[Proj] cs2cs's +towgs84 usage

[Ilumas]

Dear Frank,

thanks a lot for your prompt assistance!

br,

ilumas



On Wed, Apr 21, 2010 at 7:13 PM, Frank Warmerdam <warmerdam at pobox.com>wrote:

> Ilumas wrote:
> > Dear list,
> >
> > could you kindly confirm that the cs2cs's +towgs84 parameters remain the
> > same for a certain datum regardless of the direction of the
> > transformation. Some people (e.g.
> >
> http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propberly-td3410479.html
> )
> > <
> http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propberly-td3410479.html
> >seem
> > to suggest that the values need to be negated when going FROM wgs84 TO
> > something else, but this seems a bit strange to me. Unfortunately all
> > the examples in the man pages seem to be about changing the coordinates
> > TO wgs84...
>
> Ilumas,
>
> The parameters are always expressed in terms of what it takes to transform
> to WGS84.  If PROJ is transforming from WGS84 it will internally reverse
> the sense of the transformation.
>
> Keep in mind there is also an issue with the handedness of the parameters.
> Some folks, mostly european, express the parameters with a different
> handedness.  The scripts that generate the "epsg" init file already know
> about this case and convert to the usual PROJ.4/North American handedness
> but if you are researching parameters yourself from various sources you
> may need to be aware of it.
>
> Best regards,
> --
>
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam<http://pobox.com/%7Ewarmerdam>
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
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