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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.

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    SRID = Spatial Reference ID. It refers to a large collection of Coordinate reference systems used around the world. Each SRID contains the specifications for projecting features on the curved surface of the earth to a flat paper or computer screen. It's certainly possible to have a feature in one SRID overlap a feature in another SRID. But you will usually see that either the shape, length, directions will be different when you display. – Micha May 24 '12 at 10:10
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    You question should actually be "What is a spatial reference system?" SRID is just a number identifying a particular SRS. – Igor Brejc May 24 '12 at 18:31
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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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    and if you encounter a srid without the authority, you can usually assume its EPSG (this is the case for PostGIS and ogr/gdal at least) – atlefren May 24 '12 at 14:10
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Wikipedia:

A Spatial Reference System Identifier (SRID) is a unique value used to unambiguously identify projected, unprojected, and local spatial coordinate system definitions.

In other words, a system of indentifiers for spatial reference system, so that i can tell a GIS application (like postGIS) that my data is in WGS84 Geographic coordinates by saying "EPSG:4326"

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In Layman's Language, a spatial reference system describes how to convert spatial coordinates into real world locations.

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I inserted a point in PostGIS, with SRID 300 and then inserted the same point with SRID 350, I found the two points showing the same place, in QGIS viewer.

There is no SRID 300 or 350. QGIS just defaulted to the same CRS for both points. That's why they showed up in the same place.

QGIS comes with a CRS database which it uses to determine how to project the input coordinates. The SRID is the identifier of one entry of this database. If the SRID cannot be found, QGIS picks the default CRS (defined in the program options).

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