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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 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 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 at 16:08
up vote 7 down vote accepted

Both examples are coordinate systems. The difference is that WGS 1984 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.

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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 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 at 19:41
    
Thank you @Jyler. What is the difference between a projected coordinate system and projected CRS (coordinate reference system?)? – marco Feb 26 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 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 at 21:26

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