Subject: [Geopriv] How do you return error

From: "Rosen, Brian" <[email-masked]>
Date: Thu, 17 Jul 2008 21:26:18 -0400

So far, in the discussions on error, the way it's proposed to specify
error is that the location return is a shape representing the
confidence, and uncertainty is returned as a scalar.

I'd like to question that proposal.

In every other engineering system I'm familiar with, you return the
nominal value and you express the error.  So you might get a measurement
of 1.077 meters, plus or minus .03 meters.

Why are we not returning that kind of response here?

If we ask for a location, we should get a point and an error.  The error
may be a shape, but 99% of applications would be satisfied with a few
scalers (circular or ellipsoidal error).

You shouldn't have to do calculations at the watcher to get the nominal
point, especially if the shape is not circular or ellipsoidal.

Most consumer systems work that way.  Most real apps want data that way.

Specific proposal:

Return location normally as a point (2 or 3 space).
Return error as metadata: a shape with optional distribution & optional
confidence

It's conceivable that we need OGC to help with this.  Fine.

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From: "Winterbottom, James" <[email-masked]>
Date: Thu, 17 Jul 2008 20:46:54 -0500

No.

The shape is the uncertainty and the confidence is a scalar

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] On
Behalf
> Of Rosen, Brian
> Sent: Friday, 18 July 2008 11:26 AM
> To: [email-masked]
> Subject: [Geopriv] How do you return error
> 

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From: "Dawson, Martin" <[email-masked]>
Date: Thu, 17 Jul 2008 21:02:03 -0500

Sea below

-----Original Message-----
Behalf Of Rosen, Brian

So far, in the discussions on error, the way it's proposed to specify
error is that the location return is a shape representing the
confidence, and uncertainty is returned as a scalar.
[[MCD]] Are you sure you don't mean the shape represents the uncertainty
and a (percentage/fractional) scalar represents the confidence?

Why are we not returning that kind of response here?
[[MCD]] I'm not sure why you think we aren't. A circular uncertainty is
a point + radius. It's a Lat +/- metres and a Long +/- metres. It could
just as easily be +/- degrees but since the size various depending on
the location on the Earth, that doesn't tend to be the convention.

If we ask for a location, we should get a point and an error.  The error
may be a shape, but 99% of applications would be satisfied with a few
scalers (circular or ellipsoidal error).
[[MCD]] Right - isn't that what PIDF-LO profile provides? Some basic
assumed representations along with how to encode them?

Most consumer systems work that way.  Most real apps want data that way.
[[MCD]] I question the "shouldn't" aspect. Fact is that a single timing
measurement from a single base station with a known quantization error
gives you an arc-band as the most efficient shape to cram the maximum
probability into. If you insist that a LIS convert it into a circle,
then the size of the uncertainty area has to increase to maintain at
least as much confidence. If you want a point, the calculation is
simple, pick any point in the arc-band - including the edges; no point
is more special than the other as far as that shape encoding is
concerned.

[[MCD]] If you want to argue that applications should be allowed to be
lazy and only understand "point" then that's different. On the other
hand, I believe the point of the PIDF-LO profile spec is that the
minimum subset of shapes you should understand is larger than that - and
here's how you'll find them encoded.

Specific proposal:

[[MCD]] I agree that PIDF-LO should have a confidence field. There
should be a way to explicitly say "not specified". That might not be
helpful but it's better than implying that it's actually 95% or some
other default when it isn't.

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From: "Rosen, Brian" <[email-masked]>
Date: Fri, 18 Jul 2008 12:11:37 -0400

Sorry for reversing confidence and uncertainty.  I DO know which is
which, but I goofed here.

I think there is a substantive difference between returning a point and
a circular, ellipsoidal, arc-band or polygonal uncertainty boundary, and
returning just the uncertainty boundary and implying the point.

I want to return a point as a GML object and include the uncertainty
boundary as metadata.  The uncertainty would be a shape.

While I understand the issues involved in an arc-band shape of
uncertainty, I'm willing to bet that 95+% of applications and users just
don't care, and if you have an arc-band, they will be much happier with
the corresponding enclosing circle or ellipse.  However, since there are
some small percentage of apps who might care, I am fine if the metadata
includes the whole range of shape data we permit in PIDF-LO.  But, I
want the nominal point, and I want the uncertainty as metadata.

>From draft-thomson-geopriv-uncertainty-01.txt, that would be true only
with a rectangular error distribution function.  That document describes
algorithms to get the appropriate point given an arc band, if that is
the actual original source information from measurement.  I understand
that in at least some systems, the measurement gives a point, and
algorithms that understand error sources construct the arcband around
the point. 

Brian

> From: Dawson, Martin [mailto:[email-masked]]
> Sent: Thursday, July 17, 2008 10:02 PM
> To: Rosen, Brian; [email-masked]
> Subject: RE: [Geopriv] How do you return error
> 
> Sea below
> 
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From: "Dawson, Martin" <[email-masked]>
Date: Fri, 18 Jul 2008 21:42:33 -0500

> returning just the uncertainty boundary and implying the point.

If you're talking about a point with no uncertainty - then I agree;
there is a difference. Of course there is uncertainty but it's not being
communicated. That's the difference.

If you're talking about the point +/- x metres (which was the example
you gave), why do you think this is any different than a circle of
uncertainty? (and whatever happened to the word "substantial"?)

> with a rectangular error distribution function.

That's right - and without additional metadata that indicates what the
probability density function is, the actual convention - and the
reasonable convention - is to assume it's rectangular.

Cheers,
Martin

-----Original Message-----
From: Rosen, Brian [mailto:[email-masked]] 
Sent: Saturday, 19 July 2008 2:12 AM
To: Dawson, Martin; [email-masked]
Subject: RE: [Geopriv] How do you return error

Brian