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[aprssig] RE: Position Ambituity in APRS!

Keith VE7GDH ve7gdh at rac.ca
Fri Jan 11 05:58:28 UTC 2008


Alex KF4LVZ wrote...

(this seems to have disappeared into the ether, so I'm sendng it again)

> However, you didn't have to waste time quoting my entire
> message either just to take a jab at me.

I know you know I didn't do that, but starting off the quoted text with
my name, callsign and email address visible was ambiguous enough to make
it look to someone not paying attention that you were talking about me.

> If I were lost in the woods and I knew I was somewhere
> between 35 and 36 degrees north and 85 and 86 west, I'll
> transmit 35 N 85 N [WE KNOW YOU MEANT WEST]
> and hope that the SAR people will search a box.

If to the best of your knowledge you were somewhere between 35 and 36 N,
and somewhere between 85 & 86 W, to me a more accurate representation
would be to send 35 30 N / 85 30 W with the appropriate spaces for the
ambiguity. Of course, if you just don't know you were approximately half
way between 35N and 36N and approximately half way between 85W and 86W,
it would be misleading. I can understand why ambiguity is written into
the spec, but it is very ambiguous!

To give another example, if you knew you were very nearly 35 N and very
nearly 85W, would you still send 35N / 85W or would you enter more
digits... as many as you were reasonably confident about? I don't think
it should be a box. I think it should be a circle centred on the
position that to the best of the knowledge of the person entering the
coordinates is the correct position.

To me, an ambiguous position of 35N 85W means somewhere between 34N and
36N and somewhere between 84W and 87W. My suggestion would be to enter
as many digits as you were confident about when entering your position.

> So I end up dying in the wilderness and coming back to
> haunt everyone.

Well, no. If you get lost in the wilderness and end up dying there, it's
your responsibility. If word gets through to SAR that you are lost and
in need of assistance, they will do their best to find you and evacuate
you to safety. If they don't find you because you couldn't give them
better directions, it's too bad. I feel sorry for you, but I would feel
sorrier for the burden that you (I know this is hypothetical) placed on
them by asking them to search for you. If you meet your demise, you have
gone to a better place... or not. Those that you asked to search for you
would have to deal with not being to save everyone they looked for, but
I digress. That's not really anything to do with APRS. As an ex-SAR
member, I thought that I would throw that in, but I just wouldn't want
you to come back and haunt me when it wasn't justified.

> Every other coordinate system on this planet uses
> grids for uncertainty. The grid square system (it's
> right there in the name, square), UTM, Maidenhead, all
> of those are some kind of polygon not circles.

For the ground pounders, the UTM grid sure makes life a lot easier. APRS
of course uses degrees, minutes and decimal minutes. As soon as you are
working with other agencies, it's nice to know that it's as easy as
pushing a few buttons on a GPS to change the format.

> I would suggest as you modify the specifications...

Even though the APRS spec has been around a while, there are probably a
few things that should be re-worded. This is an opportunity to hammer it
out. Of course, I have never in my life been ambiguous about a position.
In the old days, I had a map and compass. These days I have a map and
compass AND a GPS receiver. Of course, with a GPS, I know exactly where
I am even if I am lost. If the batteries die, it still makes a pretty
good paper-weight to hold the map down if it's breezy.

When it comes down to it, ambiguity probably doesn't mean much to a lot
of people involved in APRS. It does to Bob because he probably uses
APRSDOS. I know from messages in the past that he is in the habit of
punching in an approximate lat/long at airports and while hiking without
a GPS. Both instances are examples of why ambiguity might sometimes be
used. To me - someone that has hardly ever gone anywhere without a GPS
(whether hiking, skiing, driving, on a SAR task or mountain biking)
since I got my first one - ambiguity doesn't make a lot of sense.

For someone that is travelling without a GPS, it allows them to
participate in APRS e.g. with a D7. For someone with a D7 and a failed
GPS or one with dead batteries, it again makes sense. I understand it. I
would just never use it - unless experimenting!

When I say it doesn't mean much to a lot of people involved in APRS, I
am referring to the huge majority of APRS users that use UI-View at
home... and some on the road. It just doesn't display ambiguity as Bob
envisions it. It acts as if the spaces are filled zeroes. Because the
majority of people looking at your ambiguous position won't know it's
ambiguous unless they take a closer look at it, it would be to your
advantage - if you were like Alex in his example above - if you were to
use the "unknown position" symbol... \.  At least people looking at you
in a graphical environment will see that without even looking for the
missing digits, whether they are using UI-View or not.

I've been reading the discussion about ambiguity but not paying a lot of
attention to it. If I were to make any argument at all, it would be that
the ambiguous position would be represented by a circle and not a box,
with the diameter determined by the amount of ambiguity the sender
intended.

So... was any of the above ambiguous? hi

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"






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