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[aprssig] Position Ambituity in APRS!

Robert Bruninga bruninga at usna.edu
Wed Jan 9 23:05:46 UTC 2008


> So in your mind, 35  .  N\083  .  W 
> is the same as 3500.00N\08300.00W 
> with a 60 mile ambiguity circle?

Not not at all.  How many times do we have to go over this.

35  .  N and 083  .  W
Is not the same as 3500.00N and 8300.00.00W!

It is 35 degrees north and 83 degrees west and in my opinion
should be displayed at that location with an ambiguity circle of
about 60 miles.  Since it is only good to the nearest degree.

The position 3500.00N/08300.00W is the APRS packet STRING that
specifies a very precise position to the nearest 60 feet.  There
is no way they are the same thing.  

As we are taught in middle school science, when we write a
number, we are conveying both a magnitude, a sign, and a degree
of precision.  There is a difference if we put 10 gallons in our
tank, compared to putting in 10.00 gallons.  The first is an
approximation good to the nearest gallon.  The second one is
measured with an expensive instrument that can resolve volume to
the nearest hundredth of a gallon

APRS in a sense is just trying to preserve those three elements
of a number (including precision) from sender to receiver.  If
someone has a two digit number, it is NOT the same as that
number with a bunch of zeros added to it.  That changes the
precision.

Bob, WB4APR

> 
> >
> > Your software then has to use whatever internal math it
needs to
> > be able to activate the right pixels on the screen.  This
> > includes offsets, scale, mercator projection distrotion
> > elimination, etc.  But in my code, the entry for for "35"
> > degrees is just 35.
> >
> > Good luck.
> > Bob
> >
> >> >> 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.
> >>
> >> Yes, I was thinking in base 100, not 60 when I made up
those
> > numbers.
> >>
> >> [big snip]
> >>
> >> --
> >> William McKeehan
> >> KI4HDU
> >> http://mckeehan.homeip.net
> >>
> >>
> >
> >
> >
> 
> 





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