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Position Ambiguity [was] Re: [aprssig] findu.com Location off by 100 miles.

Robert Bruninga bruninga at usna.edu
Tue Jun 6 18:33:15 UTC 2006


>>> kc2mmi at verizon.net 06/06/06 1:48 PM >>>
>Within APRS, if I understand the spec,...
> ambiguity is presented by simple truncation [of]
>the rightmost digits.  Suppose, instead of truncating them, 
>we filled those positions... by "A" instead of a number. 
>"A" for "Ambiguity here!" 

That is exactly what APRS does!  APRS transmits a SPACE
character to represent the degree of ambiguity.  That is
just like you write it with a pencil.  The APRS spec says
to transmit only the digits that you know, and transmit
SPACES for the digits you dont know:

DDMM.HH if you know the position to 60 feet
DDMM.H_ if you are not sure of the last 600 feet
DDMM.__  if you only know the nearest mile
DDM_.__   if you only know the nearest 10 miles
DD___.__  if you only know the degrees.

*This does not mean truncation*.  It means transimt
what you know to the number of digits of resolution 
that you know or that you want displayed on receipt.

Of course, the Kenwood and any other Mic-E device
cannot transmit  SPACES since the Mmic-E encoded
format is highly compressed.  Thus we added just a
few bits to indicate the number of "spaces" that the
position does not have..

>APRS packet would esssentially be the same, all that is needed 
>is to have the software parsing the packet become a bit smarter 
>and say "Oh, look, an "A"Ambiguity marker here! Pass it on."

I inclluded these levels of precision in APRS since the very first
packet in 1992.  The problem is just too many APRS clones have
ignored these and many other detials of the protocol and have
just have not "passed along" this information on receipt 
to the recepient and properly implemented a display of this
information to the end user.

>Until *all* the applications that use GoogleMaps, NavTeq maps, 
>TIGER databases, and other sources of imprecise online mapping 
>stop using "pinpoints" and start using "blobs", they will remain 
>very impressive kiddie coloring books--but not cartography.

Agree completely!
Exactly why the original APRS shows symbols that either
cover the area of the ambiguity as the map zooms, or
the symbol itself gets bigger to cover the area of ambiguity
or the symbol goes away and is replaced by a circle when
the zoom is below the point where the size of the symbol
is smaller than the ambiguity.  All of these techniques
clearly avoid pinpoint resolution where such resolution does
not exist.    But again, most clones ignored most of  this
detail.

> It takes little effort on an author's part to say "Oh, this 
>datum, this source, is known to have an error of xxx feet, 
>therefore I won't use pinpoints, I'll use something else." 
>Very little extra effort--and no extra effort if the author 
>has any professional pride.
>
>What say Bob? How would using "A" instead of a "fuzzy 
>digits here" impact APRS?

Like I said, it is already there.  THe indicator is the SPACE
character.  ANd not only did the original APRS display the
ambiguity on RECEIPT it also included it on transmit
automatically if the sender created an object with his
mouse pointer.   The original APRS would look at any such
cursor placement and look at the range scale of the map
that was currently in use, and would not generate a
PRECISE position unless the map was zoomed into sufficiently
to display said resolution. 

THus, if someone moves a cursor to place a thunderstorm on
the map while looking at the 128 mile range scale, the original
APRS would only give that object a position precision of
a mile.  SImilarly  If someone entered an object that they
saw on the National Weather Service as 38 59N and
076 29W, again, that would be TRANSMITTED with only
that precision indicating a 1 mile ambiguity.  This is MUCH
different than transmitting a position that reads as
3859.00N and 07629.00W which is a very precise position
(to 60 feet).  WHich is just plain dumb for a thunderstorm.

Again, the original APRS accounts for all of this as well as does
the SPEC.  It is just that many of the follow-on clones may have 
ignored some of these details.

Bob, WB4APR






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