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[aprssig] Position Ambituity in APRS!

William McKeehan mckeehan at mckeehan.homeip.net
Tue Jan 8 22:05:30 UTC 2008


I'm not fighting about how things should be, I'm just trying to understand
your thought process, please bear with me thru a few more questions...

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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