Le lundi 24 mars 2014 à 13:24 -0700, Hamish a écrit : > > Hi, > > First of all, avoid using Google's "spherical mercator" projection for > anything other than necessary data import/export if you can possibly help it. > But sometimes we don't have a choice, so.. >
I wish I could never have heard about it... till I had to cope with french geographical web services : their wmts service is based on this projection system, so I need to define a spherical mercator location to correctly retrieve tilesets. > Google's spherical Mercator projection (formerly the unofficial > epsg-ish code 900913 from the esri.extra file) isn't really WGS84, it just > borrows the major axis of the Earth used in WGS84 for both the major and > minor axes of the ellipsoid, making it a sphere larger than Earth really is. > So it's just a simple Mercator on a sphere, which happens to have a > particular radius. > > Different ellipsoids (and a sphere being a type of ellipsoid) squash the > Earth north-south, so the northing value changes the most. Since Mercator > doesn't deform east-west (lines of longitude are all parallel and perfectly > vertical), in that projection changing the ellipsoid does not change the > easting value at all. > Yes, in my case I noticed a n-s 30 km gap > Next thing to know is the +nadgrids=@null hack to get around the datum > transform. See > http://trac.osgeo.org/proj/wiki/GenParms#ThenullGrid > > and this basically explains the rest: > > http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator I believed the nadgrid was a shifting matrix applying to ellipsoid center (in cartesian geocentric coordinates). Does this mean +nadgrids=@null implicitly tells cs2cs : keep the from-ellipsoid (ie ignore the to-sphere), and shift it (of 0,0,0) ? > > > good luck, > Hamish > Your explanations and links on this topic were very welcome ! Thank you, Vincent _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
