ANNEX
GUIDANCE ON CHART DATUMS AND THE ACCURACY OF POSITIONS ON CHARTS
1 Many different definitions of a horizontal datum (also known as geodetic datum) exist. However, a practical working definition in use is:
"A horizontal datum is a reference system for specifying positions on the Earth's surface. Each datum is associated with a particular reference spheroid that can be different in size, orientation and relative position from the spheroids associated with other horizontal datums. Positions referred to different datums can differ by several hundred metres."
2 The practical result is that a given geographical position, not associated with a specific datum, could refer to different physical objects. In other words, a physical object can have as many geographical positions as there are datums. For example, South Foreland Lighthouse, United Kingdom, has the following positions:
GEOGRAPHICAL POSITION HORIZONTAL DATUM
51°08.39' N 01°22.37' E
referred to OSGB(36) Datum (the local datum for the United Kingdom)
51°08.47' N 01°22.35' E
referred to European (1950) Datum (the continental datum)
51°08.42' N 01°22.27' E
referred to World Geodetic System 1984 (WGS84) Datum (the world-wide datum used by Global Positioning System (GPS))
3 Most charts are not yet referred to WGS84 Datum. This means that, in those cases, positions obtained from satellite navigation receivers will not be directly compatible with the chart and must not be used without adjustment. Hydrographic offices are attempting to refer as many new charts as possible to WGS84, but there remain many areas of the world where information does not exist to enable the transformation to be performed.
4 When known, the horizontal datum of the chart is usually named in the chart title albeit, on its own, this information is of limited benefit to the mariner. Since 1982 many hydrographic offices have been adding "Satellite-Derived Positions" notes (usually situated close to the title) when charts have been revised. This note provides a latitude and longitude adjustment to be applied to positions obtained directly from satellite navigation systems (such as GPS) to make them compatible with the horizontal datum of the chart.
