Thanks Frank, I think I found a bug in OGRSpatialReference::IsSameGeogCS(), which is called by IsSame(). The code looks for identical "DATUM" values, but doesn't check for DATUM="unknown". In my test, even thought the spheroid values are identical the test fails.
Filed a bug report and one-liner fix http://trac.osgeo.org/gdal/ticket/4281 I'm not confident in commiting myself before confirmation that it's ok. if( pszThisValue != NULL && pszOtherValue != NULL + && !EQUAL(pszThisValue,"unknown") && !EQUAL(pszOtherValue,"unknown") && !EQUAL(pszThisValue,pszOtherValue) ) return FALSE; regards, Etienne On Fri, Oct 7, 2011 at 9:49 PM, Frank Warmerdam <warmer...@pobox.com> wrote: > Etienne, > > You should in theory just use OGRSpatialReference::IsSame() method to > see if two coordinate systems are essentially equivelent, but it is not > really all that perfect. > > Best regards, > > > On Fri, Oct 7, 2011 at 4:36 PM, Etienne Tourigny > <etourigny....@gmail.com> wrote: >> Hi all, >> >> I would appreciate any hints as to how I can compare 2 >> WKT/OGRSpatialReference objects. >> >> My goal is to make sure that a more detailed WKT (with authority codes >> and explicit datum) corresponds (numerically) to a simpler one, such >> as in the following examples. >> >> I understand that the oSRSGDAL.exportToPrettyWkt() function can remove >> AXIS, AUTHORITY and EXTENSION nodes, but how can I guarantee that the >> PARAMETER nodes are in the same order? Does FixupOrdering() solve that >> problem? >> >> Furthermore, How could I compare the datums? >> >> >> >> >> CF=[PROJCS["unnamed", >> GEOGCS["unknown", >> DATUM["unknown", >> SPHEROID["Spheroid",6378137,298.2572221010002]], >> PRIMEM["Greenwich",0], >> UNIT["degree",0.0174532925199433]], >> PROJECTION["Lambert_Conformal_Conic_2SP"], >> PARAMETER["standard_parallel_1",-18], >> PARAMETER["standard_parallel_2",-36], >> PARAMETER["latitude_of_origin",0], >> PARAMETER["central_meridian",134], >> PARAMETER["false_easting",0], >> PARAMETER["false_northing",0]]] >> >> GDAL=[PROJCS["GDA94 / Geoscience Australia Lambert", >> GEOGCS["GDA94", >> DATUM["Geocentric_Datum_of_Australia_1994", >> SPHEROID["GRS 1980",6378137,298.2572221010002, >> AUTHORITY["EPSG","7019"]], >> TOWGS84[0,0,0,0,0,0,0], >> AUTHORITY["EPSG","6283"]], >> PRIMEM["Greenwich",0], >> UNIT["degree",0.0174532925199433], >> AUTHORITY["EPSG","4283"]], >> PROJECTION["Lambert_Conformal_Conic_2SP"], >> PARAMETER["standard_parallel_1",-18], >> PARAMETER["standard_parallel_2",-36], >> PARAMETER["latitude_of_origin",0], >> PARAMETER["central_meridian",134], >> PARAMETER["false_easting",0], >> PARAMETER["false_northing",0], >> UNIT["metre",1, >> AUTHORITY["EPSG","9001"]], >> AUTHORITY["EPSG","3112"]]] >> >> >> >> Another option is exporting to PROJ.4 strings, but there are the >> +towgs84 that shows up , plus some differences in +datum vs. +ellps. >> >> e.g. >> >> CF=[+proj=utm +zone=55 +ellps=WGS84 +units=m +no_defs ] >> GDAL=[+proj=utm +zone=55 +datum=WGS84 +units=m +no_defs ] >> or >> CF=[+proj=lcc +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=134 +x_0=0 +y_0=0 >> +ellps=GRS80 +units=m +no_defs ] >> GDAL=[+proj=lcc +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=134 +x_0=0 >> +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs ] >> >> >> >> many thanks, >> Etienne >> _______________________________________________ >> gdal-dev mailing list >> gdal-dev@lists.osgeo.org >> http://lists.osgeo.org/mailman/listinfo/gdal-dev >> > > > > -- > ---------------------------------------+-------------------------------------- > I set the clouds in motion - turn up | Frank Warmerdam, warmer...@pobox.com > light and sound - activate the windows | http://pobox.com/~warmerdam > and watch the world go round - Rush | Geospatial Software Developer > _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
