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Spherical/Web Mercator: EPSG code 3785

I just received an update from the EPSG mailing list:

New to Version 6.15 are (among other things): Added spherical Mercator coordinate operation method and associated CRS as seen in popular web mapping and visualisation applications.

It looks like they FINALLY added the spherical Mercator / Web Mercator projection used in Virtual Earth and Google Maps.

This is a big surprise. EPSG’s earlier statement whether to include it was this:

"We have reviewed the coordinate reference system used by Microsoft, Google, etc. and believe that it is technically flawed. We will not devalue the EPSG dataset by including such inappropriate geodesy and cartography.

Guess they changed their mind, or did they just devalue their dataset? Then again, judging from the remarks EPSG put in there, their arrogance still shines through. There´s absolute nothing wrong with using a sphere instead of a flattened sphere. Sure it's not as accurate as for instance WGS84, but then again WGS84 is not accurate either - no ellipsoid is. But we know the exact differences between the two, and as always you will need to take these things into account so I don´t see the real issue. Viisually the distortion is far less than what you would notice, and when doing area, distance and bearing calculations you would first of all never use the mercator units without taking the projection distortion into account, and if you do your calculationg in long/lat it's more or less just as easy to use WGS84 as a base for your calculations (since no datum transform is really needed).

Anyway, finally we get an official code for "Web Mercator": EPSG:3785 

Here are the details of the entry:

Full WKT with authorities (untested!):
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", 0.0174532925199433, AUTHORITY["EPSG", "9102"]], AXIS["E", EAST], AXIS["N", NORTH], AUTHORITY["EPSG",4055]], PROJECTION["Mercator"], PARAMETER["False_Easting", 0], PARAMETER["False_Northing", 0], PARAMETER["Central_Meridian", 0], PARAMETER["Latitude_of_origin", 0], UNIT["metre", 1, AUTHORITY["EPSG", "9001"]], AXIS["East", EAST], AXIS["North", NORTH], AUTHORITY["EPSG",3785]]
  

Projected CRS
COORD_REF_SYS_CODE: 3785
COORD_REF_SYS_NAME: Popular Visualisation CRS / Mercator   
AREA_OF_USE_CODE: 3544   
COORD_REF_SYS_KIND: projected   
COORD_SYS_CODE: 4499       
DATUM_CODE:
SOURCE_GEOGCRS_CODE: 4055 (see below)
PROJECTION_CONV_CODE: 19847   
CMPD_HORIZCRS_CODE:
CMPD_VERTCRS_CODE:
CRS_SCOPE: Certain Web mapping and visualisation applications.   
REMARKS: Uses spherical development. Relative to an ellipsoidal development errors of up to 800 metres in position and 0.7% in scale may arise. Some applications call this WGS 84. It is not a recognised geodetic system: see WGS 84 / World Mercator (CRS code 3395)   
INFORMATION_SOURCE: Microsoft.
DATA_SOURCE: OGP
REVISION_DATE: 3/14/2008
CHANGE_ID:
SHOW_CRS: TRUE
DEPRECATED: FALSE

Geographic CRS:
COORD_REF_SYS_CODE: 4055
COORD_REF_SYS_NAME: Popular Visualisation CRS
AREA_OF_USE_CODE: 31262
COORD_REF_SYS_KIND: geographic 2D
COORD_SYS_CODE: 6422
DATUM_CODE: 6055 (see below)
SOURCE_GEOGCRS_CODE:
PROJECTION_CONV_CODE:
CMPD_HORIZCRS_CODE:
CMPD_VERTCRS_CODE:
CRS_SCOPE: Certain Web mapping and visualisation applications.
REMARKS: Some applications erroneously call this WGS 84. It uses a sphere with a radius having the same value as the semi-major axis of the WGS 84 ellipsoid. There is no geodetic recognition of this system.
INFORMATION_SOURCE: Microsoft.
DATA_SOURCE: OGP
REVISION_DATE: 3/13/2008
CHANGE_ID:
SHOW_CRS: TRUE
DATUM_CODE: FALSE


Datum:
Code: 6055       
Datum Name: Popular Visualisation
Datum Type: geodetic
Origin Description: Not specified in the classical sense of defining a geodetic datum.
Datum Epoch:
Ellipsoid Code: 7059 (see below)
Prime Meridian Code: 8901
Area Code: 1262
Datum Scope    : Used by certain popular Web mapping and visualisation applications.
Remarks: Not recognised by geodetic authorities.
Information Source: Microsoft.
Data Source: OGP   
Revision Date: 13-Mar-08
Change ID:
Deprecated: No


Ellipsoid:
Code: 7059
Ellipsoid Name: Popular Visualisation Sphere   
Semi-major axis (a): 6378137   
Axes units code: 9001       
Inverse flattening (1/f):
Semi-minor axis (b): 6378137   
Ellipsoid?: No   
Remarks: Sphere with radius equal to the semi-major axis of the GRS80 and WGS 84 ellipsoids. Used only for Web approximate mapping and visualisation. Not recognised by geodetic authorities.   
Information Source: Microsoft.
Data Source: OGP
Revision Date: 14-Mar-08
Change ID:   
Deprecated?: No

Comments (4) -

  • James Fee

    5/15/2008 8:17:12 PM |

    Well how about that?  I'm looking for pigs flying out side and I don't see any.

  • bmc

    5/29/2008 6:50:39 PM |

    How long before we start seeing GIS applications add this?  I'd particularly like to see a datum transformation in Arc.  I'm trying to display shapefiles on top of Google Map tiles, and the accuracy you get from shapefiles in WGS84 is pretty bad.

  • mark2atsafe

    6/10/2008 12:34:09 AM |

    > How long before we start seeing GIS applications add this?

    It's supported by FME (2009-beta) if you count that as a GIS app.
    www.fmepedia.com/.../Whats_Great

    It's a big thing for users of our VirtualEarthTiler transformer.

  • Morten

    6/10/2008 12:44:21 AM |

    Well that all depends on how fast they ship out a service pack / update. Note though that most applications ALREADY support it. It's just a matter of you defining the projection manually. This is all because there's not really anything new to this - Spheres instead of ellipsoids are just a matter of modifying the parameters of the ellipoid and Mercator projections are already supported by most GIS'. It's just the combination and/or the EPSG code that's not defined in there.  For instance ArcGIS 9.3 will have this projection in there, but since it's about to ship, I doubt that this EPSG code made it into the release. You have also been able to define it in ArcGIS manually longer than this spatial reference has existed.

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