[gdal-dev] Encoding EPSG:3857 (WebMercator) in GeoTIFF, and ArcGIS interoperability

[Even Rouault]

> The ArcGIS (or QGIS in qpj) save 3857 in such prj file:
> 

I hoped that this thread could remain focused just about GeoTIFF encoding. 
Shapefile encoding issues are a different matter (see 
http://trac.osgeo.org/gdal/ticket/3962)

> 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,
>              AUTHORITY["EPSG","8901"]],
>          UNIT["degree",0.0174532925199433,
>              AUTHORITY["EPSG","9122"]],
>          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"]],
>      AXIS["X",EAST],
>      AXIS["Y",NORTH],
>      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"]]
> 

Are you 100% positive that ArcGIS generates such a .prj ? This doesn't look 
like classical ESRI WKT (which generally lacks most AUTHORITY nodes and has no 
EXTENSION PROJ4 stuff), but like GDAL WKT.

> And GDAL opens such shape file and set the spheroid.
> But if I save the shape file in 3857 via GDAL I get ellipsoid:
> 
>    PROJCS["WGS_84_Pseudo_Mercator",
>      GEOGCS["GCS_WGS_1984",
>          DATUM["D_WGS_1984",
>              SPHEROID["WGS_1984",6378137,298.257223563]],
>          PRIMEM["Greenwich",0],
>          UNIT["Degree",0.017453292519943295]],
>          PROJECTION["Mercator"],
>          PARAMETER["central_meridian",0],
>          PARAMETER["false_easting",0],
>          PARAMETER["false_northing",0],
>          UNIT["Meter",1],
>          PARAMETER["standard_parallel_1",0.0]]

Yes, that's the result after morphToESRI(). Which is probably not the right 
ESRI WKT for WebMercator, anyway... Should rather be something like the 
following according to http://trac.osgeo.org/gdal/ticket/3962 :
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",0.0174532925199433]],
    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],
    AUTHORITY["ESRI","102100"]]

> 
> And both GDAL(QGIS) and ArcGIS open this shape file and set the ellipsoid!

I don't understand what you mean here.

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