[MetaCRS] Google Spherical Mercator (EPSG:3785)

[Frank Warmerdam]

Folks,

I am trying to add support for EPSG:3785 (Google Spherical
Mercator) to GDAL/OGR and PROJ.4 per bug report:

  http://trac.osgeo.org/proj/ticket/9

The problem i see is that I end up with the WKT definition:

PROJCS["Popular Visualisation CRS / Mercator",
    GEOGCS["Popular Visualisation CRS",
        DATUM["Popular_Visualisation_Datum",
            SPHEROID["Popular Visualisation Sphere",6378137,0,
                AUTHORITY["EPSG","7059"]],
            TOWGS84[0,0,0,0,0,0,0],
            AUTHORITY["EPSG","6055"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.01745329251994328,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4055"]],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    AUTHORITY["EPSG","3785"],
    AXIS["X",EAST],
    AXIS["Y",NORTH]]

This definition includes TOWGS84[0,0,0,0,0,0,0].

My understanding is that we really want to treat the lat/long values
on datum EPSG:6055 as if they were equivelent to lat/long values
on the WGS84 datum.

However, the way I apply TOWGS84[] translations is to convert
the lat/long values to geocentric coordinates on the source spheroid,
apply the offset, and then transform back to lat/long on the destination
spheroid. The vast difference between the WGS84 and spherical
spheroids is resulting in a large latitude shift.

So my question is what is in error?

1) Is the EPSG dictionary wrong in suggesting a TOWGS84[0,0,0]
transformation?

2) Am I wrong in applying TOWGS84[] transformations by using
the different spheroids to go from and to geocentric coordinates?

3) Am I going wrong in how I look things up in the EPSG database?

Advice appreciated.

Best regards,
-- 
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I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush | Geospatial Programmer for Rent