Changes between Initial Version and Version 1 of FAQ/projections

Jan 24, 2011 4:37:17 PM (9 years ago)



  • FAQ/projections

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     1=== What is the difference between an ellipsoid, a geoid, a datum and a projection ===
     3These 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.
     5=== Ellipsoids (or spheroids) ===
     7This 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.
     9Another 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.
     11Examples of ellipsoids include:
     12 * [ GRS80] Used as part of the GPS system
     13 * [ WGS84] Note this is as distinct from the datum of the same name - the ellipsoid used is very similar to GRS80.
     14 * The Airy spheroid, used with the OSGB36 datum in the previous definition of the British National Grid projection
     16Examples of geoids include [ EGM96]
     18=== Datums ===
     20These 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.
     22Examples of datums include:
     23 * [ 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).
     24 * OSGB36 - See link below for British National Grid projection for details on this.
     25 * [ ETRS89]
     27=== Projections ===
     29These 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.
     31Examples of projections include:
     32 * [ UTM]
     33 * [ British National Grid]
     35[wiki:FAQ Back to FAQ]
     39Related articles:
     40 * [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?