The Ordnance Survey are marking the 80th anniversary of the Retriangulation of Great Britain, which began on this day in 1936. More from BBC News. Events include the Trig Pillar Trail Challenge, which invites people to post pictures to social media of one of 25 selected trig (triangulation) pillars (the #TrigPillar80 hashtag is very busy this morning). There are Flickr galleries of various trig pillars from Flickr user Andrew (who took the one above in 2013) and (of course) the Ordnance Survey.