An interesting fact I learnt only yesterday. Did you know that around the island of Great Britain, sea level – the reference point for the height of everything on land, from your front door step to Ben Nevis – is measured relative to the head of a brass bolt screwed to the floor inside a shabby red and white hut on the end of a pier in Newlyn, Cornwall. The bolt head is 4.75 metres above the average level of the sea, as measured every hour over the six-year period between 1915 and 1921. Oceanographers refer to this definition of sea level as ODN, which stands for Ordnance Datum Newlyn; when planners and housebuilders talk about land height they use the acronym AOD, or Above Ordnance Datum. The century-old bolt is still the reference for sea level, literally ground zero, as far as the UK is concerned. The bolt sits on a bed of granite.