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[aprssig] Local Traveler Repeater Freq Objects

Curt Mills archer at eskimo.com
Thu Feb 1 03:38:26 UTC 2007


On Wed, 31 Jan 2007, Robert Bruninga wrote:

> That is why APRS has always supported position ambiguity.  It
> has ambiguity settings of 0.1, 1 and 10 miles.  Just input the
> repeater object with an ambiguity of 1 mile.  The format for
> ambiguity is simply placing SPACE characters in the least
> significant bytes.  
> 
> Instead of 3859.11N/07629.11W which is an exact position to 60
> feet, instead enter 3859.  N/07629.  W and it will yield only a
> 1 mile accurate position.  Unfortunately many folow-on
> APRSclones never implemented this fundamental principle and will
> plot the station INCORRECTLY at 3859.00N/07629.00W which is
> WRONG.
> 
> Nothing should be plotted there, unless the station is on a map
> scale such that the ICON itself is larger than 1 mile.  Then it
> can be plotted.  Below that mapscale, it only plots as a circle
> of the size of the ambiguity.  This way there is absolutely no
> potential for missinterpretation on the receive end.
> 
> Only a few APRS clients do it properly.

"Properly" (mathematically) would be as a rectangle of 0.11, 1.15,
11.51, or 69.09 miles.

If the clients plot them as circles or ellipses they're doing it
incorrectly, but we've had this discussion before.  Xastir plots
such things as rectangles, with the symbol at the center and lines
going from the symbol to each corner.  They are very unmistakable
from anything else on the map, and we know the symbol is _somewhere_
inside that rectangle or along an edge.

-- 
Curt, WE7U.				archer at eskimo dot com
http://www.eskimo.com/~archer
  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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