[Proj] cs2cs's +towgs84 usage

[Mikael Rittri]

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
and watch the world go round - Rush    | Geospatial Programmer for Rent

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