Christopher Schmidt wrote: > On Mon, Dec 24, 2007 at 11:20:04AM -0500, Mike Adair wrote: > > Note that at the time of FOSS4G, EPSG:900913 projections worked > flawlessly. I confirmed this by testing them against the OpenLayers > internal reprojection formulas. >
I think it works only for transformations from/to projections that use a WGS84 datum. >> So what I've done in proj4js is to allow a 'datum=none' parameter (and >> added that to the Google def string) which will bypass the datum >> transformation if it is set for either source or destination coordinate >> system. >> I do not think that this is a good idea, for the reasons described below. > I believe that this matches up to the [EMAIL PROTECTED] hack described at > http://proj.maptools.org/faq.html#sphere_as_wgs84 . > Hm. From looking at the OpenLayers reprojection code between 4326 and Spherical Mercator, I would guess that conversions between the two are just a conversion from degrees to meters with a stretch in y-direction, increasing with the distance to the equator. This leads me to the conclusion that Spherical Mercator is based on a WGS84 datum, with a different ellipsoid shape in y-direction only. >> From my test cases, I'm still seeing differences on the order >> of 100m in the y value and I'm wondering if that's the best we can do in >> this case. >> This is the usual y difference between projections based on the MGI Bessel datum (like EPSG:31285) and WGS84-based projections. You will experience the same difference when converting from EPSG:31285 to EPSG:4326 without datum transformation. > Probably not, though it may mean pursuing a different behavior for > proj4js than for proj. (This is, for example, the path that GeoServer > has taken, defining an entirely new projection type specifically for > Google.) > I think this should also be done in proj4js: if 900913 is requested as source or target projection, reproject to/from 4326 instead, and apply the OpenLayers transformation code before/after that conversion. Of course, this is just an idea from me, knowing that I do not know too much about reprojecting coordinates. Regards, Andreas. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ mapbuilder-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/mapbuilder-devel
