[gdal-dev] Spherical Mercator Projection mismatching major/minor axis

[jratike80]

Hi,

This does not really belong to my knowledge area but I'll have a try anyway.

Check what you have after reading the proj string instead. Here with Python

>>> from osgeo import osr
>>> spatialRef = osr.SpatialReference()
>>> spatialRef.ImportFromProj4("+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
>>> +towgs84=0,0,0,0,0,0,0 +wktext +no_defs")
>>> print(spatialRef)
PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            TOWGS84[0,0,0,0,0,0,0],
            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["Easting",EAST],
    AXIS["Northing",NORTH],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0
+x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs"]]
    

It means that GDAL makes a clever guess and considers that the Proj string
that you provided means EPSG:3857. You probably mean the same. And in the
EPSG database EPSG:3857 is based on WGS84 ellipsoid
https://epsg.org/crs/wkt/id/3857

 ELLIPSOID["WGS 84",6378137,298.257223563,

and that it behaves in spherical way is handled by a special conversion

CONVERSION["Popular Visualisation Pseudo-Mercator",

I am not sure what to do if you definitely want to keep the ball with
diameter of 6378137 meters but perhaps dropping +nadgrids=@null could do it.
If EPSG:3857 is what you really need then it takes less writing to import by
the EPSG code but your way gives the same result.

-Jukka Rahkonen-





Andreas Roth wrote
> Hi,
> 
> I call OSRImportFromProj4 with
> "+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 +towgs84=0,0,0,0,0,0,0 +wktext +no_defs"
> 
> and with the returned handle i get the major and minor axis (
> OSRGetSemiMajor/OSRGetSemiMinor). I would expect the both axis to be
> equal,
> but they aren't.
> Instead i'm getting the following result:
> semi_major_axis=6378137
> semi_minor_axis=6356752.314
> 
> Build on Ubuntu 20.10 (groovy; amd64) with libgdal-dev,
> version 3.1.3+dfsg-1ubuntu2) and libproj-dev:amd64 with version 7.1.0-1
> 
> The full example application code:
> https://pastebin.com/xf8QpVhM
> 
> Regards,
> Andreas
> 
> _______________________________________________
> gdal-dev mailing list

> gdal-dev at .osgeo

> https://lists.osgeo.org/mailman/listinfo/gdal-dev





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