[webmap-discuss] Re: Google Map projection and EPSG codes

[Paul Neave]

I'd like to second this request - Mercator projection for WMS is
something that I'm finding I desperately need for similar applications
I'm developing that use imagery from different sources (that don't use
plate carrée).  The hack that's currently available leaves large areas
of the map distorted at zoomed-out levels.

I don't know how difficult it would be to implement a new EPSG code on
NASA's WMS server, but judging other examples around on the web it
looks at though it is possible.  Does anyone have any experience with
this?

Thanks,
Paul.


On 27/09/06, Cameron Shorter <cameron.shorter at gmail.com> wrote:
> One of the commonly requested features for a webmapping client is to
> render WMS layers over Google Map basemaps.
>
> Unfortunately, this is tricky to implement because the projection used
> by Google Maps doesn't have a corresponding EPSG code which means that
> the projection cannot be selected using a standard WMS query.
>
> John Deck describes the problem, and a hack to get around it at:
> http://johndeck.blogspot.com/2005/09/overlaying-mercator-projected-wms.html
> John defines his own EPSG code in a mapserv config file.
>
> Questions:
> ==========
> 1. geoserver-devel, Is there a similar hack that can be used in Geoserver?
>
> 2. webmap-discuss, has anyone worked out a "less hacky" solution?
>
> 3. What is the process for defining a new EPSG code? Is this something
> we can make happen?
>
> Proposal:
> =========
> webmap-discuss, geoserver-devel, mapserver people,
> If we cannot force a new EPSG code to be officially created, I suggest
> that all the Open Source projects define a de-facto EPSG code for Google
> Maps which Geoserv, Mapserv and the various webmapping clients all support.
>
> Craig Bruce wrote:
> > "Kralidis,Tom [Burlington]" <Tom.Kralidis at ec.gc.ca> wrote:
> >
> >
> >>Question: has there been any definition of an EPSG code as applied to
> >>the way Google Maps displays their data (I'm not exactly sure what the
> >>projection params, but I did find one def from the postgis-users mailing
> >>list [see:
> >>http://postgis.refractions.net/pipermail/postgis-users/2005-November/009
> >>920.html])?
> >>
> >>I'm not sure if CubeWerx has defined one in the 42xxx series of
> >>'defacto' EPSG codes, or whether it's defined already, for that matter.
> >
> >
> > The closest match we have found is a simple Mercator projection.  One is
> > defined in the OGC 41xxx space as:
> >
> > EPSG:41001:
> >
> > PROJCS["WGS84 / Simple Mercator",
> >     GEOGCS["WGS 84",
> >         DATUM["WGS_1984",
> >             SPHEROID["WGS_1984",6378137,298.257223563]],
> >         PRIMEM["Greenwich",0],
> >         UNIT["Decimal_Degree",0.0174532925199433]],
> >     PROJECTION["Mercator"],
> >     PARAMETER["latitude_of_origin",0],
> >     PARAMETER["central_meridian",0],
> >     PARAMETER["false_easting",0],
> >     PARAMETER["false_northing",0],
> >     UNIT["Meter",1]]
> >
> > This doesn't appear to be an exact match, however, as the mismatch grows
> > from the equator from zero to a few kilometers.  There might be a scale
> > factor/secant involved.
> >
> > --------------------------+----------------------+--------------------------
> > Dr. Craig S. Bruce        | Ph 819-771-8303 x205 |             CubeWerx Inc.
> > Senior Software Developer |   Fax 819-771-8388   |  Gatineau, Québec, Canada
> > csbruce at cubewerx.com      |  http://csbruce.com/ |  http://www.cubewerx.com/
> > --------------------------+----------------------+--------------------------
> >     "Q: How many of your previous romantic relationships were failures?
> >      A: All of them.
> >      Q: So, what are the chances for your present relationship?
> >      A: Well, let's check the statistics..."
> >
>
>
> --
> Cameron Shorter
> http://cameron.shorter.net
>
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