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

[Paolo]

I realize I still need to learn much.
Thanks for taking time to explain.
Paolo

Il 07/11/2013 14:59, G. Allegri ha scritto:
>
>
>     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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