On 18 April 1936, a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby, Northamptonshire, and began the retriangulation of Great Britain.
Ordnance Survey (OS) is celebrating the 80th anniversary of the triangulation pillar, most often known as a "trig pillar" or "trig point" and a welcome sight to many a walker as they reach the peak of their walk.
There were once about 6,500 trig pillars, built by the early surveyors at OS.
The pillar was devised by Brig Martin Hotine to provide a solid base for the theodolites used by the survey teams and to improve the accuracy of the readings obtained.
Though the OS no longer uses the trig pillars, maintaining them is still its responsibility; about 6,000 remain. Here is a selection of those.
Never underestimate the efforts of the surveyors working on the retriangulation of Great Britain.
Reaching this trig pillar when out walking in the Snowdonia National Park feels like an achievement - but imagine carrying all the equipment up there to build it and then the days it could take to survey and take measurements too.
Andrew McConochie's photo was an OS Photofit competition winner and now features on the cover of OS Explorer Map OL23 for Cadair Idris and Llyn Tegid, Gwynedd.
Trig pillars are often on peaks of hills, as George Ewen's photo from the cover of OS Explorer map for Crieff, Comrie and Glen Artney, Perthshire, shows. But the lowest, at Little Ouse, Norfolk, sits at 3ft (1m) below sea level.
Trig pillars were usually constructed from concrete, but some were built from stone available on location. Tom Henman's photo from the cover of OS Explorer map OL53 for Lochnagar, Glen Muick and Glen Clova, in the eastern Highlands of Scotland, shows an example of this.
Trig pillars often provide a 360-degree viewpoint - perfect for capturing a sunrise or sunset, as Colin Harvey did in this photo. Mr Harvey was an OS Photofit competition winner, and his photo now features on the cover of OS Explorer map 166 for Rhondda and Merthyr Tydfil.
Ed Fielden captured one of the most northerly trig points, on the Shetland Islands.
Few people can resist a trig pillar photo, and if you check Instagram you'll see many with people standing on them, planking, popping their dog on top, and a good few yoga poses too. Aleks Kashefi-Mofrad managed a pistol squat on his recent visit to High Seat, Cumbria, in the Lake District.
To celebrate the 80th anniversary of the trig pillar, some of Ordnance Survey's #GetOutside champions have nominated their favourite trig pillar and written an accompanying walk for others to follow. Dave Roberts's walk takes in Foel-fras, Gwynedd, Wales' 11th highest peak.
Bagging Wales' highest trig pillar is an achievement thousands attempt each year. Experienced mountaineer Jason Rawles captured this beautiful sunrise from Snowdon's summit, in Gwynedd.
Jeremy Harris captured the flush bracket on the trig pillar at Trevose Head in Cornwall. Flush brackets, found in trig pillars and on buildings, have a unique number on them and provide a network of points at which height above sea level had been precisely measured.
OS does not condone the redecoration of trig pillars, but could not fail to be impressed by the level of effort put into these two. The Minion pillar was spotted near Barnsley by Siobhan Brennan, and the other picture was taken by Chris Tibbert at Hurlet Hill, Glasgow.
The vast majority of trig pillars follow the standard Hotine design, but in some areas, particularly in Scotland, there are some "Vanessas", taller, cylindrical concrete pillars. Scott MacLucas-Paton took this photo of a Vanessa on top of Ben Tianavaig, Skye, last winter.
The exposed nature of this trig pillar at Ben Ledi, Perthshire, led to a fantastic capture of some rime ice, formed by freezing fog, by Calum Menzies.
Two of the most common decorations OS sees on trig pillars are the English rose and the Welsh dragon. Mathew Elliston captured this trig pillar during a fell run across the West Pennine Moors atop Winter Hill, Lancashire.
Win Hill, Derbyshire, in the Peak District, affords trig baggers a fantastic view, shown by Shaun Fradsham. There are many dedicated trig baggers out there, logging their finds at trigpointing.uk. Rob Woodall recently completed his 14-year mission to bag all of Britain's trig pillars - clocking up 6,190 in total. The number is in constant decline, as land use changes and their conditions deteriorate.
Trig pillars are quintessentially British, and even made it on to Bill Bryson's list of favourite British items in his 2015 book The Road to Little Dribbling. This is Worcester Beacon trig pillar at sunset, photographed by Dan Grant.
Photographs courtesy Ordnance Survey.