Le vendredi 19 octobre 2012 16:41:14, jcurru a écrit : > Hello! > > I have an old problem with TOWGS84 and OGRSpatialReference. Most likely > it's because I'm no expert in GDAL. It is important for my project, and > limits many of its capabilities. The fact is that, for unknown reasons (to > me), proj4 does not do well conversions between coordinate system if there > are not a TOWGS84 nodes inside the OGRSpatialReference(s). > > First of all: is this so? Do I really need that node? If not properly > added, clearly some conversions fail.
The best is to point you to some reference documentation : http://trac.osgeo.org/proj/wiki/GenParms#towgs84-DatumtransformationtoWGS84 http://docs.geotools.org/stable/javadocs/org/opengis/referencing/doc- files/WKT.html#TOWGS84 > > I continue: Imagine getting an OGRSpatialReference a shapefile or a TIF. To > assign the node TOWGS84, what I do is look into the file "EPSG", found in > many sites related to GDAL, that contains a list of EPSGs with > corresponding parameters. Knowing the EPSG, I can generate the TOWGS84 > node smoothly. > > The problem comes when I do not know the OGRSpatialReference’s EPSG, > something very common: AutoIdentifyEPSG () gets nothing most of the time, > at least with the systems in which I work. > > So what to do then? I need the TOWGS84 node but, with my knowledge, I can > only get it from the EPSG. This is indeed a difficulty that is not easily solved. The WKT in .prj that come alongside with shapefiles is in ESRI WKT that stripps TOWGS84 information. And most geotiff don't include TOWGS84 : the GeoTIFF format has only been extended very recently to convey TOWGS84 : http://trac.osgeo.org/geotiff/wiki/TOWGS84GeoKey And indeed AutoIdentifyEPSG() only works a few common cases, mainly EPSG:4326, EPSG:326XX & EPSG:327XX (WGS84 UTM projections), EPSG:267XX (NAD27 UTM projections) and EPSG:269XX (NAD83 UTM projections). _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev