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[aprssig] Position Ambituity in APRS!

Robert Bruninga bruninga at usna.edu
Tue Jan 8 02:12:31 UTC 2008


> So let's say you're at 35 58.12 N, 119 45.70 W.

Woaha.  If you are at that location, then trnsmit that location.
If you are not sure about the precise digits, then transmit what
is closest to where you think  you are.

In the above case, you appear to be a little bit north of 35 deg
58m North.  And just a little east of 119 deg 46m w.  So in this
case, if you do not have any better information, then do not
transmit any more precision than you have.  That is what APRS
Ambiguity is all about.  You transmit your position only to the
degree you best know it, or only to the degree you want others
to see it. you transmit 3558.  N/11946.  W.  


>  Does APRSdos round that to the closest degree (36 N, 120 W)

If someone wants to turn a PRECISE position into an ambiguous
position, yes, then of course, they have to round up or down.

> I'd rather have an estimated exact location, 
> and a 50% CEP radius or similar metric for 
> ambiguity.  That'd fit nicely with vicinity 
> tracking - the point would be the location 
> of the receiving digipeater (which is 
> precisely known) and the radius would be 
> proportional to the range of the digi.  

No, you cannot place people randomly in that great a range with
icinity tracking because they would be all over the city.  To
plot vicinity plots, my recommendation is to place them randomly
within one mile of the digi (so they are not all on top of each
other) but then give each one an ambiguity level (1, 10, or 60)
that is relative to the PHG circle of the digi.  Maybe that is
what you said anyway...

> As it is, if your digi is right in the middle 
> of the rectangle, its only option is to provide 
> a corner of that rectangle as the starting point, 
> which is going to be less accurate.

That is why everyone must get away from thinking of precise
rectangles when talking about ambiguous positions.  Yes, plot th
evicinity plot within one mile of the didi (again, do not plot a
call, just a question mark on the map).  It is only a marker for
a station heard via that digi that has not indicated his
position.  His call is only referenced to that question mark
symbol if one does a SEARCH or FIND on that callsign, then that
question mark is the indicator of that call.  Otherwise it is
just a map marker...

The call can appear when the question mark is sselected...

Bob

> Scott
> N1VG
> 
> Robert Bruninga wrote:
> >>> (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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> >> 
> > 
> > 
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> 
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