=== Q: What is the difference between an ellipsoid, a geoid, a datum and a projection ===

'''A:'''[[BR]]
These terms tend to get used almost interchangeably it seems sometimes (not helped by the fact sometimes the same name is used for more than one), but they are actually different. All are used for mapping.

=== Ellipsoids (or spheroids) ===

This defines an approximation for the shape of the Earth (or other planet you're measuring). You need to know this to know where you are on the planet, but you need a datum to know where you are (it's no good to try and work out where on the ellipsoid London is if you've defined the ellipsoid on the Moon, for example). It's a regular shape (3D ellipse, sphere or just to be confusing occasionally something more like a pear) defined using equations.

Another way of approximating the shape of the Earth is using a geoid. This defines a surface along which gravity is equal, and tends to be what was determined with surveying techniques prior to satellite navigation (because it's what you get if you hold a plumb line). These are still used for some applications.

Examples of ellipsoids include:
 * [http://en.wikipedia.org/wiki/GRS_80 GRS80] Used as part of the GPS system
 * [http://en.wikipedia.org/wiki/World_Geodetic_System WGS84] Note this is as distinct from the datum of the same name - the ellipsoid used is very similar to GRS80.
 * The Airy spheroid, used with the OSGB36 datum in the previous definition of the British National Grid projection

Examples of geoids include [http://en.wikipedia.org/wiki/EGM96 EGM96]

=== Datums ===

These use one or more known tie points to match a particular ellipsoid to a known point on (or close to) the surface of the Earth. This then allows you to know where everywhere else on the Earth is using that ellipsoid and relative to the matching points. A particular datum therefore implies use of a particular ellipsoid.

Examples of datums include:
 * [http://en.wikipedia.org/wiki/World_Geodetic_System WGS84] Note this is as distinct from the ellipsoid of the same name (and usually when people refer to WGS84 they mean the datum, not just the ellipsoid).
 * OSGB36 - See link below for British National Grid projection for details on this.
 * [http://en.wikipedia.org/wiki/European_Terrestrial_Reference_System_1989 ETRS89]

=== Projections ===

These define a method of "projecting" a (roughly) spherical surface onto a 2D plane (ie a map). Generally there are particular equations and starting information required to do this. Often a particular projection will normally be used with a particular datum (for instance UTM is often used with the WGS84 datum, and when BNG was originally defined it was associated with the OSGB36 datum), but there is no requirement for this - a map of any projection can be generated using any datum.

Examples of projections include:
 * [http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system UTM]
 * [http://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid British National Grid]

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Related articles:
 * [wiki:FAQ/ostn02 If I load my ARSF data with a GPS dataset in a GIS, they don't match. Why not?] Otherwise known as, why is British National Grid projection not where it should be, and what is OSTN02?