Re: [Geopriv] ResponseTime

From: Henning Schulzrinne ^lt;hgs@cs.columbia.edu>
Date: Thu Aug 30 2007 - 10:11:00 EDT

The point has been made before, but it probably bears repeating.
There are at least two kinds of uncertainty/accuracy specifications,
namely for a particular technology and for a particular point.

Take a technology that produces a 100 m accuracy with 90% confidence.
Taken naively, this implies that every point is somewhere in a circle
with diameter 100 m.

Unfortunately, this error is not the same everywhere. Some locations
will presumably see a much larger error, possibly with non-zero mean
error. For example, if a timing error is absolute (+/- 1 ns), it
affects short distances much more than longer ones. Or multipath
makes a particular location always seem further away than it really is.

Thus, you could easily get a situation where the location for a
particular place is *always* outside that circle, even though the
technology, on average, performs as advertised. I don't know if
anybody has studied this problem in detail to see how much this
matters in practice.

http://www.comsoc.org/pci/private/2000/oct/bulusu.html shows an
example of this. ("The localization error is lowest at the position
corresponding to the centroid of the region and increases toward the
edges of the region.")

http://www.gmat.unsw.edu.au/snap/publications/lib_etal2005d.pdf
http://www.ieee-infocom.org/2004/Papers/55_5.PDF
also show behavior like that.

Btw, http://www.locatemodelcities.org/documents/
LOCATE_Final_Report.pdf is of some relevance here. In particular, it
contains definitions for terms.

Henning

On Aug 29, 2007, at 9:57 AM, Stuard, Doug wrote:

> In practice, “uncertainty” is usually taken to mean “accuracy”,
> although, as pointed out, accuracy is really a combination of
> uncertainty and confidence. Given that most folks (and I’m talking
> about call takers, not system administrators) think “accuracy” and
> not the uncertainty/confidence continuum, confidence is worse than
> of no use, it is confusing, so it is either not provided or is
> suppressed. Nevertheless, the reported “accuracy” (uncertainty)
> should be based on an underlying common confidence value (even if
> it is not displayed or otherwise used).
>
>
>
> While it may be true that accuracy/uncertainty is not used TODAY by
> individual service providers, do we want to ensure that it can
> NEVER be used??? As familiarity with geolocation concepts grows,
> and as more automated systems are deployed (i.e., those that
> understand the confidence/uncertainty tradeoff and plot big (or
> little) circles on maps), greater use of the information can be
> expected.
>
>
>
> Or maybe not.
>
>
>
> The point I’m trying to make (and to get back to the original
> subject) is that by providing optional time and “accuracy” values
> (actually, uncertainty at some a commonly agreed to confidence
> level), with defined meanings similar to those I suggested earlier,
> the flexibility to adapt to a wide range of applications (emergency
> and otherwise) and technological advances can be accommodated going
> forward.
>
>
>
> All we need is an agreement as to what the confidence level should
> be (oh, and world peace, too).
>
>

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Received on Thu, 30 Aug 2007 10:11:00 -0400

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