[OSRS-PROJ] Re: [GRASSLIST:4621] datums transormation and dX dY dZ values

Markus Neteler neteler at itc.it
Fri Oct 4 05:37:14 EDT 2002

On Thu, Oct 03, 2002 at 08:28:36AM +0200, giorgio wrote:
> Hi all
> I'm not a very expert in datums shift and projections so, eventually, I'm sorry for my basic question.  
> I have a map where are drawed the grids in two different datums Gauss
> Boaga (Rome 40 datum) and UTM (eur50 datum). I'm not sure that parameters
> (dx dy dz) for the datum shift indicated in the internal datum tables or
> epsg or NIMA tables are very good for my little area of work so I would
> like know if is it possible desume (and in which way) my user defined dx
> dy dz for the molodensky datum shift R40 --> ED50 directly from the two
> grids drawed onto map or if I need of other parameters?
> Many thanks in advance.

Hi Giorgio,

I think that

cs2cs +init=epsg:26591 +towgs84=-225,-65,9 +to +init=epsg:23032

should do the job (R40 --> ED50).

Note that epsg:26591 does not contain the Rome datum, that's why
I used '+towgs84=-225,-65,9' here.

The underlying values are stored in the PROJ4 source code (nad/epgs file)
or whereever you installed PROJ4.

Note also that GRASS's [mrs].proj do not (yet) perform datums shift except
for NAD, at time (I hope) the code upgrade is ongoing within the
GRASS 5 libraries.

Hope this helps, please verify above with some known points
and post the results.

Best regards,

 Markus Neteler
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