[aprssig] APRS LAT/LONG standards

Curt Mills archer at eskimo.com
Sun May 29 18:14:57 EDT 2005

On Sat, 28 May 2005, VE7GDH wrote:

> You are probably familiar with the UTM that I mentioned. Universal
> Transverse Mercator is based on 60 zones that are 6º (latitude) wide and
> with the "easting" and "northing" in metres for the coordinates. It is a
> very easy one to use with paper maps that have the UTM grid printed on them.

Careful there.  In our area of the woods the 6º statement is
accurate.  In Europe and north of there you have to specify whether
you're using the regular 6º grid or the NATO grid, as some of the
zones are different widths in that region.  Weird, but that's the
way they do it.  There are grids of 3º, 6º, 9º, and 12º in that
region.  A lot of the docs on the web about UTM gloss over this.

> For APRS, the "standard" is WGS84. In spite of that, if you are referencing
> maps calibrated with another datum, I would think you would be better off
> using that same datum in the GPS. I wasn't sure if a GPS always puts out
> WGS84 in NMEA mode or if it puts out a position based on the datum that the
> GPS is set to.

Depends on the GPS.  Most Garmins that I've used change the NMEA
serial stream datum as you change the displayed datum.

> I just did a search and came across a page that said "older
> GPS Receiver firmwares always output WGS84, independent of the chosen map
> datum..." so I assume that means that newer receivers might put out
> coordinates based on whatever datum the receiver is set for. If this is the
> case, setting your GPS for APRS to your local datum would mean that there
> would be errors in your position on the APRS-IS & findu.com etc. if that is
> the case. When it comes down to it, the errors will be small if you are
> looking at a map the size of a city. However, if you were zoomed down to
> look at an area of just a few city blocks, it could be significant.

Correct.  It sucks, but that's what we have.

Curt, WE7U.				archer at eskimo dot com
  Lotto:  A tax on people who are bad at math. - unknown
Windows:  Microsoft's tax on computer illiterates. - WE7U.
The world DOES revolve around me:  I picked the coordinate system!"

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