[aprssig] The best resolution of position from APRS

scott at opentrac.org scott at opentrac.org
Tue Jan 3 15:55:01 EST 2006

> Would it not make sense that THE best accuracy would be 
> obtained by sending
> the RAW GPRMC string in its entirity ?

Depends on your input format.  Garmins (in their native binary protocol)
send their position as double-precision floating point, in radians.  I
haven't bothered to figure out what sort of spatial resolution that works
out to, but it's absurd.  The waypoint integer format (which happens to be
the same as the OpenTRAC position format) is 32 bits and works out to around
2 cm resolution.

However, I'm not aware of any APRS tracker devices, save one prototype on my
workbench, that speak Garmin binary.  I'm not even sure if any desktop APRS
apps do.

The newer NMEA format supports extra precision - though I understand that
caused some trouble with the Kenwoods when it started appearing.  I don't
know if that was a problem only for their local GPS connection, or for NMEA
reception on-air as well.

But before you go nuts trying to get your icon to show up in the right place
on your maps, take some time to learn about your maps.  Are they in the
Australian AGD66 datum?  If so, you need to convert the registration point
coordinates to WGS84.


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