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The background to this question is in Understanding difference between Coordinate System and Projection?

There @mdsumner commented:

WGS84 is not a cs, it's a datum.

On a couple of places online I have found: "WGS84 geographic coordinate system" mentioned. Is this an incorrect terminology?

WGS84" is a datum, but "WGS84" is also a geographic coordinate system defined by the "WGS84" datum?

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  • Thank you @nmtoken. I am not sure how that topic answers the exact question from this one. The reply by "underdark" talks about what "WGS84 comprises of". The reply by "Dan S." talks about the differences between "EPSG 4326 spatial reference" and "WGS 84" term. But can "WGS84" be a term used to name both the geographic coordinate system, and a datum? – marco Mar 2 '16 at 18:24
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    Doesn't WGS84 consist of a coordinate system, a datum, and a geoid? – Barbarossa Mar 2 '16 at 18:45
  • @Barbarossa: if "WGS84" is consisted of those three, then what exactly is the the "WGS84"? – marco Mar 2 '16 at 19:05
  • A geodetic reference system that attempts to model the size, shape, gravity, and geomagnetic fields of Earth. – Barbarossa Mar 2 '16 at 19:15
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    I don't think there's a widespread agreement on whether there's any difference between a "geodetic reference systems" and a "geographic coordinate system." – mkennedy Mar 2 '16 at 20:14
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Both. It's also a geographic coordinate reference system and an ellipsoid, and sometimes a spheroid, and a bunch of other related/similar terms that are used in the fields of geodesy, geomatics, geography, etc.

The US military started the process by using the same name "World Geodetic System 84" for an earth gravitational model, an ellipsoid, and a geodetic datum.

Since then, terminology has moved on, with ISO and OGC using coordinate reference system that contains a datum plus a "coordinate system." For them, a coordinate system is the axes order and direction plus names and abbrevations and the unit.

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  • Thank you @mkennedy. Just to see if I understood you on this: A "coordinate reference system" is a "coordinate system" (geographic or projected) plus a datum? – marco Mar 2 '16 at 20:34
  • Yes, according to ISO (International Standards Organisation). – mkennedy Mar 2 '16 at 20:49
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I think it's sloppy terminology to use it as a cs, it is not technically one just because it's used that way. If I hear 'GDA94', I have to ask what UTM zone is implied, or if the datum is really what was meant. The sloppiness is reflected in interchange formats where it is implied rather than recorded explicitly. Practitioners can guess solutions, but beginners are understandably confused.

It's a vicious cycle.

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  • Thank you for the reply @mdsumner. What makes a "WGS84 geographic coordinate system" a sloppy term? Doesn't it immediately imply that this geographic coordinate system is defined by a WGS84 datum? – marco Mar 2 '16 at 22:29
  • Ok, to label the WGS84 datum, the term: "WGS84 the datum" needs to be used. And to label the geographic coordinate system, which is defined by the "WGS84 the datum", one would need to use, which term @mdsumner ? – marco Mar 3 '16 at 0:01

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