Glossary

Height datums and statistical water-level terms used on the map and in the texts.

Datums are national or international zeros. Statistical values name the same kind of high water, low water or mean, with a different abbreviation by country and by tidal or inland water. Hover an abbreviation in the text for the short form; a click opens the entry. From the map, a link can bring the current view back with this button.

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Kind
Country or region
Water

Datums

Zeros for heights and charts. A national datum belongs to one country; LAT and HAT are international.

NHN

Normalhöhennull

  • National
  • Germany
  • Coast and tide
  • Inland and Baltic

Official German height reference (DHHN92 / DHHN2016). Land heights in Germany are given relative to NHN.

Same idea elsewhere: MSL

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PNP

Gauge zero (Pegelnullpunkt)

  • National
  • Germany
  • Austria
  • Coast and tide
  • Inland and Baltic

The zero of one gauge, not a national height system. The water level is the height above this zero. Also written PN.

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SKN

Chart datum (Seechartennull)

  • National
  • Germany
  • Coast and tide

Zero of depths on German nautical charts. In the North Sea it is usually the same level as LAT.

Same idea elsewhere: LAT

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NAP

Normaal Amsterdams Peil

  • National
  • Netherlands
  • Coast and tide
  • Inland and Baltic

Dutch national height datum. It is widely used as a reference around the North Sea.

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LAT

Lowest Astronomical Tide

  • International
  • International
  • Coast and tide

Lowest water level that can be predicted from the tide alone, without wind or air pressure. Nautical charts and the sea-level map use it as a common zero.

Same idea elsewhere: SKN, OLW

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HAT

Highest Astronomical Tide

  • International
  • International
  • Coast and tide

Highest water level that can be predicted from the tide alone, without wind or air pressure.

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TAW

Tweede Algemene Waterpassing

  • National
  • Belgium
  • Coast and tide
  • Inland and Baltic

Belgian height datum for land and coastal measurements.

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DVR90

Dansk Vertikal Reference 1990

  • National
  • Denmark
  • Inland and Baltic

Danish national height system for land elevations.

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Statistical values

The same kind of value, named differently at home and abroad. LAT and HAT, the astronomical extremes without weather, are listed with the datums.

Thw

Tide high water

  • National
  • Germany
  • Coast and tide

One high water at a German tide gauge (North Sea and estuaries), not a long-term mean.

Same idea elsewhere: HW

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Tnw

Tide low water

  • National
  • Germany
  • Coast and tide

One low water at a German tide gauge. Also written ThL.

Same idea elsewhere: NW, LW

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MThw

Mean tide high water

  • National
  • Germany
  • Coast and tide

Long-term mean of high waters at a tide gauge. On this map the line is a ten-year mean, drawn dotted in the station chart.

Same idea elsewhere: GHW, MHW

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MTnw

Mean tide low water

  • National
  • Germany
  • Coast and tide

Long-term mean of low waters at a tide gauge. Mean tidal range is MThw minus MTnw.

Same idea elsewhere: GLW, MLW, MNW, MTR

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HHThw

Highest tide high water on record

  • National
  • Germany
  • Coast and tide

The highest high water measured so far at that German tide gauge.

Same idea elsewhere: HHW, HThw

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NNThw

Lowest tide high water on record

  • National
  • Germany
  • Coast and tide

The smallest high water measured so far at that gauge — not the lowest low water.

Same idea elsewhere: NThw, NNW

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HThw

Highest tide high water in a period

  • National
  • Germany
  • Coast and tide

Highest high water in a stated span, for example one year. Not necessarily the all-time maximum.

Same idea elsewhere: HHThw

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NThw

Lowest tide high water in a period

  • National
  • Germany
  • Coast and tide

Smallest high water in a stated span, for example one year.

Same idea elsewhere: NNThw

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NTnw

Lowest tide low water in a period

  • National
  • Germany
  • Coast and tide

Smallest low water in a stated span, for example one year. The low-water counterpart of HThw.

Same idea elsewhere: Tnw, HThw

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MLWS

Mean low water springs

  • International
  • International
  • Coast and tide

Average of the low waters at spring tides, when the tidal range is largest.

Same idea elsewhere: MTnw, MLW

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MTR

Mean tidal range

  • National
  • Germany
  • Coast and tide

Mean rise and fall of the tide: MThw minus MTnw.

Same idea elsewhere: MThw, MTnw

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MW

Mean water level

  • National
  • Germany
  • Inland and Baltic

Long-term mean level on German inland waters and the Baltic, where there is little or no tide.

Same idea elsewhere: MSL

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HW

High water

  • National
  • Germany
  • Netherlands
  • Coast and tide
  • Inland and Baltic

In Germany, a high water on inland waters and the Baltic. In the Netherlands, HW (hoogwater) is the high water of a tide; the German tidal name for that is Thw.

Same idea elsewhere: Thw, GHW

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NW

Low water

  • National
  • Germany
  • Inland and Baltic

A low water on German inland waters and the Baltic. In tidal waters the German name is Tnw.

Same idea elsewhere: Tnw, LW

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MHW

Mean high water

  • National and international
  • Germany
  • International
  • Coast and tide
  • Inland and Baltic

In Germany, the mean of high waters on inland waters and the Baltic. Internationally, the same kind of average of ordinary high waters. In German tidal waters the matching term is MThw; in the Netherlands, GHW.

Same idea elsewhere: MThw, GHW, MHHW

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MNW

Mean low water

  • National
  • Germany
  • Inland and Baltic

Mean of low waters on German inland waters and the Baltic. The international tidal name is MLW; the German tidal name is MTnw.

Same idea elsewhere: MTnw, MLW

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HHW

Highest water level on record

  • National
  • Germany
  • Netherlands
  • Coast and tide
  • Inland and Baltic

Germany: the highest level ever measured. Netherlands: hoogst gemeten waterstand, the same idea.

Same idea elsewhere: HHThw

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NNW

Lowest water level on record

  • National
  • Germany
  • Inland and Baltic

The lowest level ever measured at a German inland or Baltic gauge.

Same idea elsewhere: NNThw, LLW

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HSW

Highest navigational water level

  • National
  • Germany
  • Inland and Baltic

An upper level used as a limit for shipping, not the highest level ever measured.

Same idea elsewhere: NSW, OLW

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NSW

Lowest navigational water level

  • National
  • Germany
  • Inland and Baltic

A lower level used as a limit for shipping and available depth.

Same idea elsewhere: HSW

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LW

Laagwater

  • National
  • Netherlands
  • Coast and tide

One low water in Dutch tidal waters.

Same idea elsewhere: Tnw, NW

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GHW

Gemiddeld hoogwater

  • National
  • Netherlands
  • Coast and tide

Dutch mean high water. The German tidal equivalent is MThw; the international name is MHW.

Same idea elsewhere: MThw, MHW

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GLW

Gemiddeld laagwater

  • National
  • Netherlands
  • Coast and tide

Dutch mean low water. The German tidal equivalent is MTnw; the international name is MLW.

Same idea elsewhere: MTnw, MLW

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LLW

Laagst gemeten waterstand

  • National
  • Netherlands
  • Coast and tide

Lowest water level measured at a Dutch station.

Same idea elsewhere: NNW

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OLW

Overeengekomen laagwater

  • National
  • Netherlands
  • Coast and tide

An agreed low water used in the Netherlands for depth and draught, not the lowest astronomical tide.

Same idea elsewhere: LAT, NSW

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MLW

Mean low water

  • International
  • International
  • Coast and tide

International mean of ordinary low waters. German tidal waters use MTnw; the Netherlands use GLW.

Same idea elsewhere: MTnw, GLW, MNW, MLLW

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MHHW

Mean higher high water

  • International
  • International
  • Coast and tide

Mean of the higher of the two daily high waters, where the tide is mixed and the two highs differ.

Same idea elsewhere: MHW

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MLLW

Mean lower low water

  • International
  • International
  • Coast and tide

Mean of the lower of the two daily low waters, where the tide is mixed and the two lows differ.

Same idea elsewhere: MLW

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MSL

Mean sea level

  • International
  • International
  • Coast and tide
  • Inland and Baltic

A long-term average of the sea surface. It is not a chart datum, and countries do not all compute it the same way.

Same idea elsewhere: MW, NHN

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Official references

The definitions on this page follow these services. They are the places to check a value for navigation or a warning.