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

Hamish hamish_b at yahoo.com
Thu May 29 15:17:11 AKDT 2008


by chance this was posted today on the Proj4 list; explains the +nadgrids=@null part.  --H


--- On Thu, 5/29/08, Frank Warmerdam <warmerdam pobox.com> wrote:

> From: Frank Warmerdam <warmerdam pobox.com>
> Subject: Re: [Proj] Problem overlaying georeferenced images in Google,Maps(projection problem!)
> To: "PROJ.4 and general Projections Discussions" <proj lists.maptools.org>
> Date: Thursday, May 29, 2008, 5:19 PM
> 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.
> 
> Folks,
> 
> I haven't followed this issue closely, but please be
> sure you are aware of:
> 
>  
> http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toVirtualEarthMercator
> 
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam pobox.com
> light and sound - activate the windows |
> http://pobox.com/~warmerdam
> and watch the world go round - Rush    | President OSGeo,
> http://osgeo.org
> 
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