[OSRS-PROJ] Geocentric systems and ellipsoids

[czaht at ocag.ch]

Frank,

Thanks for your swift response. I actually already use it the way you
suggest, and now
I can do it with a good conscience as well ;-)

/Thomas

-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Dienstag, 13. Januar 2004 15:04
To: osrs-proj at remotesensing.org
Subject: Re: [OSRS-PROJ] Geocentric systems and ellipsoids


czaht at ocag.ch wrote:
> Hi,
> 
> I have a question regarding the handling of geocentric system libproj. 
> It seems that I need to define an ellipsoid together with a geocentric 
> system, in my case the WGS84 Cartesian system. Why?
> In my opinion, a geocentric system is defined by the centre of the earth
and
> a rotation relative to the WGS84 Cartesian system.
> 
> Could someone shed a bit of light on this matter?

Thomas,

The geocentric support in PROJ.4 (as opposed to Gerald's newer libproj4) is
quite new and somewhat experimental.  The need for an ellipse is really an
artifact of internal assumptions of the library before geocentric support
was added.

Just select the same ellipsoid/datum as the output coordinate system you
will be transforming to (or the input system you are transforming from). You
are correct that the geocentric coordinates define the location without any
real need for an ellipse.

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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