[OSRS-PROJ] Geocentric systems and ellipsoids

[Gerald Evenden]

PROJ's derived its name from "projection," that operation relating a  
point
on a sphere or ellipsoid to a two dimensional, cartesian system.  What   
seems to
be discussed below is the transformation between three dimensional
systems and thus has no relation to the projection operation
in the original and definition of PROJ's operation.

Sorry, this mixing of apples and oranges within the OSRS distribution
of PROJ has always been a sore point with me.  It is also the
prime bottleneck in updating OSRS PROJ with the expanded libproj4
material---the later stays within the initial boundaries.

Alas.

On Jan 13, 2004, at 9:03 AM, Frank Warmerdam wrote:

> 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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_____________________________________
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