[Xastir-dev] Re: [Xastir]

[Gerry Creager]

Oh, how I hate it when I find these gems late in the game.

Brad Douglas wrote:
> 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.

Try "Just too much e-mail mixed with poor connectivity on travel?"

> 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.

Essentially correct.  One other minor difference is that NAD83 is 
adjusted every 2-3 years, now, while WGS84 is adjusted annually.  Thus 
you can see an increased error budget over time, but they usually 
resolve to centimeters when the adjustments match up.

> 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."

In point of fact, the North American Datum of 1983 has been consistently 
valid for the North American and Central American landmasses, and 
adjustments are made using those and (as Brad quotes) other sites.

> 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.

Since the datum parameterizations are so similar to WGS84 and the 
ellipsoids are essentially identical, the results, as another poster has 
already mentioned, are effectively identical.  The few among us who'd 
care about the differences are probably using cartesian coordinates and 
raw observables, including carrier phase, anyway.

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

Good paper.

gerry
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
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