Horizontal Datum

Out of the numerous systems used, about 170 reference systems are important for marine applications. Although the reference system of most sea charts is known, there are also some cases where it is unknown. Knowledge of the local reference system was not relevant for a couple of centuries, as the inaccuracies of fixes were larger than the differences between reference systems. With the advent of terrestrial radio positioning systems like Decca, Omega or Loran C the accuracy of fixes improved considerably, so that it became necessary to use identical reference systems for charts and transmitter stations . Initially hyperbola were printed on the chart as the receivers were not able to calculate and display geographical coordinates. As modern receivers now can directly provide these coordinates the imprint of hyperbola is superfluous.

The reference system WGS 84 has been adopted as the compulsory horizontal datum for ENCs. Hydrographic offices are working intensively to convert charts to WGS 84 or an WGS 84 compatible datum.

The usage of ECDIS is limited in areas which are not covered by ENCs. Under these circumstances ECDIS systems can be used for navigation in RCDS mode. Here datum conversion problems occur; mariners have to determine the difference between the WGS 84 related position from the positioning system and its equivalent related to the reference system of the chart.

If charts with different horizontal datum are used in RCDS mode the conversion between the WGS 84 position of the GPS receiver and the relevant chart datum is necessary. The horizontal datum is described under the remark “Positions” on each raster chart. Most GPS receivers offer the possibility to convert between horizontal datums.

ECDIS systems should give an alarm in both ECDIS or RCDS mode if there are differences between the GPS horizontal datum and the horizontal datum of the chart.