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[aprssig] Position Ambituity in APRS!

Robert Bruninga bruninga at usna.edu
Mon Jan 7 21:36:45 UTC 2008


>> (too bad position ambiguity 
>> is designed for squares corresponding to 
>> fractional degrees of lat/long rather than 
>> a center point with an "uncertainty" radius).

What?  The above is missinformation... Based on erroneous
interpretation of the APRS Ambiguity.  APRS Ambiguity -is- an
approximate position with a surrounding area of ambiguity.

One digit of ambiguity is an approximate location and about a
tenth of a mile ambiguity around it.  Two digits of ambiguity is
an approximate location and about 1 mile of ambiguity...  Three
digits is an approximate location and approximately a 10 mile
ambiguity.  Four digits is an approximate location and about 60
miles of ambiguity.  These are -not- rectangles of LAT/LONG by
any stretch of the imagination.

The way the original APRSdos and other properly implemented
clients display this to end users is a random placement within
the range of ambiguity with a circle showing approximately the
range of that ambiguity.  This way, multiple ambiguous posits in
the same ambiguous area do not display right on top of each
other and hide each other, but instead are randomly placed in
that area of ambiguity so that ther is no missinterpretation to
the recepient.

The circle is *not* a discrete exact boundary.  It is a
graphical display AIDE to the end user that this position could
be anywhere within *approximately* the size of that circle to
the indicated position.

SECONDLY, no ICON is displayed if the ICON is smaller than the
range of the ambiguity on any given zoom.  This is to avoid any
possible missinterpretation as to the exact position of the
symbol.

Any interpretation of LAT/LONG rectangles is incorrect.

Bob, WB4APR

> 
> I'm being _very_ nitpicky here, but they're rectangles in most
> (all?) parts of the world.  Actually, depending on the
projection
> and scale they might be _like_ rectangles but with curved
lines.
> ;-)
> 
> --
> Curt, WE7U: <www.eskimo.com/~archer/>     XASTIR:
<www.xastir.org>
>   "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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