Automatically swap X/Y coordinates in GML using urn: srs syntax

[xificurk]

ST_AsGML always outputs coordinates in order X,Y.

⇒ SELECT st_asgml(st_setsrid(st_makepoint(15, 50), 4326), 15, 0);

<gml:Point srsName="EPSG:4326"><gml:coordinates>15,50</gml:coordinates></gml:Point>

⇒ SELECT st_asgml(st_setsrid(st_makepoint(15, 50), 4326), 15, 1);

<gml:Point srsName="urn:ogc:def:crs:EPSG::4326"><gml:coordinates>15,50</gml:coordinates></gml:Point>

⇒ SELECT st_asgml(st_setsrid(st_makepoint(15, 50), 4326), 15, 16);

<gml:Point srsName="EPSG:4326"><gml:coordinates>15,50</gml:coordinates></gml:Point>

On the other hand, ST_GeomFromGML swaps coordinates, if given srsName in format like 'urn:ogc:def:crs:EPSG::4326' instead of simple 'EPSG:4326'. Although the examples use 4326 SRS, it does not depend on this exact setting.

⇒ SELECT st_astext(st_geomfromgml('<gml:Point xmlns:gml="http://www.opengis.net/gml/3.2" srsName="urn:ogc:def:crs:EPSG::4326"><gml:coordinates>15,50</gml:coordinates></gml:Point>'));

POINT(50 15)

ruian⇒ SELECT st_astext(st_geomfromgml('<gml:Point xmlns:gml="http://www.opengis.net/gml/3.2" srsName="EPSG:4326"><gml:coordinates>15,50</gml:coordinates></gml:Point>'));

POINT(15 50)

Tested on 64bit Gentoo setup with fresh postgis install on top of PostgreSQL 9.1.5. POSTGIS="2.0.1 r9979" GEOS="3.3.3-CAPI-1.7.4" PROJ="Rel. 4.8.0, 6 March 2012" LIBXML="2.8.0"

[strk]

You need to add 16 to the st_asgml options to get the swap. Indeed I'm not sure why GeomFromGML swaps based on presence of the "url" prefix.

Is is a GML prescription that "urn"-prefixed srsName imply a lat/long order ?

[pramsey]

Yes, that's exactly what the guidance from OGC is: urn-formed srsNames should obey the "new order of things", epsg formed srsNames can do whatever you like (gah!)

[pramsey]

I think auto-swapping constitutes a new feature, since the desired behaviour here is already available if the correct parameter is provided.

[xificurk]

Well, the first point was that the option 16 (at least on my build) does not swap the coordinates. The second point was that PostGIS swaps the coordinates when reading GML with urn-formed srsName, but NOT when writing (option 1) GML with urn-formed srsName.

[pramsey]

The doco says the 16 option is only useful for GML3 output, and I did find I had to request GML3 in order to see the behaviour, but see it I did:

select st_asgml(3, 'SRID=4326;POINT(10 20)'::geometry,8,16);

I think this still qualifies as an enhancement request, not a defect.

[robe]

Milestone renamed