GPS Coordinate transformations

[John Halleck]

> [...]

> > As others suggested, you could just type coordinates into a GPS and read
> > them back in the other system.  However I've read that some GPS units use
> > simplified coversion formulae which only approximate the correct answer.
> 
>   And most don't say what Datum is being used for the conversion (which
>   makes an accurate conversion look different on units from different
>   companies using different reference datums.)  What I'm saying here is
>   that differences you see may not be issues of simplified conversion, but
>   may be differences between the underlying reference frame that they
>   choose to use.
> 
>   If you are interested I can point you to good discussions on the topic
>   (And NGS has some papers on line about it.)

  Note also that the "correct answer" is a fiction (approximation).
  There are current debates (Usually involving the US High Accuracy Refrence
  Network (Harn) about WHERE the center of mass of the earth is...  Arguments
  about values that vary only by a few milimeters.   If you were going
  for "CORRECT" you'd also have to take into acount the solid earth tides
  (also a few mm most locations).

  Anything practical in surveying is an approximation.  Any measurement is
  clearly an apporximation.

  EVERYTHING is an apporximation of the "correct" answer.  The question
  usually boils down to how close an approximation one wants to deal with.

> [...]

  And I stand corrected, there are actually 15 meter differences between
  some of the datums for some places on the planet.  (So the "correct"
  answer depends on your underlying model.)