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A WFS server I was using offered boundaries data in the following EPSG codes:

  • EPSG:6.9:27700
  • EPSG:6.9:4326

...defined in the XML giveCapabilities spec like this:

<DefaultCRS>urn:ogc:def:crs:EPSG:6.9:27700</DefaultCRS>
<OtherSRS>urn:ogc:def:crs:EPSG:6.9:4326</OtherSRS>

As I understand it, EPSG 4326 is the "standard" latitude and longitude ellipsoid used by GPS systems, and EPSG:27700 is the older British system behind the Ordnance Survey National Grid.

But what's the 6.9: mean?

I tried inputting it into ogr2ogr and it just got confused:

{"errors":["ERROR 6: EPSG PCS/GCS code 6 not found in EPSG support files. Is this a valid","EPSG coordinate system?","Failed to process SRS definition: EPSG:6.9:4326",""]}

...and I can't find any reference to what the component parts of an EPSG code mean, only lists of 'standard' EPSG codes that don't have these initial numbers.

  • Version of the database. – user30184 Jul 24 '16 at 19:27
  • There's nothing 'old' about British National Grid, it's a valid current projection system. – nmtoken Jul 25 '16 at 9:02
  • @nmtoken I didn't mean "old" as in outdated or invalid; it is old in that it was defined in 1936 based on models going back to 1830 and has been used in UK cartography since well before EPSG:4326 (1984) or its predecessors: it predates the "WGS" efforts to devise an international standard which started in the 1950s. Old doesn't imply invalid. – user568458 Jul 25 '16 at 9:19
  • Hoisted by my own petard :) I agree it is old in its definition. My comment probably should have been that using the word 'old' here doesn't add anything to the question (it doesn't matter that its old, as it's still valid and in active use). – nmtoken Jul 25 '16 at 17:15
  • @nmtoken I'll go with "older"... :-) – user568458 Jul 25 '16 at 17:26
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Note: this is part of an answer already given here, but it seems fitting to post it again.

The 6.9 means that the SRS 4326 specified in version 6.9 of the EPSG database, which you can find here.

5

The question has an accepted answer, but for completeness, the OGC document

Definition identifier URNs in OGC namespace, 07-092r1, 2007

Tells us that:

To identify the definition of a single object, the URN shall have the form:

urn:ogc:def:objectType:authority:version:code

the URN value for an anyURI that references one object in the European Petroleum Survey Group (EPSG) database shall have the form:

urn:ogc:def:objectType:EPSG:version:code

So 6.9 here is the version of the authority definition.

Note that:

The urn, ogc, def, and six: parts of this URN are fixed.

'This' references the URN for the definition of a single object, that means that urns without the optional version should have a double colon in them like:

urn:ogc:def:crs:EPSG::27700

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