On Thu, Jul 31, 2014 at 8:48 AM, Andreas Bergmann <bergmann...@gmail.com>
wrote:
> Hi Jody,
>
> I expected 3857, as far as I know the data has been created with QGIS
> Desktop. I will check this and if this the case I will ask the QGIS team if
> they plan to enhance their definition.
>
>
The proper 3857 definition is:
PROJCS["WGS 84 / Pseudo-Mercator",
GEOGCS["WGS 84",
DATUM["World Geodetic System 1984",
SPHEROID["WGS 84", 6378137.0, 298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
UNIT["degree", 0.017453292519943295],
AXIS["Geodetic longitude", EAST],
AXIS["Geodetic latitude", NORTH],
AUTHORITY["EPSG","4326"]],
PROJECTION["Popular Visualisation Pseudo Mercator",
AUTHORITY["EPSG","1024"]],
PARAMETER["semi_minor", 6378137.0],
PARAMETER["latitude_of_origin", 0.0],
PARAMETER["central_meridian", 0.0],
PARAMETER["scale_factor", 1.0],
PARAMETER["false_easting", 0.0],
PARAMETER["false_northing", 0.0],
UNIT["m", 1.0],
AXIS["Easting", EAST],
AXIS["Northing", NORTH],
AUTHORITY["EPSG","3857"]]
Note that there is the authority and code at the end.
Here is the QGis one:
PROJCS["WGS_84_Pseudo_Mercator",
GEOGCS["GCS_WGS_1984",
DATUM["D_WGS_1984",
SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
PRIMEM["Greenwich", 0.0],
UNIT["degree", 0.017453292519943295],
AXIS["Longitude", EAST],
AXIS["Latitude", NORTH]],
PROJECTION["Mercator_2SP"],
PARAMETER["standard_parallel_1", 0.0],
PARAMETER["latitude_of_origin", 0.0],
PARAMETER["central_meridian", 0.0],
PARAMETER["false_easting", 0.0],
PARAMETER["false_northing", 0.0],
UNIT["m", 1.0],
AXIS["x", EAST],
AXIS["y", NORTH]]
As you can see they are not even using the same projection. Meractor_2SP is
not the google projection,
Google took it as the base, but the simplified its math to make it work as
if the earth was a sphere.
>From the EPSG projections guide, page 33:
Popular Visualisation Pseudo Mercator
(EPSG dataset coordinate operation method code 1024)
-----------------------------------------------------------------------------------
This method is utilised by some popular web mapping and visualisation
applications. It applies standard
Mercator (Spherical) formulas (section 1.3.3.1 above) to ellipsoidal
coordinates and the sphere radius is
taken to be the semi-major axis of the ellipsoid. This approach only
approximates to the more rigorous
application of ellipsoidal formulas to ellipsoidal coordinates (as given in
EPSG dataset coordinate operation
method codes 9804 and 9805 in section 1.3.3 above). Unlike either the
spherical or ellipsoidal Mercator
projection methods, this method is not conformal: scale factor varies as a
function of azimuth, which creates
angular distortion. Despite angular distortion there is no convergence in
the meridian.
Cheers
Andrea
--
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