The Concrete Pillars On Top Of British Hills: Trig Points

http://tomscott.com - http://twitter.com/tomscott - Around the United Kingdom there are odd concrete pillars on the top of hills, built to last for decades if not centuries. They've got a cryptic marking on them, and the words "Ordnance Survey Triangulation Station".

What are they? (They're trig points.) Who put them there? (Brigadier Martin Hotine and thousands of people working with him.) And why? (To get an accurate map of the UK, with maths.)