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Coordinate Reference System
The Coordinate Reference System is used to define how geographic data is to be interpreted as a location on the earth. A CRS is required for all spatial data, in a similar manner the a unit (like "Meter" or "Feet") is required when measuring length.
Most coordinate reference systems start with an understanding of how the earth is shaped and provide a coordinate relative to this shape. Coordinates are expressed as measurements along two or more axis. The units for this measurement (and indeed the order of the axis) are defined as part of the Coordinate Reference System.
A coordinate reference system may also include Projection information, define a valid range for values and so on.
The European Petroleum Standards Group was formed in 1986 to collection the definition of Coordinate Reference Systems. uDIg makes use of "EPSG" codes where possible as a short hand notation for common coordinate reference systems.
