[gdal-dev] Flattening in EPSG:3857

Frank Warmerdam warmerdam at pobox.com
Fri Nov 19 12:10:59 EST 2010

Jukka Rahkonen wrote:
> Hi,
> Has EPSG changed again something with the Web Mercator projection? 
> The EPSG report about EPSG:3857 shows that it is using a flattened 
> WGS84 ellipsoid.
> http://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3857&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3857
> Compare with 
> http://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3785&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3785
> I thought that 3857 was just the same as EPSG:3785 used to be and the same than
> the fake EPSG:900913. What should I believe now? Is there any valid EPSG code
> for a spherical world or is it better to go with the self made EPSG:900913?


In EPSG:3785 web mercator was expressed as a normal Mercator 1SP projection
on a spherical GCS.  However, this lost the fact that the lat/long coordinates
you produced when unprojecting the mercator coordiantes were supposed to be
treated as WGS84, not lat/long on the sphere.

So that was deprecated and EPSG:3857 (it freaks me out that they are the
same digits just rearranged!) was introduced.  This is based on WGS84, but
they define a custom projection method "Popular Visualization Psuedo
Mercator" which seems to have the spherical earth model for the projection
algorithm "baked in", but is otherwise based on a normal WGS84 GCS.  This
makes it easy for applications to understand that the unprojected
coordinates are to be treated as WGS84.

For what it's worth, GDAL has custom logic to treat them as equivalent.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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