Ordnance Survey Department in March, 1844, but subsequent
records taken in May and June, 1859, by a self-recording gauge
on St. George's Pier, showed that the true mean level of the
sea at Liverpool is 0.068 ft below the assumed level. The
general mean level of the sea around the coast of England, as
determined by elaborate records taken at 29 places during the
years 1859-60, was originally said to be, and is still,
officially recognised by the Ordnance Survey Department to be
0.65 ft, or 7.8 in, above Ordnance datum, but included in these
29 stations were 8 at which the records were admitted to be
imperfectly taken. If these 8 stations are omitted from the
calculations, the true general mean level of the sea would be
0.623 ft, or 7.476 in, above Ordnance datum, or 0.691 ft above
the true mean level of the sea at Liverpool. The local mean
seal level at various stations around the coast varies from
0.982 ft below the general mean sea level at Plymouth, to 1.260
ft above it at Harwich, the places nearest to the mean being
Weymouth (.089 ft below) and Hull (.038 ft above).

It may be of interest to mention that Ordnance datum for
Ireland is the level of low water of spring tides in Dublin
Bay, which is 21 ft below a mark on the base of Poolbeg
Lighthouse, and 7.46 ft below English Ordnance datum.

The lines of "high and low water mark of ordinary tides" shown
upon Ordnance maps represent mean tides; that is, tides halfway
between the spring and the neap tides, and are generally
surveyed at the fourth tide before new and full moon. The
foreshore of tidal water below "mean high water" belongs to the
Crown, except in those cases where the rights have been waived