[GRASSLIST:4622] Re: datums transormation and dX dY dZ values

[Eric G. Miller]

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.  Giorgio.  

They are the amount of translation (in meters) to apply in the
respective x, y, z directions relative to the WGS84 geocentric
ellipsoid/datum.  NIMA pub TR8350.2 (available for download in PDF)
will explain this better.  With the origin located at the center 
of the earth's mass, the axes are approximately positive Z (north pole),
positive X (greenwich meridian at equator), positive Y (90 degrees at
equator).  It's a "right-handed" three dimensional cartesian coordinate 
system. So, it's a little bit complicated to derive the shift values;
however, both Rome 1940 and European 1950 have datum shift
transformation paramaters relative to WGS84 (Rome 1940 is listed as 25
meters error in each of the three directions, pretty large error...).

Don't know if that helps any...

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