From: Henning Schulzrinne ^lt;*hgs@cs.columbia.edu*>

Date: Fri Feb 02 2007 - 11:47:23 EST

Date: Fri Feb 02 2007 - 11:47:23 EST

Let me try a new angle that just might bridge the point vs. area

discussion. I think both somewhat miss the point (sorry). We need to

think probabilistically, not deterministically.

For all measurements, we're talking about a probability distribution

of measurements. There are three cases:

(1) The probability of being outside the area is zero. This is only

true if the area circumscribes, say, a property line and there is no

realistic chance that a device associated with that area can be

outside that area. This has nothing to do with measurement errors as

such, just physical limitations of where, say, Ethernet sockets are.

Obviously, even in buildings with DHCP, this is sometimes fuzzy,

e.g., if there's a wireless hotspot that radiates outside the

building. Nothing is said about the likelihood distribution within

that area.

A fuzzed (privacy-obfuscated) area is a special case. In other words,

the actual measurement is much more precise than what's conveyed, so

the object is (virtually) guaranteed to be somewhere inside the

indicated ranges of coordinates, with equal probability.

(2) The circle or other area describes a probability distribution

induced by measurement errors. In almost all cases, this really means

that x%, for some large x (say 90 or 95, but could be 66 in some FCC

regs), of the time the true location is indeed inside that area and

for (100-x)% it is outside. This seems to be the typical definition.

However, nothing is said about the probability distribution within

that area - the recipient of the information has to assume that the

likelihood of the object being at any given point within the area is

equal.

In reality, the latter isn't often quite true. Thus, the most likely

location is close to the centroid of the area. However, just having

an area doesn't tell you one way or the other.

(3) A point with an uncertainty region. Here, the assumption is that

the highest-probability location is at the point, with an uncertainty

region of the x% kind around it. This is the typical measurement

uncertainty/significant figures case.

Thus, the answer is not "point or area", but "both".

Unfortunately, 3825 and many other specs are often unclear what

assumptions they make on the probability distribution. 3825 addresses

both the fuzzed case and (3), but the probability interpretation

differs.

Henning

On Feb 2, 2007, at 11:07 AM, Brian Rosen wrote:

*>> Thanks very much for giving your opinion (I'm rather shocked you
*

*>> withheld it in the first place :) ). Just one question, do the
*

*>> values conveyed in 3825 constitute a point or an area?
*

*> A point
*

*>
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*>
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Received on Fri, 2 Feb 2007 11:47:23 -0500

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