Blencathra From Skiddaw Trig Pillar. OS BM S1543. Photo by Andrew (CC licence).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.أكثر...