob’s quest as a “seriously impressive achievement”.

6 fascinating facts about trig pillars



1. The highest trig pillar sits on the top of Ben Nevis in the Scottish Highlands. 

2. The lowest trig pillar is at Little Ouse, sited at -1m, Cambridgeshire, England.

3. Trig pillars are usually made of concrete 

4. They are shaped like truncated square pyramids or obelisks that taper towards the top. 

5. Like an iceberg, there is a large part of the trig pillar below the surface.

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

Fiona Russell

Fiona Russell is a widely published adventure journalist and blogger, who is better known as Fiona Outdoors. She is based in Scotland and is an all-round outdoors enthusiast with favourite activities including trail running, mountain walking, mountain biking, road cycling, triathlon and skiing, both downhill and back country. Her target for 2021 is to finish the final nine summits in her first round of all 282 Munros, the Scottish mountains of more than 3,000ft high. Aside from being outdoors, Fiona's biggest aim is to inspire others to enjoy the great outdoors, especially through her writing. She is also rarely seen without a running skort! Find out more at Fiona Outdoors.