[Proj] cs2cs's +towgs84 usage

[Frank Warmerdam]

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,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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