OGR Python API – Getting EPSG Value from OSR SpatialReference Class

ogrpostgispython

When reading a layer from a OGR PostGIS connection I can get the SpatialReference of the layer, but is it possible to get the EPSG value? Is there any documentation on this?

For example:

lyr = conn.GetLayerByName(tbl) # Where conn is OGR PG connection
srs = ly.GetSpatialRef()
print srs

Returns:

PROJCS["OSGB 1936 / British National Grid",
GEOGCS["OSGB 1936",
    DATUM["OSGB_1936",
        SPHEROID["Airy 1830",6377563.396,299.3249646,
            AUTHORITY["EPSG","7001"]],
        AUTHORITY["EPSG","6277"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4277"]],
UNIT["metre",1,
    AUTHORITY["EPSG","9001"]],
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],
AUTHORITY["EPSG","27700"],
AXIS["Easting",EAST],
AXIS["Northing",NORTH]]

So how do I get the EPSG value for the projection? E.g.:

srs.GetEPSG()
print srs
27700

I've tried srs.GetAttrValue('AUTHORITY'), but this just returns 'EPSG'.

Best Answer

It's a bit buried, but there is a second parameter to GetAttrValue() which returns the value at that ordinal. So I can do:

In [1]: import osgeo.osr as osr

In [2]: srs = osr.SpatialReference()

In [3]: srs.SetFromUserInput("EPSG:27700")
Out[3]: 0

In [4]: print srs
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"]]

In [5]: srs.GetAttrValue("AUTHORITY", 0)
Out[5]: 'EPSG'

In [6]: srs.GetAttrValue("AUTHORITY", 1)
Out[6]: '27700'

After a bit of playing about, I've found you can get the value for any parameter using a pipe | as a path separator:

In [12]: srs.GetAttrValue("PRIMEM|AUTHORITY", 1)
Out[12]: '8901'

Which may be of use in finding the geographic coordinate system of a projected CS:

In [13]: srs.GetAttrValue("PROJCS|GEOGCS|AUTHORITY", 1)
Out[13]: '4277'
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