A tidal datum is a reference elevation of sea level, defined in terms of a certain phase of a tide. They are primarily used to estimate water heights and/or depths (e.g. on a nautical chart). Tidal datums are limited to local area because they are estimated from observations of sea level made at a particular tide gauge site. Tidal ranges vary considerably spatially depending on local topographic and hydrodynamic characteristics (e.g. tidal range may be much larger at the head of an estuary compared to the mouth), and hence tidal datums vary accordingly. They should not therefore be extended into areas with different oceanographic characteristics without sufficient observations or numerical modeling to support any such extrapolation.
Tidal datums are usually estimated statistically from sea level observations made over a tidal datum epoch. This is the interval of time recommended to calculate tidal datums, usually at least 19 years to ensure the full 18.6 year lunar nodal cycle is included in the analysis. The tidal datum is the average sea level over a tidal epoch of water height at a particular tidal phase.
For the UK, sea levels are often quoted relative to chart datum (CD), the lowest level water level which can be predicted to occur under any combination of astronomical conditions, or Ordnance Datum Newlyn (ODN), defined as the mean sea level at Newlyn in Cornwall between 1915 and 1921. The heights of CD relative to ODN in the UK are listed here.
Figure 1: Sea level at Newlyn over a two day period, relative to Chart Datum (CD) on the left hand axis and Ordnance Datum Newlyn (ODN) on the right hand axis.