[GRASS-user] projection issue

[Vincent Bain]

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