Seasons and sky

Tidal Datum

Direct answer

Tidal Datum: A Tidal Datum is a local reference elevation derived from a defined phase or statistic of observed water levels over a specified epoch.

A Tidal Datum is a local water-level reference based on tide observations.

Seasons & Earth AlmanacThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Seasons & Earth Almanac
Related: Tidal Range, Semidiurnal Tide, Astronomical Season, Equinox

Four countrymen round a great flat sundial in a field under the sun, one sowing and one reaping — Earth and season · visual guide **Tidal Datum**
A Tidal Datum is a local water-level reference based on tide observations.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.

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The story behind Tidal Datum

Long tide-gauge records enabled national agencies to establish consistent local datums such as Mean Lower Low Water.

Why Tidal Datum matters

Charts, coastal boundaries, flood thresholds, and engineering elevations need a stable stated zero.

Tidal Datum in practice: an example

A charted depth referenced to Mean Lower Low Water is not measured from the same zero as an elevation referenced to a land datum.

Where readings of Tidal Datum go wrong

It is a mistake to extend a local Tidal Datum into hydrologically different waters without measurements and an authorized transformation.

Sources and editorial method

Research references used for this entry include:

NOAA Tides and Currents Glossary - Operational definitions for tides, tidal currents, ranges, datums, bores, and amphidromic systems.

Source: The Great Almanac Dictionary
Category: Seasons & Earth Almanac
Last updated: 2026-09-26

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Tidal RangeTidal Range is the height difference from high water to low water. Semidiurnal TideA Semidiurnal Tide has two broadly similar highs and lows each lunar day. Astronomical SeasonAn Astronomical Season runs from one equinox or solstice to the next. EquinoxAn Equinox occurs when the Sun crosses the celestial equator. SolsticeA Solstice marks the Sun's greatest apparent northward or southward position. SunriseSunrise is the calculated first appearance of the Sun's upper edge at the horizon.

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