[Proj] Problem overlaying georeferenced images in Google, Maps(projection problem!)

Hamish hamish_b at yahoo.com
Thu May 29 19:25:16 EDT 2008

> Eric Miller wrote:
> > Yes, Google maps uses a spherical model rather than
> > ellipsoidal (hence, +a == +b in the destination coordinate
> > system).  So, it's not really WGS84.  However, my
> > understanding is that GMaps projection assumes there
> > isn't any difference.  So, source coordinates should be
> > converted to Geodetic on the WGS84 ellipsoid/datum and then
> > projected onto the Google Mercator "WGS84" sphere
> > ignoring the fact that the models are really different.  So,
> > the +towgs84 parameter on the destination should probably be
> > defined as +towgs84=0,0,0.
Frank wrote:
> I haven't followed this issue closely, but please be
> sure you are aware of:
> http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toVirtualEarthMercator

ah, ok. I happened to be looking at this problem yesterday WRT OpenStreetMap's PostgreSQL exporter -m flag (->Mapnik):

One thing I don't understand in that is what the "+over" term is for.
It is new to me.

Also, Jorge- is that 3785 EPSG term for Google's merc a new official EPSG one or a custom thing? if a custom thing maybe it is better to stick with 
<900913> "Chris' funny epsgish code for the google mercator"
(from /usr/share/proj/esri.extra)



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