[Proj] cs2cs's +towgs84 usage

Fri Apr 23 14:02:00 EST 2010

Interesting to note that the "European" convention is left-handed in geodetic 7-parameter transformations just as it is left-handed in their photogrammetric transformations and also was with their old European opto-mechanical stereoplotters.

The "American' convention is right-handed in geodetic 7-parameter transformations just as it is right-handed in their photogrammetric transformations and also was with their old American opto-mechanical stereoplotters.

Both sides of the Atlantic have been pretty consistent for over a century ... that's not arbitrary to me.

Clifford J. Mugnier, C.P., C.M.S.
Past National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing 
and 
Chief of Geodesy,
Center for GeoInformatics
Department of Civil Engineering 
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY 
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic] 
Voice and Facsimile:  (225) 578-8925 [Research] 
Honorary Life Member of the 
Louisiana Society of Professional Surveyors 
Member Emeritus of the ASPRS 
Member of the Americas Petroleum Survey Group
======================================================
http://www.asprs.org/resources/GRIDS/
======================================================

________________________________

From: proj-bounces at lists.maptools.org on behalf of strebe
Sent: Fri 23-Apr-10 13:47
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] cs2cs's +towgs84 usage

On Apr 23, 2010, at 2:37:28 AM, "Mikael Rittri" <Mikael.Rittri at carmenta.com> wrote:

	The terms "left-handed" and "right-handed" are
	just as bad. 

Are they? Those terms are standard in mathematics, where "Position Vector Transformation" and "Coordinate Frame Rotation" are unknown. There is no ambiguity in their meaning or usage.

(Not that I would advocate using the terminology in this context, since EPSG's terminology is Position Vector Transformation versus Coordinate Frame Rotation. But at least it is not arbitrary like "American" versus "European".)

Regards, 
- daan Strebe 

On Apr 23, 2010, at 2:37:28 AM, "Mikael Rittri" <Mikael.Rittri at carmenta.com> wrote:

From:	 "Mikael Rittri" <Mikael.Rittri at carmenta.com>	
Subject:	 Re: [Proj] cs2cs's +towgs84 usage	
Date:	 April 23, 2010 2:37:28 AM PDT	
To:	 "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>	
	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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