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[aprssig] Position Ambituity in APRS!

Robert Bruninga bruninga at usna.edu
Tue Jan 8 18:44:03 UTC 2008


> 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,

Which would be pretty silly wouldn't it...

Nit pickers note:  

If you go back all the way through the last 10 years of this
debate, I have always referred to these degrees of precision as
60 mile, 10 mile, 1 mile and .1 mile ambiguity (nautical miles).
Because this is exactly what they are.  In Latitude, this
applies everywhere on earth from the equator to the poles.

This was always interpreted in the original APRSdos as a 60, 10,
1 and 0.1 mile circles of ambiguity.  That is how I intended for
them to be drawn. The spec does not fully convey this background
and gives the reader the incorrect interpretation in this
sentence: "The station may be located anywhere in a bounding
box..."

I wish I had caught the potential error of that sentence 10
years ago.  But I was working extra long hours   building a
satellite while an independent technical editor was trying to
get APRS down into a spec.  It was clear how APRSdos displayed
this as circles at the time, and so I trusted he would properly
capture the escence of that system into the spec.

But since I was not allowed to include any descriptions of how
things were to be displayed on receipt, my recommendations in
this paragraph were omitted.

> 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.

The engineer refused to misslead all viewers by sending a
"precise best guess" when PRECISION was not avaialble.  Instead
the engineer designed APRS to send the same lack of precision
from the sender to the receiver so that there could be no
mistake in interpretation.

Bob, WB4APR





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