[ gpsdrive ] Google's mercator projection

[Hamish]

Hi,

I just got a link from Markus with a good definition of Google's Mercator
projection:

  http://spatialreference.org/ref/user/6/

Usable from GDAL or PROJ.4 using ESRI-addon EPSG code 900913,
see /usr/share/proj/esri.extra

#
#  Chris' funny epsgish code for the google mercator
#
<900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
+y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs <>


+nadgrids=@null is used as apparently they use a spherical ellipsoid. As
we (and GPS) use the a WGS84 datum/ellipsoid (the earth is not a perfect
sphere) converting downloaded maps before plotting could help...
(probably the importance of needing that depends on location and map
scale)

e.g. convert Google map -> lat/lon WGS84
   gdalwarp -s_srs +init=epsg:900913 -t_srs +init=epsg:4326


fyi,
Hamish



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