Chart Datum

Note: The following is taken from a response from the UKHO for clarification of the phrase "Predicted heights are in metres above chart datum", which is something we're asked about frequently.

Chart Datum is the plane to which all tidal heights are referred. It is also the plane below which all depths are published on a navigational chart, so that adding the tidal height to the charted depth the true depth of water is determined. By international agreement it is defined as being a level so low that the tide will not frequently fall below it.

In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide (LAT). LAT is the lowest level which can be expected to be predicted under average meteorological conditions and under any combination of astronomical conditions ; this level will not be reached every year. LAT is not an extreme level, as storm surges may cause considerably lower levels to occur.

The specific phrase "Predicted heights are in metres above chart datum" means that the tidal heights are predicted i.e. computed values and they are in metres above Chart Datum which is the zero point or reference plane from which the tidal heights are measured as defined above.

Please note that predicted tidal times and heights are representative of average meteorological conditions and can differ significantly from the actual tide time and height due to the local meteorological conditions.

Onshore winds will pile the water up on the coast and offshore winds will have the opposite effect. Winds blowing along the coast may advance or retard the time of High Water. In extreme conditions the wind can set up a storm surge, which when combined with a high spring tide can dramatically increase the height of tide actually experienced.

Changes in Barometric pressure cause the sea level to rise, a difference of 34 millibars from the average can cause a difference in height of about 0.3m. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds, moreover, to the average change in pressure over a considerable area. Changes in level due to barometric pressure seldom exceed 0.3m but, when mean sea level is raised or lowered by strong winds or by Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur), this effect can be important.