aborruso wrote > Dear all, > I have a geospatial png file - http://ge.tt/7tmMR7r/v/0?c -and gdalinfo > give me this projection info: > > PROJCS["Popular Visualisation CRS / Mercator", > GEOGCS["Popular Visualisation CRS", > DATUM["Popular_Visualisation_Datum", > SPHEROID["Popular Visualisation Sphere",6378137,0, > AUTHORITY["EPSG","7059"]], > TOWGS84[0,0,0,0,0,0,0], > AUTHORITY["EPSG","6055"]], > PRIMEM["Greenwich",0, > AUTHORITY["EPSG","8901"]], > UNIT["degree",0.01745329251994328, > AUTHORITY["EPSG","9122"]], > AUTHORITY["EPSG","4055"]], > UNIT["metre",1, > AUTHORITY["EPSG","9001"]], > PROJECTION["Mercator_1SP"], > PARAMETER["central_meridian",0], > PARAMETER["scale_factor",1], > PARAMETER["false_easting",0], > PARAMETER["false_northing",0], > AUTHORITY["EPSG","3785"], > AXIS["X",EAST], > AXIS["Y",NORTH]] > > It's EPSG:3785. > > If I run this command: > > gdalwarp -s_srs EPSG:3785 -t_srs EPSG:4326 -of GTiff "input.png" > "output.tif" > > The output file has this extent: > > Upper Left ( -0.6064394, 44.7016078) ( 0d36'23.18"W, 44d42' 5.79"N) > Lower Left ( -0.6064394, 44.6980730) ( 0d36'23.18"W, 44d41'53.06"N) > Upper Right ( -0.5975973, 44.7016078) ( 0d35'51.35"W, 44d42' 5.79"N) > Lower Right ( -0.5975973, 44.6980730) ( 0d35'51.35"W, 44d41'53.06"N) > Center ( -0.6020184, 44.6998404) ( 0d36' 7.27"W, 44d41'59.43"N) > > If I run this command, without -s_srs parameter: > > gdalwarp -t_srs EPSG:4326 -of GTiff "Avenue de Mont-de-Marsan.png" > "male.tif" > > This output has an extent that is different from the previous (look at > latitude): > > Upper Left ( -0.6064394, 44.8937072) ( 0d36'23.18"W, 44d53'37.35"N) > Lower Left ( -0.6064394, 44.8901724) ( 0d36'23.18"W, 44d53'24.62"N) > Upper Right ( -0.5975973, 44.8937072) ( 0d35'51.35"W, 44d53'37.35"N) > Lower Right ( -0.5975973, 44.8901724) ( 0d35'51.35"W, 44d53'24.62"N) > Center ( -0.6020183, 44.8919398) ( 0d36' 7.27"W, 44d53'30.98"N) > > Is it normal?
At least it isn't a bug. Without -s_srs, the CRS info from the png file is used which specifies TOWGS84[0,0,0,0,0,0,0]. When specifying the EPSG code via -s_srs, the parameter +nadgrids=@null is used (in both cases EPSG:3857 and EPSG:3785). In the former case, a "change of ellipsoid" is performed during coordinate transformation (from the sphere to WGS84), in the latter case, this isn't done. The difference is a latitude-only offset of around 20 km. What you usually want is the latter transformation, i.e. no change of ellipsoid. Hermann -- View this message in context: http://osgeo-org.1560.x6.nabble.com/A-strange-thing-with-a-png-geospatial-file-and-gdalwarp-Is-it-a-bug-tp5075948p5075998.html Sent from the GDAL - Dev mailing list archive at Nabble.com. _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
