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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