Ordnance datum

The First Geodetic Levelling of England and Wales (1840–1860) needed to define a datum plane from which to specify spot heights.

By the time of the Second Geodetic Levelling (1912–1921) the importance of stability was better appreciated and so it was decided to use Fundamental Bench Marks (FBMs) installed in solid rock, rather than on buildings as before.

To measure average MSL around Great Britain three tide gauges were employed: at Dunbar, Newlyn and Felixstowe.

Newlyn has certain practical advantages: it is set in granite bedrock, is far from major rivers, and it better represents deep ocean sea levels.

But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum.