Chart datum & ordnance datum

Differences at selected UK and Irish ports

Tidal levels on this site (and in 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 nearly always 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.

Download a tidal datums information sheet →

Read more about ordinance datum at Newlyn →

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