Subject: [Ecrit] Geodetic interpretations

From: "James Winterbottom" <[email-masked]>
Date: Thu, 31 Mar 2005 09:44:20 +1000

Hi All,

I am going to change the subject a bit here from what has been going around
the list to date about "validated locations".

I want to ask some opinions about geodetic information, and how we should
seek to interpret this. For IETF-62 I presented a paper in the GeoPriv
working group that was authored by me, Hannes Tschofenig and Martin Thomson
on, amongst other things, what might be a required minimal set of GML to
implement to support routing applications.
(http://www.ietf.org/internet-drafts/draft-winterbottom-geopriv-pdif-lo-prof
ile-00.txt)

In the presentation I indicated that we were going to look at uncertainty in
more detail in the next edition, and confidence. The difference being that
an uncertainty is usually defined as being with X metres of a provided
Latitude/Longitude, and a confidence factor is a percentage chance that you
will reside in that area. PIDF-LO does not support the notion of a
confidence factor today, but this is something that we plan to look at in
the next iteration of the above draft in the form of suggestions.

The wireless standards, at least those used in North America, generally
provide both. And the FCC for regulations are really only interested in 2
confidence factors, the 67% and the 90%. This tends to lead to the location
representations as being closed shapes, circles, polygons, arcbands etc,
rather than single points. While a circle with uncertainty can be expressed
a s point with radius, neither a polygon or an arcband can. 

So where is this leading?
1) Do we care about confidence in location for routing?
2) If so, what is our minimal confidence level?
3) Do we have preferences in how location is to specified (see draft above)?

While these points may be lower level requirements, they are things that we
must start considering if we want an implementable solution.

Cheers
James
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---

From: Henning Schulzrinne <[email-masked]>
Date: Wed, 30 Mar 2005 20:48:09 -0500

I'm not sure what you would do differently for routing if you knew the 
confidence interval (which this is, roughly). You'd presumably always 
route on the center of circle or other geometric shape. Clearly, the 
dispatcher might find this useful to guess whether to send one squad car 
or a search party.

I'll repeat my rant from earlier: elaborate confidence measures are, in 
my view, largely bogus for dispatch and routing. (They are useful to 
characterize the performance of a technology.) They are usually based on 
not much more than "measured across thousands of points across the 
country, 90% of measurements fell within 100 m of true location", which 
says very little about the GPS satellite shading or the number and 
relative location of cell towers (for TOA-style measurements), say, at 
the corner of Main and 5th in Podunk, NY. Thus, if you were to 
repeatedly measure location at that point, you might only be off by 10 
m, or maybe it's a bad spot and it's always off by 200 m.

This would be useful if you could bound the technology resolution in all 
cases, i.e., that it is extremely unlikely to be wrong by more than X 
meters.

Thus, worrying too much about accuracy is like providing 
pound-to-kilogram conversions on noodle packages to 7 digits.

James Winterbottom wrote:
> Hi All,
> 
> I am going to change the subject a bit here from what has been going 
> around the list to date about "validated locations".
> 
> I want to ask some opinions about geodetic information, and how we 
> should seek to interpret this. For IETF-62 I presented a paper in the 
> GeoPriv working group that was authored by me, Hannes Tschofenig and 
> Martin Thomson on, amongst other things, what might be a required 
> minimal set of GML to implement to support routing applications. 
> (http://www.ietf.org/internet-drafts/draft-winterbottom-geopriv-pdif-lo-profile-00.txt)
> 
> In the presentation I indicated that we were going to look at 
> uncertainty in more detail in the next edition, and confidence. The 
> difference being that an uncertainty is usually defined as being with X 
> metres of a provided Latitude/Longitude, and a confidence factor is a 
> percentage chance that you will reside in that area. PIDF-LO does not 
> support the notion of a confidence factor today, but this is something 
> that we plan to look at in the next iteration of the above draft in the 
> form of suggestions.
> 
> provide both. And the FCC for regulations are really only interested in 
> 2 confidence factors, the 67% and the 90%. This tends to lead to the 
> location representations as being closed shapes, circles, polygons, 
> arcbands etc, rather than single points. While a circle with uncertainty 
> can be expressed a s point with radius, neither a polygon or an arcband 
> can.
> 
> While these points may be lower level requirements, they are things that 
> we must start considering if we want an implementable solution.
> 
> Cheers
> James
> 
> 
> ------------------------------------------------------------------------
>