[Qgis-user] Difference: EPSG 3004 - EPSG 102092

[G. Allegri]

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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