[OSM-dev] Mapnik projection slightly wrong?

[Dave Stubbs]

On Tue, Sep 23, 2008 at 4:30 PM, Jan-Benedict Glaw <jbglaw at lug-owl.de> wrote:
> Hi!
>
> I just looked at applications/rendering/mapnik/generate_{image,tiles}.py
> and noticed the projection string:
>
>        +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 +no_defs +over
>
> `a' and `b' are ment to be the equatorial and polar radius of the
> earth. Since we all use GPS, I guess we want to use the WGS84
> ellipsoid, where the polar radius (b) would be 6356752.315m, instead
> of 6378137.000m?  Is this intentional?  At least, osm2pgsql uses the
> same, so all of the users are identical :)


The main osm online maps are all done in spherical mercator. As the
name suggests, a and b are the same. This is the same projection
google, yahoo and microsoft use for their web maps. You'll find the
projection often referenced as EPSG 900913, which is an impossible
number to remember until you realise you can read it...

You'll see osm2pgsql has some other projection options if you want to use them.