Letter: How high is the sea?

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The Independent Online
Sir: To answer Colin Lindsay's letter (11 May), the height of the sea is the height above chart

datum. This datum is fixed at the level of the lowest predicted tide, or LAT (Lowest Astronomical Tide), as the prediction is made using astronomical tide-generating forces. The LAT ignores adverse weather conditions, such as high winds, which can cause a lower tide than LAT (ie, the LAT assumes average meteorological conditions).

All hydrographic charts for navigation are reduced to this chart datum for obvious reasons. However, for global sea-level determination (perhaps by satellite) the sea height will possibly be referenced to a different

datum, such as a reference ellipsoid, as I think may have been the case in determining the slope (sea) between England and France. But that is another


Yours sincerely,


Newcastle upon Tyne