[GeoSciML] URN or URL encoding of srsName EPSG codes in GeoSciML [SEC=UNCLASSIFIED]

Oliver.Raymond at ga.gov.au Oliver.Raymond at ga.gov.au
Thu Jun 25 02:22:40 EDT 2015

Dear GeoSciML folk,

During our recent teleconference, I asked a question for which no-one could give an authoritative answer at the time: "If we are using HTTP-URI (eg, URL's) to identify everything in our GeoSciML WFS services, what is the appropriate way to identify an EPSG code? It appears that EPSG codes are the last bastion of URN's. Is there a standard HTTP-URI way to identify an EPSG code?"

I went looking for an answer in the OGC documentation, and found this in the WFS v2 specification (dated November 2010): srsName parameter

The optional srsName attribute may be used to assert a specific WFS-supported CRS transformation to be applied to the geometries of the features returned in a response document.
The value of the srsName parameter may be the wfs:DefaultCRS or any of the wfs:OtherCRS values listed for the feature type in a server's capabilities document (see 8.3.3). If no srsName value is supplied, then the feature geometries shall be encoded in the response document using the advertised wfs:DefaultCRS value.
This attribute has no meaning for feature types with no spatial properties and shall be ignored.
Servers that advertise more than one wfs:OtherCRS value in their capabilities document (see 8.3.3) shall be able to transform between the CRS used to store features and any CRS requested using the srsName attribute.
Servers that implement this International Standard shall be able to process srsName attribute values using the following format model:
urn:ogc:def:objectType:authority:version:<EPSG code> (see OGC 07-092r2)
In this format model, objectType shall have the value of "crs", authority shall have the value "crs" and the value <EPSG Code> is a placeholder for the actual EPSG code value.
EXAMPLE srsName="urn:ogc:def:crs:EPSG::26986".

This says that if you are going to use WFS v2, then you should use the URN pattern for identification of an srsName EPSG code.  The OGC 07-092r2 document mentioned above also notes that EPSG URN's are resolvable via the www.epsg-registry.org<http://www.epsg-registry.org> registry (eg; http://www.epsg-registry.org/export.htm?gml=urn:ogc:def:crs:EPSG::4283).

So I guess that means that we stick with URN's for EPSG srsNames (eg; urn:ogc:def:crs:EPSG::4283), and URL's for everything else (eg; vocabulary terms).


Ollie Raymond
Senior Geologist  - Information Management  |  GEOSCIENCE AUSTRALIA<http://www.ga.gov.au/>

IUGS Commission for the Management and Application of Geoscience Information<http://www.cgi-iugs.org/>

Acting Chair, GeoSciML Standards Working Group
Open Geospatial Consortium<http://www.opengeospatial.org/projects/groups/geoscimlswg>

Phone:  +61 2 6249 9575    Fax:  +61 2 6249 9971
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