[GRASSLIST:6820] Re: PROJ trasformation parametrs for datums

[Frank Warmerdam]

On 5/19/05, Luca Penasa <luca.penasa at email.it> wrote:
> Im working with Grass to georeference some maps. iv created a new
> location where to put georeferenced data. Its a Gauss-Boaga loaction and
> into the poj_info's file i see:
> towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68
> I know this is the correct towgs84 parameter but i cant understand how
> does it work. I know that it's used for the datum trasformation, but i
> also know that the gauss-boaga system is based on the internation
> ellipsoid, so...the wgs84 parameter is used for the trasformation from
> which ellipsoid?

Luca, 

The parameters are used to apply a "datum shift" - somewhat more than
just the conversion between ellipsoids is going on.  The transformation
is how to get from the datum+ellpisoid in question to the WGS84 datum.

The actually parameters are x/y/z shifts, x/y/z rotations and an overall
scaling value.  Note that this transformation is only used if you convert
between datums. 

Best regards,
-- 
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