Map projections explained
What is OSGB36?
MAGIC uses the British National Grid projection as its default coordinate reference system. The Ordnance Survey British National Grid is a system of geographic grid references used in Great Britain. It uses meters as its unit of measurement and takes the form of a ‘Transverse Mercator’ projection based on the OSGB36 coordinate reference system (Ordnance Survey Great Britain 1936). When using the MAGIC service, unless you specify a different coordinate reference system (e.g. WGS84 or ETRS89) you will be using the British National Grid projection (sometimes referred to as OSGB36). For more information on how to use the OS British National Grid follow the link. For further information on the coordinate systems of Great Britain click here.
What is WGS84?
The World Geodetic System 1984 is a geographic coordinate reference system using degrees as its unit of measurement, similar to ETRS89. Unlike OS British National Grid, WGS84 covers the entire globe, with positional coordinates given in the form of degrees of latitude and longitude. The global coverage of WGS84 (when compared with British National Grid) lends itself to the display and mapping of geographic information for areas located further away from the UK coastline, such as offshore marine data. For further information on the coordinate systems of Great Britain, including WGS84, click here.
What is ETRS89?
The European Terrestrial Reference System 1989 is a geographic coordinate reference system using degrees as its unit of measurement, similar to WGS84. ETRS89 covers all of Europe, unlike OS British National Grid which covers only Great Britain, or WGS84 which covers the entire globe. When using ETRS89 positional coordinates given in the form of degrees of latitude and longitude. The European coverage of ETRS89 (when compared with British National Grid) lends itself to the display and mapping of geographic information for areas located further away from the UK coastline, such as offshore marine data. The importance of ETRS89 to us in Britain is that this is the datum used for all Ordnance Survey GPS positioning. It is a convenient system because we can forget about the tectonic effects apparent in WGS84 (which do not concern us in British mapping), while still being able easily to convert these coordinates to WGS84 when required. For further information on the coordinate systems of Great Britain, including ETRS89, click here.
What is the difference between WGS84 and ETRS89?
Both WGS84 and ETRS89 are geographic coordinate reference systems that use degrees as their unit of measurement, with positional coordinates given as degrees of latitude and longitude. They are also based on the same underlying shape (the GRS1980 spheroid or ellipsoid). The main difference between WGS84 and ETRS89 is the fact that WGS84 covers the globe which is subject to tectonic movement, whereas ETRS89 is fixed to cover the Eurasian tectonic plate. As a result, the exact position of WGS84 fluctuates and changes over time, whereas ETRS89 is an ‘earth centred, earth fixed’ reference system that is not subject to change caused by continental drift (i.e. tectonic plate movement). In 2000, the difference between WGS84 coordinates of a point and the ETRS89 coordinates was about 25cm, increasing by about 2.5 cm per year. For this reason, the European Terrestrial Reference System 1989 (ETRS89) is used as the standard precise GPS coordinate system throughout Europe. For further information on the coordinate systems of Great Britain, including WGS84 and ETRS89, click here.