On 24/03/14 13:10, Vincent Bain wrote:
Comment on my previous message :if we have a closer look at parameters, we notice a difference between the current (grass location) projection parameters and epsg:3857 parameters : Current : a (semi-major axis) = 6378137 rf (flattening factor inverse) =298.257223563 am I wrong or as a consequence b (semi-minor axis) = 6356752 ? epsg:3857 : a (semi-major axis) = 6378137 b (semi-minor axis) = 6378137 (a sphere ?) Now if I run cs2cs with these distinct params : echo "6.46311951 45.78037144" | cs2cs +init=epsg:4326 -f "%.8f" +to +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 +wktext +no_defs returns 719471.17278927 5745223.17292760 0.00000000 and echo "6.46311951 45.78037144" | cs2cs +init=epsg:4326 -f "%.8f" +to +proj=merc +a=6378137 +b=6356752 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs returns 719471.17278927 5714587.36406099 0.00000000 that is what m.proj initially said. It looks as if grass was taking into account the flatten-shape of the ellipsoid, where epsg registry considers it is a sphere. Sorry for the noise, but I would actually enjoy understanding where I am wrong ;-(
You're not wrong. The difference comes from the GRASS file /etc/ogr_csv/pcs.csv in which there is a specific rule set for 3857.
Don't know why, though. Moritz _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
