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[aprssig] Position Ambituity in APRS!

Robert Bruninga bruninga at usna.edu
Tue Jan 8 23:01:25 UTC 2008


> please bear with me thru a few more questions...
> So how do you define "approximately at 35N 
> and 83W"? Seems to me that's a well
> defined area as described by my box.

No, it is preferred to be a circle of the right ambiguity size
centered on approximately 35N and 83W.  The word "approximately"
is because it should be randomized there.  Probably within say
half or less of the radius of ambiguity...

> So if my APRS packet says 35  .  N\083  .  W, 
> I could actually be at 3467.85N\08272.45W ?

Unfortunately, neither of those is a valid posit.  Minutes can
only be 00 to 59.  But assuming you meant 3457.85N\08253.45W,
then yes, the coordinates of  35N and 83W are closest to that
position.
 
> On Tue, January 8, 2008 2:04 pm, Robert Bruninga wrote:
> >> If I transmit my position to degree precision
> >> (for example 35  .  N\083  . W), to plot that
> >> "location" on a map, I would mark an everything
> >> from 3500.00N to 3559.59N and 08300.00W to
> >> 08359.59W, right?
> >
> > No not at all.  That is the incorrect BOX interpretation
which
> > was never intended.
> >
> > The original APRSdos plots a circle of 60 miles radius
> > approximately at 35N and 83W.
> 
> So how do you define "approximately at 35N and 83W"? Seems to 
> me that's a well
> defined area as described by my box.
> 
> > A sufficient random offset from
> > that location would be added so that if there were several
> > stations all reporting that same position ambiguity, they
would
> > not all appear as one circle but would all be visible.
> >
> > The circle is not a precise boundary of ambiguity.  It is a
> > REPRESENTATION to the VIEWER that that station's position is
not
> > well known and its ambiguity is on the order of the size of
the
> > circle presented.
> 
> Sounds like what you really need is a way to say I'm here 
> 3500.00N\08300.00W
> or within 60 miles. And to convey that message, you are 
> saying I'm at 35  . 
> N\083  .  W. Is that right?
> 
> >
> > If we had just been allowed to put those words into the
spec, we
> > would not still be having different interpretations for the
last
> > 10 years.
> >
> >> So that would be the possible area (whatever
> >> geometric shape it may be) that I can be in,
> >> right?
> >
> > Sorry, wrong.  It is exactlly that missinterpretation that
we
> > are trying to avoid.
> 
> So if my APRS packet says 35  .  N\083  .  W, I could actually
be at
> 3467.85N\08272.45W ?
> 
> >
> > It is not a box.  It is a circular image of a large amount
of
> > ambiguity on the order of a circle of radius appropriate to
the
> > missing digits of precision.
> >
> > If you want to send a SEARCH box or other precisely bounded
> > area, then send an AREA object instead.
> >
> > Bob, WB4APR
> 
> I appreciate your patience and look forward to the answers.
> 
> William
> KI4HDU
> 





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