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

Eric Miller EMiller at dfg.ca.gov
Thu May 29 12:49:15 EDT 2008

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.

Good Luck,

>>> On 5/29/2008 at 9:18 AM, Jorge <jorge.arevalo at gmail.com> wrote:
> Hi Eric
> Mmm... just now I was reading about datum convert and +towgs84. But the 
> GMaps datum is somehow different from WGS84, is'n it? Can I use +towgs84 
> instead of this?
> Thank you!
>> > gdalwarp
>> >     -s_srs "+proj=utm +zone=31 +ellps=intl +units=m +no_defs"
>> >     -t_srs "+proj=merc +lat_ts=0.0 +lon_0=0.0 +k=1.0 +x_0=0.0 +y_0=0.0 
>> > +a=6378137.0 +b=6378137.0 +units=m +nadgrids=@null +no_defs"
>> >     <raster file name> <reprojected raster file name>
>> > 
>> > Then, I did the reprojection, but I still have the problem... better 
>> > look, but still remains... Bad overlaid images
>> > 
>> > I think that the proj.4 parameters are correct. ...
>> [snip]
>> It looks like you are missing the datum transformation parameters to convert 
> from ED50 to WGS84.  You need to add the +towgs84 
>> parameter to your source projection.  However, I can't help you with 
> suggested values for that parameter for your source 
>> coordinate system.  You'll have to do some research.
>  > -- > > Eric G. Miller > Staff Programmer > CA Dept. of Fish & Game
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