THERE are 5,500 of them, they were built over a 26 year period between 1936 and 1962, they are mostly white and the last one to be used was at Thorney Gale, in Cumbria.
Please be upstanding for the trig pillar; the network of four feet high concrete monoliths that were constructed to enable map makers to retriangulate Great Britain.
Although now redundant thanks to modern map making which uses satellite navigation, trig pillars still act as beacons for weary hill-walkers; confirming they’ve reached the peak of the hill they’ve just lugged themselves up, before touching the brass-plated top as a way of saying; “done it!”. And in my case, taking a photograph.
Triangulation is a mathematical process that makes accurate map making possible, and it works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline; the baseline being the trig pillar.
A theodolite was secured to the top mounting plate and angles were then measured from the pillar to other surrounding points.
In the early 20th century map making was still based on a piecemeal collection of observations taken between 1783 and 1853, but with the rapid development going on after the World War One accurate map making was a huge challenge – and so the trig pillar was born.
From the moment they were built they stood alone, more often than not miles from the nearest road; but sadly it now looks like those closest to civilization have become the target of metal thieves.
Trig pillar TP1874 stands in Chevin Country Park, also known as Caley Deer Park, a hill that looms over Otley, in West Yorkshire.
Thieves have taken a sledgehammer to it, smashing off the identification plaque and top plate and leaving lumps of concrete strewn across the forest floor.
The Ordnance Survey owns all the trig pillars, and for a while they ran an adoption and maintenance scheme. Nowadays they let members of the public keep a watching eye on them and report any maintenance issues.
I emailed the Ordnance Survey about the plight of TP1874, and got this reply:
In the circumstances where we are notified that a pillar is in disrepair, we will make a decision whether to repair or destroy it.
We shall of course do this with the consultation of the present landowner. It is unlikely that we will need to do anything unless the pillar is in a dangerous condition.
Here’s hoping that it won’t have to be taken down.
If you spot anything and want to report it, email the Ordance Survey.
A few trig pillar facts courtesy of the Ordnance Survey:
The survey control network of trig pillars was accurate to 20 metres over the entire length of Great Britain. Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the same area.
6,500 trig pillars were built for the retriangulation of which around 5,500 are still standing. In total the retriangulation had in excess of 30,000 coordinated points. The modern OS Net network performs the same function with just 110 points.
Measuring angles by eye from a trig pillar meant the retriangulation was reliant on good weather – perhaps part of the reason it took until 1962 to complete! Modern GNSS surveying works in all weathers and is available 24 hours a day.
Trig pillars are mostly made of cast concrete but a few are built from local stone cemented together.
Like an iceberg, there is more of trig pillar below the surface than above it.