If I understand well, the datum shift parameters are required in order to > improve the accuracy of the conversion between 3004 and other RS as ED50 > UTM33, while they are not needed for conversion between 102092 and e.g. > WGS84? >
The shift parameters are alway needed to obtain higher precision for CRS trasformations where the datums are different. When a transform from datum A to datum B is required, Proj4 (the engine underlying SRS and projections) does datum A -> WGS84 -> datum B. The shift parameters (+towgs84) are used my Proj4 to manage the intermediated transformations to (and from) WGS84, which is kind of a "pivot CRS". So, they're should be always employed. 102092 does not define them, that's why you get worst results. giovanni
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