On 8 Mar 2007, at 00:24, Mike Stevens wrote: > I believe this started out with the "OS Datum" which > specified that the place from which altitdue was measured by the > Ordnance > Survey was the mean high water spring tide level in a specific > location (or > something like that).
Newlyn, in Cornwall - allegedly a painted mark on a rock, but probably rather more sophisticated these days. The location was chosen, IIRC, because it's relatively stable in the context of the UK's unnerving tendency to sink in the south-east and rise in the north-west. Baz
