[Proj] Proj4 cs2cs Program and the Geocentric Coord System

Gerald I. Evenden geraldi.evenden at gmail.com
Wed Feb 4 13:26:36 EST 2009

On Wednesday 04 February 2009 11:58:56 am Karney, Charles wrote:
> 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 Frank points out many algorithms to deal with geocentric coordinates
> break down within 50km of the center of the earth.  If you need to do
> such conversions accurately use ECEFConvert from
>   http://charles.karney.info/geographic/
> After compiling ECEFConvert, you can do
>  echo 123 655 123 | ./ECEFConvert -r | ./ECEFConvert
> which returns
>  122.9999999999995 654.9999999999982 123.0000000002855

Are we not back to the same business associated with the dead horse issue of 
TM (Transmogrification Meditation): unneeded accuracy and extended 
computational range.  The real use of XYZ<->geographic conversion is in the 
region withing a few kilometers of the Earth's generalized elliptical shape 
for most of the readers of this list.

The old methods, with all their clay feet, seem to function quite well in this 
region and certainly spend a lot fewer electron dollars in their computation.  
Thus lets simply put a careat on these simpler proceedures to warning those 
who drill holes into the molten iron core of the earth or are flying their 
space ships beyond the local solar system may need better better methods.

Certainly, those with too much time on their hands should announce their 
efforts which produce a googleplex of significant digits and function over 
the range from the microworld of quantum mechanics to that of intergalactic 
space  travel.  We can all say "Wow!"  But beyond that ...

That is just the voice of someone who spent part of his career seeking faster 
ways of doing things; part of which was looking at accuracy needs and seeking 
approximations of hyperaccurate but computational intensive procedures that 
met the real needs of a probem and did the computations many times faster.

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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