It was fitting that we had a Twitter question about trig pillars yesterday, as this month will mark 79 years since the humble trig pillar was first used for the start of the retriangulation of Great Britain. On 18 April 1936, a group of men gathered around a strange, pale obelisk in the middle of an unremarkable field in Cold Ashby, Northamptonshire. The shining white monolith would now be instantly recognised by any walker, hiker or geography pupil – and is often a welcome sign that you’ve reached the peak of your hike when you spot the trig pillar.
Trig pillars now evoke the kind of sentimentalism of something quintessentially British, but they’re largely no longer in use by us at OS. Colin Hope tweeted us about the trig point he spotted at Berry Head, that was painted purple and had sadly also been graffiti-ed. It’s a huge shame, but we no longer maintain the trig pillar network, only stepping in if the pillar becomes unsafe to the public.
What did we use trig pillars for?
Something we’re often asked now as many people know they’re something to do with OS, but not sure what. At the time they were part of a state-of-the art network built to re-write the map of Britain. Triangulation is basically a mathematical process that makes accurate map making possible. It works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline and in this case, those known points were the 6,500 trig pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the retriangulation, many rounds of angles would have been measured with the observations taking several hours.But time and technologies have moved on enormously to the point where the traditional trig pillar is now obsolete in its original guise. They still act as a beacon for weary ramblers but no longer do they help shape our maps.
So how do we map the country today?
The trig pillars and the retriangulation might now be largely redundant, but we still have a responsibility to maintain and provide access to a national mapping and survey control network. The modern equivalent is OS Net, a network of 110 Global Navigation Satellite System (GNSS) receivers. Our surveyors use OS Net and GNSS technology everyday to instantly position new map detail to within a few centimetres. The system is also used to position our two aircraft as they fly the country capturing aerial imagery. What took many hours at Cold Ashby in 1936 we can now do in seconds and to a far greater degree of accuracy.
Now, 79 years on from the start of the retriangulation, around 5,500 of the original 6,500 trig pillars are still standing. Some of them have been decorated too – we recently had these examples from Wales and Scotland shared with us.
If you do spot a trig pillar looking unsafe, do let us know, so that we can take a look and decide on the best way to remedy it. But in the meantime, just enjoy spotting the lovely old structures when you’re out and about exploring Britain. You can also see a series of photos of trigs on our Pinterest board.