Today's post is about tracking down the trig pillars in Greater London, like this one at One Tree Hill in Alperton.
A triangulation pillar is a cast concrete structure positioned in the landscape to facilitate the making of maps. Over 6500 were installed across the country between 1936 and 1962, each 1.2m tall with indented brass plate on the top face to support a theodolite. The underlying maths is (relatively) simple. If you know the precise distance between two points and two angles to a third, you can calculate the precise location of that third point and thus build up an accurate map. Trig points therefore tend to be located at high points in the landscape, because line of sight was all important.
The initial triangulation of Britain kicked off in 1784 with a line rather than a triangle. Major-General William Roy established his first baseline across Hounslow Heath and came up with a distance of 27404.01 feet (which turned out later to be within two inches of the correct length). His two historic endpoints are now marked by cannons embedded in the ground, one up a cul-de-sac in Hampton and the other beside the Northern Perimeter Road at Heathrow Airport. Below is a photo of the Heathrow cannon, taken last week while the temperature was 37½C. It's one of my favourite psychogeographic London landmarks, and you can read more about it here.
Unsurprisingly the Ordnance Survey's initial survey wasn't perfect, which is why the Retriangulation of Great Britain kicked off in the 1930s. It didn't just use concrete pillars, but also bolts embedded in the ground, rivets poking out of buildings and "intersected stations" at the apex of church spires, chimneys and masts. Southwark Cathedral's tower's flagstaff is a trig point, as is a rivet on the Heathrow Airport control tower and a disc on the ground outside Brent Cross Shopping Centre. A complete list of UK triangulation points is downloadable here, if you want to check the data for yourself.
But the advance of digital mapping has made the trig point redundant. The Ordnance Survey now employs GNSS technology to determine location data much more precisely, and the OSGB36 datum has been replaced by ETRS89 (European Terrestrial Reference System 1989). Over the last 30 years several hundred trig pillars have been lost or destroyed because the OS no longer maintains them, but the vast majority remain as much-loved curiosities. Schlepping up to the top of a hill always feels a bit more special when there's a trig pillar on top.
In researching London's trig pillars, I've discovered that there are fewer than I thought. That's not because dozens have been destroyed, but mainly because London's more blessed with tall buildings than it is with hills. In total there are 24 triangulation pillars in the capital, and only one of these is in inner London. Half of the total are in just four boroughs - Havering, Harrow, Bromley and Croydon. Some seem ridiculously close to each other, others leave surprisingly large gaps, but that's a mixed demarcation system for you.
Here then are London's two dozen trig pillars, arranged approximately geographically. The height shown is the height of the top of the pillar, not ground level. Click on the height to see an Ordnance Survey map depicting (with a blue triangle) precisely where the trig pillar is. If the pillar is publicly accessible I've coloured it green, and if it isn't I've coloured it red. It's a 50/50 split.
A more accurate map of the London area can be found here. You can also see the precise locations if you go to osmaps.ordnancesurvey.co.uk, click the Places tab and select Trigpoints from the menu down the side. This works for anywhere in the country.
Some people are obsessed by visiting trig points, and their exploits can be read at trigpointing.uk. The keenest trigbaggers aren't averse to visiting off-limits trig points by snapping padlocks, creeping under electric fences or wandering across golf courses, which is taking an obsession too far. But I recommend checking their research before heading out because it turns out several red trig points are clearlyvisible from public land and certain green trig points are an overgrowndisappointment. The best ones to visit are those in the second and fourth columns of my map. Just don't try collecting them all.
