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
2 Answers
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.
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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?– marcoFeb 26, 2016 at 19:08
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1Whether 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.– JylerFeb 26, 2016 at 19:41
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Thank you @Jyler. What is the difference between a projected coordinate system and projected CRS (coordinate reference system?)?– marcoFeb 26, 2016 at 19:45
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1They are the same. Please edit your question to include your questions from the comments and I'll edit my response to address them all.– JylerFeb 26, 2016 at 19:57
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1Area 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.– mdsumnerFeb 26, 2016 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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1A spatial data layer doesn't have to have geographic coordinate system from which data is projected, that's just one way of doing it– nmtokenJan 18, 2020 at 13:04
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 nameWorld Geodetic System 1984.WGS84is only the alias of the ellipsoid.