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I have projected a geotiff to EPSG:3857 using ArcMap 10.1 (in Arc it is called “WGS_1984_Web_Mercator_Auxiliary_Sphere”). The coordinate parameters look correct, so why does gdal not recognise the coordinate system of this file? E.g. when I open it in QGIS it says "define this layers coord ref system".

Can anyone help? What can I do to make gdal recognise this coordinate system straight away?

To be more specific:

As read from gdalinfo, the geotiff projection parameters as produced by ArcMap 10.1:

PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",
    GEOGCS["GCS_WGS_1984",
        DATUM["D_WGS_1984",
            SPHEROID["WGS_1984",6378137.0,298.257223563]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.017453292519943295]],
    PROJECTION["Mercator_Auxiliary_Sphere"],
    PARAMETER["False_Easting",0.0],
    PARAMETER["False_Northing",0.0],
    PARAMETER["Central_Meridian",0.0],
    PARAMETER["Standard_Parallel_1",0.0],
    PARAMETER["Auxiliary_Sphere_Type",0.0],
    UNIT["Meter",1.0]]

vs GDAL's projection parameters:

PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"],
    AUTHORITY["EPSG","3857"]]

GDAL's parameters have the EPSG authority embedded, while Arc's does not. i.e. Is there something I should do in ArcMap to get EPSG embedded?

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ARCMAP does not include the EPSG codes the way GDAL is doing; so no chance on that side.

There is some discussion going on at the gdal-dev mailing list on this bug:

http://osgeo-org.1560.x6.nabble.com/gdal-dev-GDAL-v1-10-1-Bad-output-from-gdalwarp-works-with-1-10-0-td5099504.html

http://trac.osgeo.org/gdal/ticket/3962

The problem is that G**GLE Mercator does not follow the rules for mercator projection on a ellisoid, and simply using a sphere is also not correct.

Seems like GDAL uses a hack for this when calling EPSG:3857 by adding the complete proj string.

EPSG:900913 behaves differently, although the proj parameters are the same.

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