Le mercredi 26 mars 2014 à 15:48 +0000, Glynn Clements a écrit : Thank you Glynn for the clarification
> The default behaviour is to perform a matrix-based datum > transformation based upon the source and destination datums. > > The nadgrids parameter overrides this. Normally, it is used to specify > a data file containing the datum-transformed positions of a set of > points; this allows for a more accurate transformation than a > matrix-based transformation. yes, here in France we often need to switch between two geodetic systems, the elder being based on the Clarke 1880 ellipsoid. One can choose between a standard datum shift and a grid-based transformation. Many gis users still use the old geodetic system and don't pay attention to the transformation method when it comes to retrieve data from a gps... In the place I live in the Alps, the error observed between standard- and grid-based transformation is about 40 meters in EW direction. VB _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user