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.
Lowest Astronomical Tide (LAT)
The standard chart datum for nautical charts is the Lowest Astronomical Tide (LAT). 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.
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 separation between MSL and LAT depends on the location.
Approximately LAT (ALAT)
For estuaries, the discharge of river water influences water levels to such an extent that LAT is not a suitable chart datum. Instead, a level called Approximately LAT is used. It establishes a gradual change between LAT at Hoek van Holland and the Agreed Low Water of the Waal River at Tiel. All river levels are also expressed in NAP reference level, which allows for conversions.
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 and LAT relate to each other. This is necessary, as port authorities and lockkeepers often give water levels in NAP.
It is also possible to define LAT 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, the LAT 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 LAT level is different at the maritime boundaries of individual North Sea countries. Some hydrographic offices even use a different definition of chart datum.