I've blogged about benchmarks before and here's one in Coniston. I can't find it on the benchmark spotters website but I did find it on the Ordnance Survey website. They reckon that there are about half a million of these left in the UK. Apparently this one is 50.823 metres above sea level. That's very precise. Think about how big a wave on the sea can be. You'd think that the nearest metre would be fine. But here the height is given to millimetre precision. Now mean sea level must be a construct but even so, with the continents moving and sea level shifting, that level of precision is potentially controversial. That said, if you don't measure to that level of precision perhaps it will be harder to detect changes in sea level or earth movements.
