I am trying to understand the concept behind the Spatial Reference System.
Why is it used and what is so specific about the SRID number?
I went through many definitions and almost all went over my head.
I thought, it was a reference to draw maps from a real world place, expressed in map, but then I inserted a point in PostGIS, with SRID 300 and the same point with SRID 350. I found the two points showing the same place, in QGIS viewer.
Answer
Following on from the answer given by @atlefren, an SRID usually is made of two components, an "authority" and an identifier. The authority is just the name of the organization that catalogues the identifiers. The most common authority you'll see is EPSG, which sands for the "European Petroleum Survey Group", and they have a comprehensive database of coordinate systems, datums, ellipsoids, projections, and so on, each with a code.
There are other authorities however, each with their own identifiers that may be different from, yet represent the same information, as an EPSG code. IGNF is one such authority supported by Quantum GIS for instance.
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