Subject: RE: [Geopriv] ResponseTime

From: "Thomson, Martin" <[email-masked]>
Date: Mon, 10 Sep 2007 02:32:30 -0500

The "accuracy" for a technology is only interesting to people like the FCC, purchasers of new technology and R&D folks.  Most-times we are interested in the "accuracy" of any one positioning result.

When we calculate a position based on GPS measurements, we take a number of factors and determine an uncertainty for that point.  This is determined based on the quality of the measurements (which is also measured) and a number of other factors.  Based on our testing, and the mathematical models we employ, we can be reasonably sure that the actual point is within the area of uncertainty at the given percentage of confidence.  That is, if we say within this area 90% of the time, 90% of the time that will be right.

For this, you need statistics and mathematical models that approximate your results.  Whether the error is in timing, environment or a data entry stuff-up, errors can be accounted for and these are reflected in the final confidence value.

And when you say the point is "*always* outside the circle", there is only one point, but it is only outside of the 90% circle 10% of the time.  How do we know?  Statistics.

> -----Original Message-----
> From: Henning Schulzrinne [mailto:[email-masked]]
> Sent: Friday, 31 August 2007 12:11 AM
> To: Stuard, Doug
> Cc: GEOPRIV WG
> Subject: Re: [Geopriv] ResponseTime
> 
> 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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