[Xastir-dev] Re: [Xastir] (was: Is traffic encryption possible?) grids/datum

[Brad Douglas]

On Thu, 2007-11-15 at 06:54 -0800, Curt wrote:
> The white error boxes are useful to see where a station _might_ be,
> instead of where they're telling us they are.  Without the datum
> conversions though we'd be reporting the position incorrectly, so we
> might as well keep the larger error box until we are converting the
> datums.
> 
> Second thought:  If the datum is reported as WGS84 we could reduce
> the size of the error box 'cuz we wouldn't need to do a conversion.
> 
> What about NAD83?  I know that's very close to WGS84, but that's
> only for NA right?  If someone on the other side of the world was
> reporting NAD83 datum then the error between it and WGS84 could be
> significant, right?  Is NAD83 available world-wide?  Could I be in
> Japan or Europe and have a Garmin or Magellan GPS read out my
> position in NAD83?

I missed most of the thread due to "erroneous" subject line.  If the
topic changes in a conversation, I generally do what I did to this one.
Not such a big deal for Xastir, but extremely beneficial on larger
projects. YMMV.

Indeed, NAD83 and WGS84 are nearly identical.  NAD83 uses the GRS80
ellipsoid and WGS84 uses the WGS84 ellipsoid.  Initially, they were
deemed to be identical, but some approximations, calculation methods,
and other error factors yielded small differences.  The rule of thumb is
if your error is >= 3m, they are identical.

This should help answer the question of validity[1]:
"Some investigators have suggested that a difference between NAD83 and
WGS84 is that NAD83 is valid only within North America, while WGS84 is
valid worldwide. This is incorrect.

If one has an accurate method of extending NAD83 outside of North
America, then there is no reason not to do so, nor is there any reason
to think that the resulting coordinates would differ from WGS84
coordinates. In fact, as part of the NAD83 adjustment, Doppler
observations were used to extend the datum outside of the contiguous
survey networks to isolated areas such as Greenland, Puerto Rico, and
Hawaii."

I don't believe NAD83 has been extended globally.  I'd be curious to see
what shift European users report switching between the two on a GPS.

[1] NOAA Professional Paper #2: "North American Datum of 1983"


-- 
73, de Brad KB8UYR/6 <rez touchofmadness com>