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
by special grants. Mean high water is, strictly speaking, the
average height of all high waters, spring and neap, as
ascertained over a long period. Mean low water of ordinary
spring tides is the datum generally adopted for the soundings
on the Admiralty Charts, although it is not universally adhered
to; as, for instance, the soundings in Liverpool Bay and the river
Mersey are reduced to a datum 20 ft below the old dock sill, which
is 125 ft below the level of low water of ordinary spring tides.
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