Ticket #3962 (new defect)
Revisiting Web Mercator
| Reported by: | warmerdam | Owned by: | warmerdam |
|---|---|---|---|
| Priority: | normal | Milestone: | |
| Component: | OGR_SRS | Version: | unspecified |
| Severity: | normal | Keywords: | web mercator. |
| Cc: |
Description
ESRI now uses the following definition for web mercator:
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"]]
NOTE: The Auxiliary Sphere Type parameter accepts 0 (use semimajor axis or radius of the geographic coordinate system), 1 (use semiminor axis or radius), 2 (calculate and use authalic radius), or 3 (use authalic radius and convert geodetic latitudes to authalic latitudes).
Erdas is now using the EPRJ_PSEUDO_MERCATOR:
"Pseudo Mercator" {
Internal 68
"Spheroid" <spheroid>
"Longitude of central meridian" <4:angle ew:dd>
"Latitude of true scale" <5:angle ns:dd>
"False easting at central meridian" <6:distance ew:meters>
"False northing at origin" <7:distance ns:meters>
}
EPSG now has a custom projection method, 1024, for google mercator (assuming major axis sphere I believe).
GDAL currently translates EPSG:3857 as:
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"]],
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],
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"],
AXIS["X",EAST],
AXIS["Y",NORTH]]
The EXTENSION element with a special proj.4 string that forces use of the semi-major sphere for projection calculations, but treated as WGS84.
I am thinking that we should move to treating the pseudo-mercator projection method better in WKT, and tying the various systems together properly. Either use a custom projection like ESRI ("Mercator_Auxiliary_Sphere") or introduce the "Auxilary_Sphere_Type" parameter to standard Mercator.
