[Proj] Proj4 cs2cs Program and the Geocentric Coord System

[Frank Warmerdam]

Ung wrote:
> Hi All,
> 
> I found that running the following command:
> cs2cs +proj=geocent +ellps=WGS84 +to +proj=geocent +ellps=WGS84
> 
> with input "123 655 123" yields "-122.87 -654.29 -122.86"
> 
> As I understand it, the command would project a point from a geocentric 
> frame to the same frame, which should result in a point with the same 
> coordinates. Why the difference in output? I have only recently stepped 
> into the shallow parts of the deep waters of geodesy, having only learnt 
> the term "geodetic" two weeks ago...So while it may not be meaningful to 
> call the command above, I wonder if the output is as expected, and if 
> so, I would definitely appreciate someone setting my understanding right.

Ung,

I reproduced similar results, but I find the results are much more stable
further from the center of the Earth.


cs2cs +proj=geocent +ellps=WGS84 +to +proj=geocent +ellps=WGS84
123 655 123
-122.87 -654.29 -122.86
12300 65500 12300
12300.00        65500.00 12300.00

Based on other discussions on geocentric calculations here on this list
I'm under the impression that some methods become inaccurate very close
to the origin and that is what seems to be happening here.

For the purposes of cartographic mapping I think locations so close to the
geocentric origin are rather irrelavent so I'm not personally too concerned.

Best regards,
-- 
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