Can somebody please explain what is the difference between the Coordinate system (WGS 84 for example) and a Projection (Universal Transverse Mercator for example)?

What is the difference between a projected coordinate system and projected CRS

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    WGS84 is not a cs, it's a datum. Loose terminology is rife here. – mdsumner Feb 26 '16 at 21:23
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    This question is very nearly the same as the one in the highly-voted thread at gis.stackexchange.com/questions/664 . – whuber Feb 27 '16 at 19:19
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    @mdsumner: Doesn't "WGS84" represent both? "WGS84" is a datum, but "WGS84" is also a geographic coordinate system defined by the "WGS84" datum? – marco Mar 1 '16 at 16:08
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    In the QGISS Project Coordinate Reference System selection dialog, Under "Coordinate reference systems of the world" WGS 84 is included as a selection. If you select it, it displays "Selected CRS: WGS 84". So there is some evidence that WGS 84 is the name of a CRS. – That60sKid May 28 '18 at 16:45
  • @marco Just for a laugh. WGS 84 (that spelling) is the name of an ellipsoid EPSG::7030, the name of a projection EPSG::4326, and the alias of a datum EPSG::6326 official name World Geodetic System 1984. WGS84 is only the alias of the ellipsoid. – nmtoken Jan 18 '20 at 12:57

Both examples are coordinate systems. The difference is that WGS 84 is a geographic coordinate system, and UTM is a projected coordinate system. Geographic coordinate systems are based on a spheroid and utilize angular units (degrees). Projected coordinate systems are based on a plane (the spheroid projected onto a 2D surface) and utilize linear units (feet, meters, etc.).

More here: Difference between Geographic and Projected coordinate systems?

To answer your second question, a coordinate system (whether geographic or projected) and a coordinate reference system refer to the same thing.

  • Thank you @Jyler. Can it be said that a large number of vector and raster GIS files "are" in geographic coordinate system? And in order to use them, we need to project them to some projected coordinate system? – marco Feb 26 '16 at 19:08
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    Whether or not you need to project (or transform when going from geographic to projected CRS) depends on what you want to do. Linear and areal measurements should be measured in a projected CRS. Both have their pros/cons when it comes to cartographic production, such as maintaining size/distance/angular relationships. GIS data can be stored in either depending on how the data are used. – Jyler Feb 26 '16 at 19:41
  • Thank you @Jyler. What is the difference between a projected coordinate system and projected CRS (coordinate reference system?)? – marco Feb 26 '16 at 19:45
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    They are the same. Please edit your question to include your questions from the comments and I'll edit my response to address them all. – Jyler Feb 26 '16 at 19:57
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    Area and distance can only be sensibly calculated in a projection if/where the projection properties support it. They rarely do for distance, and often don't for area. – mdsumner Feb 26 '16 at 21:26

A spatial data layer has to have a coordinate reference system (CRS). This CRS has to have a geographic coordinate system (GCS) that is composed of a latitude and longitude(degrees) which represents the real places on the earth. You can derive the Projected Coordinate System(PCS) from a geographic coordinate system.

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    A spatial data layer doesn't have to have geographic coordinate system from which data is projected, that's just one way of doing it – nmtoken Jan 18 '20 at 13:04

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