GPS Coordinate transformations

John Halleck
Thu, 5 Sep 2002 09:02:44 -0600 (MDT)

On Thu, 5 Sep 2002, Olly Betts wrote:

> Date: Thu, 05 Sep 2002 15:08:01 +0100
> From: Olly Betts <>
> To: J Wood <>
> Cc:
> Subject: Re: GPS Coordinate transformations
> Sorry for a very belated reply (nearly 8 months later!)
> On Sun, Jan 13, 2002 at 09:05:36PM +0000, J Wood wrote:
> > 1. Can anyone point me in the direction of the appropriate transformation
> > from GPS Lat, Lon, Height to UK OS grid reference?

   The US National Geodetic Survey (NGS) has the authority and responsibility
   to handle such issues for the US.  They offer many free software packages
   (often with souces) for the PC for such tasks.

   There are issues about what Datum is being used in such conversions.
   (US GPS units sometimes use NAD 83, but some do some other datum)
   If you aren't going for "Exact"  (Sub meter resolution) then such
   issues are moot.  (Yes, I know NAD83 is also the US full Least Squares
   adjustment of the United States... but it also defines a reference
   frame for the coordanates.)  There are also other newer datums that
   don't agree with NAD83.  (Assumed north, the underlying elipsoid used,
   and exactly where the earth's center of mass is, are all items that
   vary between the Reference frames.)

   European Geodesists prefer to use ITRF00 for a reference frame,
   US ones seem to most often use WGS84.

> GEO can do conversions between pretty much any pair of coordinate
> systems.  It's a bit cryptic though - you need to write a script file
> to convert coordinates:
> 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.)

> Which units and how approximate I don't know I'm afraid.

  I don't know either.
  But, there are at least  two meter systematic differences between some
  of the reference frames.

> > 2. What was the software used for the CUCC presentation at Hidden Earth
> > last autumn?
> It was a development version of Survex, using opengl to do the 3d stuff.
> The version you saw was hardwired to view the particular terrain and map
> overlay used and had a number of shortcomings which I carefully avoided
> highlighting in the presentation.
> It showed us that a different approach was needed to get opengl working
> well.  Mark has since restructured Aven in the light of the experience
> and implemented opengl support much more cleanly.  I'm hoping we can get
> out a stable test version of this which does 3d passages by the end of
> this month so people can try it out and comment on it.
> Once that's done, we can incorporate the terrain and draped maps fairly
> easily.
> Cheers,
> Olly
> -- 
> Cave-Surveying