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We know what SRID 4326 means and what 3857 means but I do not know what these numbers mean. What is the minimum and maximum number for these SRIDs?

I know it is a standard but how did they select 3857 for Mercator, or 4326, or 900913...?

In fact I think this is not an ordinary list of numbers, it could be some kind of binary number or things like that. I have searched but could not find any answers.

  • You can download the full EPSG database from epsg.org/EPSGDataset/DownloadDataset.aspx. Why 3857 is not relevant question, perhaps they had already 3856 and 3857 was free, but there may be other reasons. What is relevant is that 3857 as CRS id is unique and what it means is thoroughly defined in the EPSG database. – user30184 Jun 26 '17 at 7:14
  • @user30184 unique for a given namespace, Not globally unique. – BradHards Jun 26 '17 at 7:17
  • Right. I meant that even within the EPSG namespace codes are unique only within the type. EPSG:4326 has two meanings: Geodetic CRS and Area of Use in United States (USA) - Wisconsin - Douglas county epsg-registry.org/… – user30184 Jun 26 '17 at 7:22
  • @user30184; OK, got it now. – BradHards Jun 26 '17 at 9:24
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They're mostly from the EPSG - https://www.epsg-registry.org/

To be unique, you have to specify where they are from (basically a namespace - EPSG:4326, not just 4326). However most people will interpret them as EPSG if not specified.

Originally, the EPSG registry was maintained in a Microsoft Access database and the well-known IDs (WKIDs) were assigned by hand. Thus you can see ranges like the WGS 84 UTM zones: 326 + zone number for the North zones, and 327 + zone number for the South zones. "326" is from WGS 84, "4326". There are similar sets for NAD83, NAD27, ED50. If new zones were added, they usually received noncontiguous WKIDs.

When the registry migrated to a DBMS, the WKID assignments became random. It was not planned that the original "web Mercator" WKID was 3785 and its replacement was 3857.

There are other namespaces - spatialreference.org provides ESRI codes, IAU2000 codes for other astronomical bodies and several thousand of its own, many of which are junk or user tests. spatialreference.org does not have every namespace though - spatialite supports some extra codes, including those defined by gfoss.it; and NGA has some codes defined too - see NGA.STND.0051_2.0_GEOPKG for their GeoPackage related codes.

EPSG codes are unique within a range of 1024 to 32766 inclusive. Codes for proprietary data range from 50000 and up, inclusive. There are several thousand codes defined and published by the EPSG - you can download the full set from http://www.epsg.org/.

So 900913 isn't an official code from EPSG - its from that proprietary range. The specific number is just google as numbers (g => 9, o => 0, and so on).

Also, 3857 isn't really Mercator, but rather a spherical approximation (sometimes "web-Mercator").

  • Thanks for your answer, So we can only have 31742 ESGPs, right? – Majid Hojati Jun 26 '17 at 7:10
  • within the unique namepace = (32766-1024 + 1). Note that definitions for what a single code means changes by version. – BradHards Jun 26 '17 at 7:16
  • epsg.org: The EPSG Geodetic Parameter Dataset is maintained by the Geodesy Subcommittee of the IOGP Geomatics Committee. If they want to add new codes they can. They can also change the structure of the database if there is a need for more codes than the current database schema can take. – user30184 Jun 26 '17 at 7:17

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