I'm trying to understand this issue myself, and keep on discovering new things. Please examine my findings.
First of all, EPSG has totally purged EPSG:3785 from their database, at least as of version 7.6. The proj.4 epsg file [1] lists identical parameters for both EPSG:3785 and EPSG:3857, though comments that EPSG:3785 is deprecated. Basically EPSG:3785 should never have existed. I think the confusion relates to datums vs. projections. OpenStreetMap stores coordinates in the WGS84 datum [2], which uses an ellipsoidal model of the earth. EPSG:3857 uses the WGS 84 datum, but it also projects that datum using a spherical model. Thus the inverse flattening that Martin mentions refers to the WGS 84 datum, not the EPSG:3857 projection from that datum. You can also read about this on the proj.4 wiki [3]. -Josh [1]: http://trac.osgeo.org/proj/browser/trunk/proj/nad/epsg [2]: http://josm.openstreetmap.de/wiki/Help/Concepts/Coordinates#Openstreetmapcoordinates [3]: http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator On Tue, Mar 8, 2011 at 7:24 AM, Tobias Wendorff <tobias.wendo...@tu-dortmund.de> wrote: > Am 25.02.2011 20:13, schrieb M∡rtin Koppenhoefer: >> >> I know that EPSG says "NOT VALID". Who cares? > > EPSG are creating and releasing the codes. You have to face *why* > they do that. They need it for surveys and for geodetic use. > > EPSG:3785 uses a sphere, which is "outdated" nowerdays. But it's > much more easy to use in OSM. A simple formular with good looking > results. > > EPSG:3857 uses an ellipsoid, which is more equal to real Earth, > since it uses WGS84 as background. > > For OSM, it would be much better to use EPSG:3857 instead of > EPSG:3785. The reason is: our coordinates are in WGS84. Normally, > you have to transform the WGS84 ellipsoid to some WGS84 sphere > to get correct results, but noone does this. > > When using EPSG:3857 you're in "one system" and you don't have > to transform anything. But the formulars to draw the map are > much more complicated, since you have to deal with integrals > or expansion in series. > > HTH > Tobias > > _______________________________________________ josm-dev mailing list josm-dev@openstreetmap.org http://lists.openstreetmap.org/listinfo/josm-dev
