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I'm trying to load an Ordnance Survey GML file into PostGIS using the ST_GeomFromGML function. The problem is that the spatial reference system that is used in the file, osgb:BNG, is not recognized by PostGIS: 

# SELECT ST_GeomFromGML('<gml:Point srsName="osgb:BNG">
         <gml:coordinates>527347.0,181346.0</gml:coordinates></gml:Point>');
ERROR:  unknown spatial reference system

Is there any workaround? Does "osgb:BNG" have an alternative name that is recognized by PostGIS? Thank you.

EDIT: Thanks to everybody for their answers. I ended up replacing "osgb:BNG" with "EPSG:27700" in the GML file (in the srsName parameter), although the other two options (modifying the spatial_ref_sys table, or using the ST_GeomFromGML function http://postgis.org/docs/ST_GeomFromGML.html with two arguments for PostGIS 2.0+) are perfectly valid too.

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The code block isn't displayed correctly: # SELECT ST_GeomFromGML('<gml:Point srsName="osgb:BNG"><gml:coordinates>527347.0,181346.0</gml:coordinates></gml:Poi‌​nt>'); ERROR: unknown spatial reference system – Orestis Tsinalis Jul 22 '12 at 14:50
add another space before it, so it will get converted to a code block. Now the site thinks you are including html and strips the gml tags. – lynxlynxlynx Jul 22 '12 at 15:25

2 Answers

up vote 3 down vote accepted

You can add this projection to the projection table postgis uses and then refer to it with the new name (97460 or SR-ORG:7460 in this case):

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 97460, 'sr-org', 7460, '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs ', 'PROJCS["OSGB 1936 / British National Grid",GEOGCS["OSGB 1936",DATUM["OSGB_1936",SPHEROID["Airy 1830",6377563.396,299.3249646,AUTHORITY["EPSG","7001"]],TOWGS84[375,-111,431,0,0,0,0],AUTHORITY["EPSG","6277"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4277"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northing",-100000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","27700"]]');

But ouch, what's all this code? I used gdalsrsinfo to get the details of the projection and http://spatialreference.org/ref/sr-org/7460/ to generate the full INSERT command for updating the spatial_ref_sys table. It could be done manually, but this way other people searching for it will be able to skip a step — it didn't include any synonims for this projection either.

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Thanks for the quick answer! What if I simply replaced "osgb:BNG" with "EPSG:27700" http://spatialreference.org/ref/epsg/27700/ in the GML file? Would that work too? – Orestis Tsinalis Jul 22 '12 at 16:00
yes, they are practically identical (last digit rounding error and no TOWGS84 tag). – lynxlynxlynx Jul 22 '12 at 16:05
Many thanks for the help! – Orestis Tsinalis Jul 22 '12 at 16:07
up vote 1 down vote

No need to insert anything special into the spatial_ref_sys table as it already contains the necessary entry for British National Grid. The British National Grid can be referenced using the EPSG code 27700 and I think the srid value in spatial_ref_sys table is the same. So, all you need to do is use the second form of the method and specify the correct srid to use:

geometry ST_GeomFromGML(text geomgml, integer srid)

So, replace text geomgml with the GML data you are getting the geometry from and integer srid with 27700. That should then work for you.

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I'm getting an error: SELECT ST_GeomFromGML('<gml:Point srsName="osgb:BNG"><gml:coordinates>527347.0,181346.0</gml:coordinates></gml:Poi‌​nt>', 27700); ERROR: function st_geomfromgml(unknown, integer) does not exist LINE 1: SELECT ST_GeomFromGML('<gml:Point srsName><gml:coordinates>5... ^ HINT: No function matches the given name and argument types. You might need to add explicit type casts. Any ideas? – Orestis Tsinalis Jul 22 '12 at 16:12
You need at least postgis 2.0 for this to work, the parameter was not available earlier. Instead just change the srsName parameter in the GML. – lynxlynxlynx Jul 22 '12 at 16:14
Cool, thanks, I have PostGIS 1.5.3 installed at the moment. – Orestis Tsinalis Jul 22 '12 at 16:16
Didn't realise that was only available in 2.0! But there is still no need to add the reference as 27700 srid exists in PostGIS 1.5.3. – CHenderson Jul 22 '12 at 16:52

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