Coordinate reference systems for depth measurement at sea
Water depth always needs to be referred to as a reference level, also known as a chart datum. In view of the fact that there are many different datums, each nautical chart states the datum used.
Approximate Lowest Astronomical Tide (ALAT)
The standard chart datum for nautical charts is the Approximate Lowest Astronomical Tide (ALAT). This is the lowest possible level based on lunar and solar gravity. Yet, water levels below LAT do occur, due to weather influences, such as high air pressure or a seaward breeze.
As of 1 January 2026, the designation of the reference plane in nautical products of the Dutch part of the North Sea, the Waddenzee, and the Eastern and Western Scheldt is changed. From that moment on Lowest Astronomical Tide (LAT) will be called Approximate Lowest Astronomical Tide (ALAT).
The reference level for both the Lower Rivers region and the Caribbean Netherlands has not changed; these were already displayed according to ALAT.
Mean Sea Level (MSL)
Other applications, such as in the offshore industry and in oceanography, also use a different vertical datum: Mean Sea Level (MSL). The annual tidal publication HP33 illustrates how NAP, MSL and ALAT relate to each other.
Amsterdam Ordnance Datum (NAP)
The national reference level for height on land is the Amsterdam Ordnance Datum, or NAP. NAP is approximately the mean water level for Amsterdam in absence of water motion. Differences with MSL are caused by meteorological and oceanographic influences.
The annual tidal publication HP33 illustrates how NAP, MSL and ALAT relate to each other. This is necessary, as port authorities and lockkeepers often give water levels in NAP.
Geoid separation
It is also possible to define ALAT with respect to the so-called geoid. The geoid is the hypothetical global shape of the surface of the oceans where water flows are only influenced by gravity and unaffected by wind and tides. Delft University of Technology is studying the relationship between these 2 levels for the North Sea.
Height measurement using GPS receivers
GPS receivers are able to establish height. To that end, the level of either MSL or the geoid first needs to be established in relation to the internal reference of GPS receivers (the ellipsoid). Subsequently, ALAT values are subtracted.
The geoid has been globally established as EGM2008 by the American National Geospatial intelligence Agency (NGA). For MSL, the Hydrographic Service recommends the DTU15 realisation of the Danish Technical University (DTU).
North Sea Hydrographic Commission
The North Sea Hydrographic Commission (NSHC) coordinates the determination of chart datums for the North Sea. However, the ALAT level is different at the maritime boundaries of individual North Sea countries. Some hydrographic offices even use a different definition of chart datum.