Coordinate reference systems for positioning at sea

Horizontal positions at sea are often expressed as latitude and longitude. They are based on coordinate reference systems, also known as geodetic datums. In Europe, the principal systems for such coordinates are: WGS84, ETRS89 and ITRS. They differ in their respective level of accuracy. Sometimes, a conversion (transformation) of the coordinates is necessary to express the coordinates in another geodetic datum.

WGS84: the standard choice at sea

For navigation at sea, WGS84 is the global standard. Modern nautical charts are published in this system.

The American National Geospatial intelligence Agency (NGA) is responsible for WGS84.

ETRS89: main datum in Europe

On the European continent, ETRS89 is the standard choice. People who use DGPS with differential stations on the European continent, obtains positions in ETRS89. Coordinates in ETRS89 are closely related to the Dutch national RD system. At sea, RD use is mostly limited to the coastal zone.

The EUREF-subcommittee of the International Association of Geodesy (IAG) manages ETRS89. 

ITRS: the most accurate worldwide system

Like WGS84, ITRS is a global system. It does not take continental drift into account. As a result, ITRS coordinates change a few centimetres a year in the Netherlands. The most recent updates of ITRS are ITRF2005 and ITRF2008. The ITRS is used for systems such as EGNOS which help GPS receivers to reach a higher level of accuracy.

The International Earth Rotation and Reference Systems Service (IERS) is responsible for the ITRS.

ED50: outdated but not fully gone

The first common geodetic datum for Europe was the ED50. This system is less accurate, and is not used frequently anymore. Older legal documents, including most maritime boundary treaties between the Netherlands and neighbouring countries, still use the ED50. Conversion to WGS84 is not straightforward, because a selection has to be made of several different sets of transformation parameters.

Local coordinate systems Caribbean

The 6 islands of the Caribbean part of the Kingdom each have their own local coordinate reference system. Transversal Mercator projections for each island allow for the expression of horizontal coordinates in metres. Local coordinates can be converted to WGS84, using the parameters published in the file ‘Transformation parameters for the islands of the Caribbean part of the Kingdom’.