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I have been able to parse a GML file and fetch the polygons I wanted. They can be plotted ok when plotted on their own. I have them stored in a GeoPandas data frame, but I am struggling to set the proper CRS to be able to use this data along with other data frames in a consistent manner.

Each feature is like this

<omi:OMIFeature>
<omi:title>MyTitle</omi:title>
<omi:description>Descr</omi:description>
<omi:linkzona></omi:linkzona>
<gml:MultiSurface srsName="GEOPOIUTM32.5" srsDimension="2">
<gml:surfaceMember>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:pos>520381.7 4648450.2</gml:pos>
<gml:pos>520435.9 4648293.8</gml:pos>
<gml:pos>520450.9 4648243.6</gml:pos>
... more positions like this

The specification of the CRS seems to be a custom one named GEOPOIUTM32.5

I did find a dictionary that seems to describe in full this coordinates reference system, but I am afraid I am not able to use this info. What info should I pick here and use it to set the CRS of this data frame with set_crs()? (I have tried 6402, 7030,4326 as EPSG but they give oddly placed results or not plot anything at all. I figured that 32633 gives x coordinate off by a huge amount, but y is nearly right).

<gml:Dictionary gml:id="OMI_Dictionary">
    <gml:description>Dictionary description</gml:description>
    <gml:name>Dictionary name</gml:name>
    <gml:dictionaryEntry>
        <gml:ProjectedCRS gml:id="ogrcrs104">
            <gml:srsName>GEOPOI / UTM zone 32.5</gml:srsName>
            <gml:srsID>
                <gml:name>GEOPOIUTM32.5</gml:name>
            </gml:srsID>
            <gml:baseCRS>
                <gml:GeographicCRS gml:id="ogrcrs105">
                    <gml:srsName>WGS 84</gml:srsName>
                    <gml:srsID>
                        <gml:name>4326</gml:name>
                    </gml:srsID>
                    <gml:usesEllipsoidalCS>
                        <gml:EllipsoidalCS gml:id="ogrcrs106">
                            <gml:csName>ellipsoidal</gml:csName>
                            <gml:csID>
                                <gml:name>6402</gml:name>
                            </gml:csID>
                            <gml:usesAxis>
                                <gml:CoordinateSystemAxis gml:id="ogrcrs107" gml:uom="urn:ogc:def:uom:EPSG::9102">
                                    <gml:name>Geodetic latitude</gml:name>
                                    <gml:axisID>
                                        <gml:name>9901</gml:name>
                                    </gml:axisID>
                                    <gml:axisAbbrev>Lat</gml:axisAbbrev>
                                    <gml:axisDirection>north</gml:axisDirection>
                                </gml:CoordinateSystemAxis>
                            </gml:usesAxis>
                            <gml:usesAxis>
                                <gml:CoordinateSystemAxis gml:id="ogrcrs108" gml:uom="urn:ogc:def:uom:EPSG::9102">
                                    <gml:name>Geodetic longitude</gml:name>
                                    <gml:axisID>
                                        <gml:name>9902</gml:name>
                                    </gml:axisID>
                                    <gml:axisAbbrev>Lon</gml:axisAbbrev>
                                    <gml:axisDirection>east</gml:axisDirection>
                                </gml:CoordinateSystemAxis>
                            </gml:usesAxis>
                        </gml:EllipsoidalCS>
                    </gml:usesEllipsoidalCS>
                    <gml:usesGeodeticDatum>
                        <gml:GeodeticDatum gml:id="ogrcrs109">
                            <gml:datumName>WGS_1984</gml:datumName>
                            <gml:datumID>
                                <gml:name>6326</gml:name>
                            </gml:datumID>
                            <gml:usesPrimeMeridian>
                                <gml:PrimeMeridian gml:id="ogrcrs110">
                                    <gml:meridianName>Greenwich</gml:meridianName>
                                    <gml:meridianID>
                                        <gml:name>8901</gml:name>
                                    </gml:meridianID>
                                    <gml:greenwichLongitude>
                                        <gml:angle uom="urn:ogc:def:uom:EPSG::9102">0</gml:angle>
                                    </gml:greenwichLongitude>
                                </gml:PrimeMeridian>
                            </gml:usesPrimeMeridian>
                            <gml:usesEllipsoid>
                                <gml:Ellipsoid gml:id="ogrcrs111">
                                    <gml:ellipsoidName>WGS 84</gml:ellipsoidName>
                                    <gml:ellipsoidID>
                                        <gml:name>7030</gml:name>
                                    </gml:ellipsoidID>
                                    <gml:semiMajorAxis uom="urn:ogc:def:uom:EPSG::9001">6378137</gml:semiMajorAxis>
                                    <gml:secondDefiningParameter>
                                        <gml:inverseFlattening uom="urn:ogc:def:uom:EPSG::9201">298.257223563</gml:inverseFlattening>
                                    </gml:secondDefiningParameter>
                                </gml:Ellipsoid>
                            </gml:usesEllipsoid>
                        </gml:GeodeticDatum>
                    </gml:usesGeodeticDatum>
                </gml:GeographicCRS>
            </gml:baseCRS>
            <gml:definedByConversion>
                <gml:Conversion gml:id="ogrcrs112">
                    <gml:coordinateOperationName></gml:coordinateOperationName>
                    <gml:usesMethod xlink:href="urn:ogc:def:method:EPSG::9807"/>
                    <gml:usesValue>
                        <gml:value uom="urn:ogc:def:uom:EPSG::9102">0</gml:value>
                        <gml:valueOfParameter xlink:href="urn:ogc:def:parameter:EPSG::8801"/>
                    </gml:usesValue>
                    <gml:usesValue>
                        <gml:value uom="urn:ogc:def:uom:EPSG::9102">12</gml:value>
                        <gml:valueOfParameter xlink:href="urn:ogc:def:parameter:EPSG::8802"/>
                    </gml:usesValue>
                    <gml:usesValue>
                        <gml:value uom="urn:ogc:def:uom:EPSG::9001">0.9996</gml:value>
                        <gml:valueOfParameter xlink:href="urn:ogc:def:parameter:EPSG::8805"/>
                    </gml:usesValue>
                    <gml:usesValue>
                        <gml:value uom="urn:ogc:def:uom:EPSG::9001">500000</gml:value>
                        <gml:valueOfParameter xlink:href="urn:ogc:def:parameter:EPSG::8806"/>
                    </gml:usesValue>
                    <gml:usesValue>
                        <gml:value uom="urn:ogc:def:uom:EPSG::9001">0</gml:value>
                        <gml:valueOfParameter xlink:href="urn:ogc:def:parameter:EPSG::8807"/>
                    </gml:usesValue>
                </gml:Conversion>
            </gml:definedByConversion>
            <gml:usesCartesianCS>
                <gml:CartesianCS gml:id="ogrcrs113">
                    <gml:csName>Cartesian</gml:csName>
                    <gml:csID>
                        <gml:name>4400</gml:name>
                    </gml:csID>
                    <gml:usesAxis>
                        <gml:CoordinateSystemAxis gml:id="ogrcrs114" gml:uom="urn:ogc:def:uom:EPSG::9001">
                            <gml:name>Easting</gml:name>
                            <gml:axisID>
                                <gml:name>9906</gml:name>
                            </gml:axisID>
                            <gml:axisAbbrev>E</gml:axisAbbrev>
                            <gml:axisDirection>east</gml:axisDirection>
                        </gml:CoordinateSystemAxis>
                    </gml:usesAxis>
                    <gml:usesAxis>
                        <gml:CoordinateSystemAxis gml:id="ogrcrs115" gml:uom="urn:ogc:def:uom:EPSG::9001">
                            <gml:name>Northing</gml:name>
                            <gml:axisID>
                                <gml:name>9907</gml:name>
                            </gml:axisID>
                            <gml:axisAbbrev>N</gml:axisAbbrev>
                            <gml:axisDirection>north</gml:axisDirection>
                        </gml:CoordinateSystemAxis>
                    </gml:usesAxis>
                </gml:CartesianCS>
            </gml:usesCartesianCS>
        </gml:ProjectedCRS>
    </gml:dictionaryEntry>
</gml:Dictionary>
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  • 1
    That crs has essentially the same definition as UTM, but with the central meridian at 12, halfway between zones 32 and 33. So you can try setting the crs with a custom string, like a proj string: +proj=tmerc +lat_0=0 +lon_0=12 +k=0.9996 +x_0=500000 +y_0=0 +datum=WGS84 +units=m +no_defs or with a WKT, take any one of the WGS84 UTM Zones' WKT and change the central_meridian to 12.
    – FSimardGIS
    Commented Jan 7, 2021 at 3:17
  • This proj string made the trick. I am very thankful and I suggest you give it as an answer so that I can mark it as the proper one.
    – Rho Phi
    Commented Jan 7, 2021 at 16:23
  • In any case I think it would be very beneficial for everyone starting out on using geopandas and related tools if we can put together an explanation on how to read this XML and get the info that is really needed.
    – Rho Phi
    Commented Jan 7, 2021 at 16:24
  • To facilitate the task I have indented the XML for better readability. I will put here in comments what I got from my own research, but I expect this can be improved and I will be glad to read a full answer.
    – Rho Phi
    Commented Jan 7, 2021 at 16:26
  • First of all I noted that the juice is really in the <gml:definedByConversion> tag. The part preceding it seems to be a definition of a "starting" CRS given in <gml:baseCRS>. The starting one is the EPSG 4326 which is like the proj string +proj=longlat +datum=WGS84 +no_defs . This can be obtained at the link for 4326. Some part of the "base" is not clear to me why is referred to as 6402 or 9901 or 9902 as these, unlike 4326, are not EPSG specifications for a full CRS, they seem to be related to definition of the rotational ellipsoid.
    – Rho Phi
    Commented Jan 7, 2021 at 16:32

1 Answer 1

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The CRS described in your file has essentially the same definition as UTM, but with the central meridian at 12, halfway between zones 32 and 33. So one possible solution is to build a custom string, like a proj string:

+proj=tmerc +lat_0=0 +lon_0=12 +k=0.9996 +x_0=500000 +y_0=0 +datum=WGS84 +units=m +no_defs

and set this as the CRS of the layer.

Or with a WKT, take any one of the WGS84 UTM Zones' WKT and change the central_meridian to 12 and adapt the WKT:

PROJCS["WGS 84 / UTM zone 32.5",
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",12],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",500000],
PARAMETER["false_northing",0],
UNIT["metre",1,
    AUTHORITY["EPSG","9001"]],
AXIS["Easting",EAST],
AXIS["Northing",NORTH]]

The meaning of the EPSG codes for the different parameters can be verified on epsg.org by searching for the code and choosing the appropriate tab corresponding to the category of the parameter in question.

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