[OSM-dev] Mapnik projection slightly wrong?

[Thomas Wood]

For webmap rendering we use the spherical mercator projection, which
assumes the earth is spherical. Although it's not particularly
accurate, the maps are good enough. I believe the spherical earth is
assumed since it simplifies client-side calculations considerably.

2008/9/23 Jan-Benedict Glaw <jbglaw at lug-owl.de>:
> 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 :)
>
> MfG, JBG
>
> --
>      Jan-Benedict Glaw      jbglaw at lug-owl.de              +49-172-7608481
> Signature of:  The course of history shows that as a government grows, liberty
> the second  : decreases."  (Thomas Jefferson)
>
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.6 (GNU/Linux)
>
> iD8DBQFI2QuEHb1edYOZ4bsRAmYjAJ0Rdae6biLZ1B2emcwx2dL59p9MAQCeLG0p
> IWx4BxFje0Vc7ECyb6iCDTk=
> =emfH
> -----END PGP SIGNATURE-----
>
> _______________________________________________
> dev mailing list
> dev at openstreetmap.org
> http://lists.openstreetmap.org/listinfo/dev
>
>



-- 
Regards,
Thomas Wood
(Edgemaster)