Frank, First of all let me tell you thanks for your work. Thanks is not enough but my poor english would put me in troubles if I tried to say something more complicated. And thank you very much for your answer. Getting help from you from the other side of the ocean (I live in Madrid, Spain) is a honor.
I am not an expert in GIS, but despite of it I am writting a big GIS application. My colleages are supposed to be, but they have been unable to give me a solution to this subject, so, please, be so kind to tell me if this is ok: Ignoring the towgs84 parameters has proved to give bad results when transforming coordinate systems using proj4, even working with standard systems like ETRS89 (used in Spain) or ED50. Giving the correct towgs84 parameter the errors dissapeared completely. Therefore, I decided to write a descendant from OGRSpatialReference whose constructors throw exceptions if the towgs84 parameter cannot be found. For example, there's a constructor that receives an OGRSpatialReference pointer as argument (obtained, for example, from a shapefile, a tif, a LAS file, etc using GDAL). My class builds the underlying OGRSpatialReference with that pointer and then tries to obtain the towgs84 parameter. Somethimes it is present in the OGRSpatialReference object, most doesn't. Then I try to use the "epsg." file if I can figure out the EPSG number. If everything fails then the constructor throws and the program asks for an EPSG to the user. And this is a pity because "almost all" the information I need is supplied by GDAL. The program is worst because it's not able to find out the famous parameter. You say: "If you want to handle a datum shift properly and you don't know how the datum of your coordinate system relates to WGS84 then you are essentially out of luck." The problem is that my application should work with any projection. I cannot make assumptions or make a list of "supported projections". My aim is to be able to get the towgs84 parameter whatever coordinate system the input files are projected into. I often use other programs to display geospatial information, for example Global Mapper. It realizes coordinate systems from (for example) shapefiles without EPSG, and applies transformations correctly. What am I missing? Sorry for my long message, but I've been overwhelmed with this problem for two years, and having you across the line gives me a hope. I'm sure there's a solution because many systems work fine. So thank you again. Sincerely yours, Jose Antonio. -- View this message in context: http://osgeo-org.1560.n6.nabble.com/Unable-to-create-OGRSpatialReference-tp5009903p5009971.html Sent from the GDAL - Dev mailing list archive at Nabble.com. _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev