[ gpsdrive ] GPSdrive SVN, Mapnik and a Patch

Hamish hamish_b at yahoo.com
Thu May 29 06:11:51 AKDT 2008


Hamish:
> * the "google projection" is as follows, and the
> likely source of Mapnik's
>   (from PROJ4's /usr/share/proj/esri.extra file)
> #
> #  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 <>


Yes, that's right, as is the guess about the spherical ellipsoid being at issue.

from the osm2pgsql source:
http://trac.openstreetmap.org/browser/applications/utils/export/osm2pgsql/reprojection.c#L14

You can see in that source that PROJ_SPHERE_MERC defines itself as srs:900913. (so in theory -m is the same as 'osm2pgsql --proj 900913')

A srs (spatial reference system?) defines the "EPSG"ish code as given in /usr/share/proj/egpg and friends. (although this one is not an offical code issued by the EPSG, it's an extra)


Hamish




      



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