[Geopriv] Location measurement error

["Rosen, Brian" <Brian.Rosen at neustar.biz>]

I'd like to start a discussion on how we deal with, in the big picture,
the notion of measurement error in general and "confidence" and
"uncertainty" in specific.

I am aware that there are a set of views that go roughly like: "there
really is no such thing as confidence and uncertainty, some vendors are
pushing this idea because they can't do what 'real' GPS devices do and
give you a 3 dimensional error in meters".

Nevertheless, in one of the most commonly encountered location
determination mechanisms (A-GPS and similar systems), confidence and
uncertainty are reported, and are the only error indicator available.  

In other systems, notably commercial GPS systems, you get a different
kind of error specification.  Sometimes it's simply error in meters,
often with a different value for Z than for x and y.  Other times you
get a more complex representation of error.

It does seem to me that when we convey measured location, that it would
be appropriate to convey the error of the measurement, and that error
indication should have the ability to represent confidence and
uncertainty.  I think we need to standardize the representation of
error.  It does occur to me to ask if such standardization would better
be done in OGC, and Carl may wish to comment.  It's also possible for
the geopriv crowd to come up with something and then OGC to incorporate
it in a future version of GML.

If we had a standardized representation of error, then we could also use
that in protocols like HELD to request a location within a specified
error bound, and we could also have a filter that requested location
within such a bound.  It would of course, be best to specify the XML for
such a bound once.

I'm specifically proposing that we allow error to be specified one of
two ways initially: with an actual error measurement, in meters for X, Y
and Z or as a shape (as in our other shape definitions) with an
"uncertainty" metric in percentage terms.  I would add it to GML if we
could do that, or additional elements in PIDF-LO.  I would then describe
an XML data structure to specify a bound, again, either in meters on X,
Y, and Z or confidence as a shape and uncertainty as a percentage.  I'd
allow the representation of both to be extended for other ways to
represent error.

Brian
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