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[aprssig] findu.com Location off by 100 miles.

Robert Bruninga bruninga at usna.edu
Tue Jun 6 01:41:57 UTC 2006


Again, position ambiguity is not for the purpose of masking or
hiding one's position.  It is for the purpose of reporting things
who's position is not precise.

THink of these examples:
1) Manually entering a posit when  you are bumped to the St Louis
AIrport and cannot get a GPS fix (or dont want to bother).
Yet you HAVE to have a position or your D7 wont transmit
a message back home saying you are stranded.
I estimate my position and then give it a 10 mile ambiguity
so that at least I show up in St Louis instead of Baltimiore
(where my flight left) and Chicago (where it was headed)...
Yet, on proper displays eveyrone sees that I am using an
estimated posit with ambiguity as much as 10 miles or more.

2) You place a thunderstorm on the map.  WIthout position
ambiguity, as soneone zooms in on the map, the thuderstorm
appears to be something only 60 feet wide and appears to be 
in someone's driveway!  WRONG.  All WX opbjects
should include position ambiguity to the map scale where they
make sense and no more!

3) If you are using a pre-defined map at an event or venu with
checkpoints and a rough lat/long grid.  Everyone with D7's
can place themselves on the map by just looking at the map
and entering their grid.  THis works indoors, underground and
all kinds of places where GPS does not wrok.  In this case,
I would use either the 1/10th or 1 mile ambiguity depending 
on the venue...

I could go on and on.  Ambiguity has NOTHING to do with
lines in the sand or discrete quanta of position.  It has to
do with the sender being able to send positions and objects
with degrees of uncertainty and having the meaning 
PROPERLY delivered to all viewers of that object or position.
It is  a major failing of some display systems to not properly
convey the senders intent to the viewer.  It undermines
the integrity of APRS to just display ICONS on a map
below the scale at which the ICON is smaller than the
ambituity that the sender intended.

de WB4APR, BOb






>>> "Curt, WE7U" <archer at eskimo.com> 06/05/06 12:05 PM >>>
On Mon, 5 Jun 2006, Robert Bruninga wrote:

> Ah, but again that interpretation is not interpreting the sender's
> intent.  Position ambiguity means the sender is only sure of his
> position to some level of accuracy.  If he uses 1 mile ambiguity
> then it does not make sense to try to force precise "edges" of that
> unknown quantity to something too precise.
>
> THe circle simply communicates that the position is unknown
> below that resolution.  A square would be fine too, but both
> should not be interpreted as precise lines'in-th-sand...

A square would _not_ be fine, 'cuz it's a rectangle according to the
definition in the APRS spec.  Well, perhaps right at the equator it
could be a square.

I understand what you are saying above, but you _could_ interpret it
as precise lines in the sand if you're watching somebody drive along
and the ambiguity rectangle all of a sudden jumps in one direction.
You know they just crossed the line and are driving into the next
rectangle.

Yes, the math works that way in the spec.  If I watched someone
drive along a major road with ambiguity on, I could gauge fairly
well where they were by when the rectangles switched.

This appears to be to be one case where the original concept and the
spec definition don't jive too well.

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer 
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"

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