Trig pillars are a welcome sight for many walkers summiting Britain’s hills and mountains. But what are they for, and why do they inspire such enduring fascination?
27th June 2025 | Words by Matt Jones | Photos by Matt Jones and Ellie Clewlow
If you’ve spent much time walking in the British countryside, you’ll undoubtedly have seen one, touched one or perhaps even stood on top of one. We’re referring, of course, to the iconic trig pillar – the squat, truncated obelisks that are frequently found atop UK hills and mountains, but which crop up in all sorts of other strange places too, from coastal cliffs and farmers’ fields to military firing ranges and even back gardens. These curious landscape features tend to spark plenty of interest – and rightly so, for they have a fascinating history, dating back to the early twentieth century.
Bagging the trig pillar on Penygadair, the summit of Cadair Idris, Eryri (Snowdonia)
Twentieth century triangulation
Today, thanks to modern technology, we have highly accurate mapping at our fingertips. But in the early 1930s, map making was still based on increasingly outdated 18th- and 19th-century data known as 'Principal Triangulation'. A more accurate and comprehensive system of observations was needed. So, in 1935, Ordnance Survey initiated an ambitious project that would become known as the 'Retriangulation of Great Britain'. Led by a Royal Engineers-trained army officer, Brigadier Martin Hotine, it would create a unified nationwide mapping projection: the basis of the metric national grid reference system and the 1:25,000 scale maps we use today.
The project relied on the mathematical process of triangulation, which determines the location of a geographical point by measuring angles to it from known points at either end of a fixed baseline. Measurements were taken using a telescope-like surveying instrument called a theodolite, which required a stable and perfectly level base. That's where triangulation pillars – or 'trig' pillars – came in. Mostly built to a standard design specified by Hotine himself, they provided a steady platform for accurate measuring, as well as a secure mounting point for the sensitive equipment.
The distinctive brass plate on the top of a trig pillar was originally a mounting point for a theodolite, a piece of surveying equipment.
Landscape icons
Hotine could hardly have known how iconic his trig pillars would eventually become. His basic design consists of a flat-topped square concrete obelisk, tapering upwards. On top, you'll normally find a brass plate with three grooved arms and a central hole, originally used to mount the theodolite. A metal plaque or Ordnance Survey Bench Mark (with the initials OSBM) and a reference number is usually set into the pillar too. There are exceptions though – some pillars are built from local stone or slate, not concrete, while others are circular, not square. In Scotland there are even some taller, cylindrical pillars of a different design, known as 'Vanessas'.
A classic white concrete trig pillar on Y Golfa near Welshpool, with an OS Bench Mark set into the base.
Today, many trigs have taken on their own identities, being creatively remodelled by enterprising locals. Famously, there's a trig pillar painted as a little yellow Minion at Monk Bretton near Barnsley, and a grey alien at Hurlet Hill, Glasgow. Other common adornments include the English or Tudor rose (found, for example, atop Winter Hill in Lancashire) and the Welsh dragon (e.g at Hay Bluff in the Brecon Beacons).
Trig pillars are unique to Britain. Though various other countries also have networks of trigonometrical or triangulation stations – including the USA, Ireland, Australia, New Zealand, South Africa and Japan – none share the same distinctive design as our squat little obelisks.
Trig pillars are unique to Britain, though many other countries have their own networks of triangulation points and geodetic survey stations, like this U.S. Geological Survey bench mark set into the summit rock on North Dome, Yosemite, California.
Trig pillar or trig point?
Some walkers call these structures 'trig points', rather than trig pillars. This isn't incorrect, but it's worth noting that though all trig pillars are trig points, not all trig points are (or were) trig pillars. Confused? Well, sometimes, there was no need for a pillar to be constructed, since another landscape feature such as a church spire, chimney or lighthouse could be used for triangulation. These were generally just sighted from other trigs – you wouldn't want to set up a theodolite on top of them! In addition, there were many other types of triangulation point, including bolts, blocks, rivets, Berntsens, Fenomarks and Curry Stools. In total, these various points total around 25,000 locations.
In contrast, there were far fewer trig pillars – though still around 6,500 in total. Each was carefully positioned, marking a point with accurate eastings and northings coordinates. When the network was complete, each trig point was also intervisible – that is, it was possible in clear weather to see at least two other trig points from any one trig point (though this is not necessarily still the case today). In addition, most trig pillars had a flush bracket at the base of the pillar that specified their height above sea level.
Trig pillars come in a variety of different designs. Many of those found across Scotland are tall, cylindrical structures, known to trig-baggers as 'Vanessas' – like this one on top of Blà Bheinn, Isle of Skye.
A heroic undertaking
The Retriangulation of Great Britain was a herculean endeavour that took nearly three decades to complete – the first pillar was positioned in 1936, the last in 1962. Their construction meant carrying heavy materials and equipment to remote sites, and labouring in difficult conditions. Some teams had to excavate as much as 15 feet underground to secure the pillars in place. The work of the surveyors was just as arduous, and even more meticulous – observations were made, checked and rechecked, often over a period of several days and even weeks. There was plenty of waiting around for clear, favourable conditions to use the theodolite. Surveyors' accounts of the task reveal that wet clothing, frozen fingers and bleary eyes were typically the order of the day (and night).
Ice-rime encrusted trig pillar on the summit of Moelwyn Mawr, central Eryri (Snowdonia), during a winter trig-bagging trip. The pillar is constructed from local slate.
The results, though, were impressive. The project was a resounding success, paving the way for modern mapping. For the time, it was also astonishingly precise. In 2015, OS surveyors returned to the trig pillar on the summit of Ben Nevis to re-measure the mountain's height, though this time using advanced Global Navigation Satellite System (GNSS) positioning rather than theodolites and triangulation. Despite the increased accuracy of modern technology, it was found that the result obtained back in 1949 was only a few centimetres short of the peak's true height. The 2015 team measured it at exactly 1,344.527 metres – though this was rounded up to 1,345m, which meant it appeared as though the Ben had grown by a metre. The new tech did make the process much quicker though – the GPS-based calculation took a mere two hours. Back in 1949, it had taken a team of seven surveyors 20 nights to obtain their measurements.
Still, the negligible difference in the height of Britain's tallest mountain is testament to the dedication and meticulousness of all those involved in the original retriangulation project. And though trig pillars are no longer used for mapping, having long been superseded by GPS satellites and aerial photography, they remain useful and much-loved local landmarks – prominent sentinels of Britain's hills, mountains, coast and countryside.
An unusual circular trig pillar on the top of Waun-Oer in the southern part of Eryri (Snowdonia). Ignore the two backpackers in the background – they were trying to look heroic…
Trig-bagging
A number of walkers have made it their mission to visit or 'bag' all 6,000+ remaining trig pillars in Britain. The first person to achieve this was Rob Woodall. Back in April 2016, Rob completed a 14-year-long personal quest to bag all of Britain's then extant trig pillars – some 6,190 in total. His final trig was at Benarty Hill in Fife – and fittingly, Ordnance Survey commemorated his achievement by presenting him with his very own OS Bench Mark at the summit. He cited his motivation for this epic feat as being down to a number of factors; namely, the diversity of different locations to visit, an interest in mapping, and the challenge of ticking off a really big list…
If those are the sort of things that inspire you to start bagging trigs too, various online resources might be useful – and much quicker than simply poring over hundreds of Ordnance Survey maps! Trigbagging.co.uk has an interactive map that can be used to discover trig points near you. If you start to find you can't walk past a trig pillar without bagging it, you might like to log your finds at trigpointing.uk. And lastly, for a comprehensive database of GPS waypoints that pinpoint all the trig pillars in the UK, visit haroldstreet.org.uk/trigpoints/
Constantly exposed to the elements and often positioned high in the mountains, many pillars are slowly crumbling away, like this example on Askival, Isle of Rum.
Trig pillar facts and figures
Trig pillars are identified on OS maps by a small blue triangle with a dot in the middle.
There were originally more than 6,500 trig pillars across Britain, though hundreds have been subsequently lost to housing developments, farming, coastal erosion and other causes.
The first trig pillar was installed back in 1936 in Cold Ashby, Northamptonshire.
There is a profusion of pillars in the Peak District. In fact, OS Landranger Sheet 119 (Buxton and Matlock) has more trig pillars than any other map, with well over 100.
Just like an iceberg, there is usually as much – or more – of a trig pillar underground than above it, since most were set in a 2½ ft x 3ft base reinforced with angle iron, underpinned by a lower centre mark fitted with a wooden cover.
The highest trig pillar in the UK is on the summit of Ben Nevis, at 1,345m.
The lowest is on the banks of the Little Ouse River on the border between Norfolk and Cambridgeshire, which is -1m below sea level.
Pillars at heights of 1, 2, 3, 4, 5 and 7m are all to be found in the Fens, though you have to go to Yorkshire if you want to bag a 6m pillar.
Many trig pillars are tricky to reach, but arguably the most inaccessible is on Bac Mòr or The Dutchman’s Cap, one of the Treshnish Isles – a remote archipelago of small islands and skerries located west of the Isle of Mull.
Trig pillars are no longer used for mapping, although they still act as a useful landmark for walkers and cyclists and continue to be maintained by the Ordnance Survey.
Trig pillars featured as one of US author Bill Bryson’s ‘favourite British items’ in his 2015 book, The Road to Little Dribbling.
The now defunct indie record label Angular Recording Corporation’s name and logo were both inspired by trig pillars. Active in the early to mid-2000s, the label launched the careers of bands such as Art Brut, Bloc Party and The Long Blondes.
