Chart datum & ordnance datum
Differences at selected UK and Irish ports
Tidal levels on this site (and most published tide tables) are given as a height above 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. The values are usually negative indicating that chart datum is below ordnance datum.
If for example, you have a tidal height for Liverpool of 10m above CD and want to convert it to be relative to OD, then add the value from the table below
10m + ( −4.93m ) = 5.07m
As ordnance datum is generally above chart datum, the numbers on a tide table when given to OD are usually lower than when given to chart datum.
Read more about ordinance datum at Newlyn →
Download a tidal datums information sheet →
- England – South
- Avonmouth
- −6.50m
- Bournemouth
- −1.40m
- Devonport
- −3.22m
- Dover
- −3.67m
- Hinkley
- −5.90m
- Ilfracombe
- −4.80m
- Newhaven
- −3.52m
- Newlyn
- −3.05m
- Portsmouth
- −2.73m
- St. Mary's (local OD)
- −2.91m
- Southampton
- −2.74m
- Weymouth
- −0.93m
- England – East
- Cromer
- −2.75m
- Felixstowe
- −1.95m
- Immingham
- −3.90m
- Lowestoft
- −1.50m
- North Shields
- −2.60m
- Sheerness
- −2.90m
- Whitby
- −3.00m
- England – West
- Blackpool
- −4.90m
- Formby
- −4.93m
- Heysham
- −4.90m
- Hilbre Island
- −4.93m
- Liverpool
- −4.93m
- Southport
- −4.90m
- Workington
- −4.20m
- Isle of Man
- Port Erin (local OD)
- −2.75m
- Channel Islands
- St. Helier (local OD)
- −5.88m
- Wales
- Barmouth
- −2.44m
- Fishguard
- −2.44m
- Holyhead
- −3.05m
- Llandudno
- −3.85m
- Milford Haven
- −3.71m
- Mumbles
- −5.00m
- Newport
- −5.81m
- Scotland
- Aberdeen
- −2.25m
- Kinlochbervie
- −2.50m
- Leith
- −2.90m
- Lerwick (local OD)
- −1.22m
- Millport
- −1.62m
- Moray Firth
- −2.22m
- Port Ellen
- −0.19m
- Portpatrick
- −1.80m
- Stornoway (local OD)
- −2.71m
- Tobermory
- −2.39m
- Ullapool
- −2.75m
- Wick
- −1.71m
- Northern Ireland
-
(OD Belfast
= OD Dublin +2.71m) -
- Bangor
- −2.01m
- Belfast
- −2.01m
- Donaghadee
- −2.26m
- Londonderry
- −1.61m
- Portrush
- −1.24m
- Éire / Ireland
- (OD Dublin)
- Cobh
- +0.13m
- Dublin
- +0.20m
- Limerick
- −0.46m
- Rosslare
- +1.02m
- Sligo
- +0.69m
- Sligo (special datum)
- −0.81m