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Chart datum & ordnance datum

Differences at selected UK and Irish ports
Tidal levels are quoted relative to chart datum (approximately the lowest level due to astronomical effects and excluding meteorological effects). The heights of chart datum (CD) relative to ordnance datum (OD, at Newlyn) in the UK are listed below.

If for example, you have a data value for Liverpool of 1m relative to CD and want to convert it to be relative to OD, then this would be -3.93m (i.e., 1m - 4.93m).

England - South Avonmouth Bournemouth Devonport Dover Hinkley Ilfracombe Newhaven Newlyn Portsmouth St. Mary's (local OD) Weymouth	-6.50m -1.40m -3.22m -3.67m -5.90m -4.80m -3.52m -3.05m -2.73m -2.91m -0.93m	England - East Cromer Felixstowe Immingham Lowestoft North Shields Sheerness Whitby Channel Islands St. Helier (local OD)	-2.75m -1.95m -3.90m -1.50m -2.60m -2.90m -3.00m -5.88m	England - West Blackpool Formby Heysham Hilbre Island Liverpool Southport Workington Isle of Man Port Erin (local OD)	-4.90m -4.93m -4.90m -4.93m -4.93m -4.90m -4.20m -2.75m
Scotland Aberdeen Kinlochbervie Leith Lerwick (local OD) Millport Moray Firth Port Ellen Portpatrick Stornoway (local OD) Tobermory Ullapool Wick	-2.25m -2.50m -2.90m -1.22m -1.62m -2.22m -0.19m -1.80m -2.71m -2.39m -2.75m -1.71m	Wales Barmouth Fishguard Holyhead Llandudno Milford Haven Mumbles Newport	-2.44m -2.44m -3.05m -3.85m -3.71m -5.00m -5.81m
Éire (OD Dublin) Cobh Dublin Limerick Rosslare Sligo Sligo (special datum)	+0.13m +0.20m -0.46m +1.02m +0.69m -0.81m	Northern Ireland (OD Belfast = OD Dublin +2.7m) Bangor Belfast Donaghadee Londonderry Portrush	-2.01m -2.01m -2.26m -1.61m -1.24m