[aprssig] APRS LAT/LONG standards (Geodesy Part 1)

[Gerry Creager N5JXS]

OK, since this is wandering back into the field of geodesy, I suppose 
it's time to come back out of the cave, and shed a little light on some 
of these topics.

VE7GDH wrote:
> Andre PE1RDW schreef:
> 
>> you mean 400 grad format? I have been using that for decades in my
>> profecional carreer along with co-ordinates in meters from a point just
>> above paris, known as the dutch "rijks driehoeks net".
> 
> etc.
> 
> I'm not familiar with "rijks driehoeks net" but it sounds like it has a lot
> going for it once you know how it works.

This sounds suspiciously like a national grid system.  For sufficiently 
small areas, planar grid systems are sufficiently accurate for surface 
geodetic calculations.  There's little distortion in 2 dimensions over a 
relatively small area of hundreds of kilometers per dimension.

>> Talking about gps standards, what datum are we using because in europe
>> wgs is default but I think the american continent has a different datum
>> as has  asia.

Stephen Smith comments in another post that WGS84 is the World Geodetic 
System of 1984.  Its origins were derived in the need for a system for 
GPS use that was reasonably accurate for use, with GPS, anywhere in the 
world.  Pretty well, they've succeeded.

Datums are generally local derivations.  Someone needs a system for 
local measurement on a non-planar system, and so derives a datum that's 
particularly suited for a given area.  the point of beginning usually 
resides within that region, and the ellipsoid is tweaked to reduce 
distortion and to place the surface of the ellipsoid as close as 
possible to the local surface of the earth.  The WGS84 ellipsoid is 
based on the GRS80 ellipsoid used by NAD83, is well suited for the 
intended purpose.  Overall, there is less than a 2m difference at the 
ellipsoidal surface between WGS84 and GRS80 paramerizations.  The 
primary difference between NAD83 and WGS84 is the frequency of 
readjustment... NAD83 is readjusted about once every 3-5 years while 
WGS84 is officially readjusted annually.  The WGS84 tweaks are small... 
think tectonic motion... and most folk never hear about them.  Thus, 
most of the GPS receivers providing autonomous fixes today are really 
out of spec with regard to the datum, BUT, and this is important, the 
amount of error caused by this failure to do the book keeping is well 
below the error budget as observed for today's receivers in Coarse 
Acquisition mode... about 6-10 meters horizontal and 9-15m vertical.

> In BC Canada, the topo maps that I have worked with up until now have been
> NAD27... North American Datum based on 1927 surveys. I understand that new
> maps (at least here) will be WGS84 or possibly NAD83 instead when they 
> are printed. Around here, there is something like 100 metres difference 
> east-west and about 200 metres north-west. When I was with search and 
> rescue, one of the things we had to deal with was that the helicopters 
> had their GPS receivers set to WGS84. This is going back about 6 years, 
> so between that and selective ability, there was the potential for some 
> fair sized errors. As I mentioned in my earlier message, it was easy to 
> change the format in the GPS if needed, and it was also easy to change 
> the datum in the GPS if I needed to translate something.

Last I heard, the next generation of your maps will be in NAD83.  The 
NAD83 datum was a North American collaborative effort.  It's generally 
valid (and 'tuned') for North and Central America.

More in Part 2
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
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Office: 903A Eller Bldg, TAMU, College Station, TX 77843