[aprssig] Position Ambituity in APRS!

Scott Miller scott at opentrac.org
Mon Jan 7 16:45:13 EST 2008

So let's say you're at 35 58.12 N, 119 45.70 W.  Does APRSdos round that 
to the closest degree (36 N, 120 W) or does it just truncate like 
everything else does?  If it's just truncating, then I think a rectangle 
is a more accurate representation than a circle centered on the given 
point - in this case  I don't think the actual point would even be IN 
the circle.

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.  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.


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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