[Proj] cs2cs's +towgs84 usage

[Ilumas]

Thanks Mikael,

what if the Method is:

 Method [Geocentric translations (geog2D domain)]

and the Sign reversible field value is YES for all three parameters?


best regards,

Ilumas






On Fri, Apr 23, 2010 at 11:37 AM, Mikael Rittri
<Mikael.Rittri at carmenta.com>wrote:

>
> About the rotation sign conventions, I would
> add that calling the two conventions "European"
> and "American" is not helpful, since there is not
> much agreement about which convention is which!
> (Both conventions are used in Europe, and there
>  is no obvious geographic pattern.)
>
> The terms "left-handed" and "right-handed" are
> just as bad.
>
> It is better to say that cs2cs uses the sign
> convention known as the
>
>     Position Vector Transformation.
>
> If you look up the +towgs84 parameters on www.epsg-registry.org
> (filter by Type: Coordinate Transformation), you will see the text
>
>  "Warning: CARE! Ensure application uses this method! See Guidance Note
> 7-1"
>
> above the parameter values.  This means that you
> should scroll down _below_ the parameter values,
> where you find the Method.  If it says:
>
>  + Method [Position Vector transformation]
>
> you can use the parameters directly in cs2cs.
> But if it says
>
>  + Method [Coordinate Frame Rotation]
>
> you should reverse the signs of the three
> rotation parameters.
>
> Best regards,
> --
> Mikael Rittri
> Carmenta AB
> SWEDEN
> www.carmenta.com
>
> -----Original Message-----
> From: proj-bounces at lists.maptools.org
> [mailto:proj-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
> Sent: den 21 april 2010 18:14
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] cs2cs's +towgs84 usage
>
> 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-propberl
> > y-td3410479.html)
> > <http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propber
> > ly-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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