You would think that measuring heights of hills and mountains would be obvious and simple. But it isn't because the zero level is not that obvious. The UK's mapping agency, Ordnance Survey, measures altitude with respect to the mean sea level as calculated during the first world war here at Newlyn. But did you know that the sea level has risen there since by about 20 cm. So the height of Mount Snowdon is really 20 centimeters lower than marked on the map.