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[aprssig] Position Ambituity in APRS!

Steve Dimse steve at dimse.com
Tue Jan 8 16:49:36 UTC 2008


On Jan 8, 2008, at 11:23 AM, William McKeehan wrote:

> Let me jump in and ask a really simple question.
>
> 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?
>
> So that would be the possible area (whatever geometric shape it may  
> be) that I
> can be in, right?

You are thinking like an engineer, not a kludger.

What Bob intends by this report is not that you are located within the  
polygon (which should indeed be indicated by shading the polygon), but  
that your best guess is your position is there, but you only know to  
an error of 60 miles. Bob shows this by drawing a 60 mile radius  
around 35.5/83.5. An engineer would display this as a polygon 60 miles  
LARGER than the polygon 35-36 by 83-84, but then again an engineer  
would have designed the protocol to send a precise best guess along  
with an uncertainty value, so the display actually meant something.

Steve K4HG





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