Lowest Astronomical Tide

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The Lowest Astronomical Tide (LAT) is an internationally standardised reference size for measuring the depth of water and to define the coastal line in sea charts.


The Lowest Astronomical Tide is the height above the lowest tidal water level, which can be used for a level alone on the basis of the contained locally dominant tidal conditions. (Tidentafeln BSH)

This is a theoretical value that is calculated in each case by the measured tide levels for the future. Bases are the tide levels for the last 18½ years. The time interval of 18 ½ years considers two oscillation periods substantial for fluctuation of tides: the nutation period of the Moon (18.6 years) in the ecliptic, and their effect, the lunar nodal tidal constituent, is the most important under the longer components of the tide. The values for LAT are computed mathematically by a harmonic analysis.


  • World-wide uniform map zero for sea charts.
  • No negative values in tide tables.
  • Secure depth data, the tide can hardly fall further.


A crucial disadvantage is that during the land surveying usual standard height datum does not agree with LAT and that this difference differs from place to place. In the Baltic Sea LAT and agrees rather exact with the standard height datum, however in Cuxhaven the difference amounts to 2.1 meters.

Depending on whether you use a chart or a map fluctuating areas like Mudflats look completely different.

LAT in Europe

The North Sea countries of Germany, the United Kingdom, France, Belgium, the Netherlands, Germany, Denmark and Norway have already introduced LAT.

LAT world-wide

In 1993 the Tidal Working Group recommended the introduction of LAT as reference size for all sea charts of the North Sea. The Tidal Working Group (TWG) is a working group of the North Sea Hydrographic Commission (NSHC) of the IHO).

Beyond the North Sea however, LAT has no meaning so far.


The depth of water measured locally by sounding line is dependent on the water level fluctuations in the tide, thus on the calendar date and the time of day. The calculated value must be corrected accordingly. The reference values are the tide tables of the reference points (40 in Germany), and the differences of the connection points (about 2000 in Germany). The water level computation map serves as an aid.

LAT is appropriate for predominantly 0.3-0.6 m with the German reference and connection places below the past, on a mean low spring tide referred to chart datum where LAT is not valid.

For electronic charts (ECDIS), world-wide chart datums are used.

Chart datums are used for waters with few tides such as the Baltic Sea, Mediterranean, Black Sea and Red Sea.

Most countries also use for chart datums for tide heights.

See also