GEOPRIV M. Thomson
Internet-Draft Andrew Corporation
Intended status: Standards Track January 19, 2010
Expires: July 23, 2010
Expressing Confidence in a Location Object
draft-thomson-geopriv-confidence-02
Abstract
A confidence element is described that expresses the estimated
probability that the associated location information is correct.
This element conveys information that might otherwise be lost about
the probability distribution represented by a region of uncertainty.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 4
2. Representation of Confidence in PIDF-LO . . . . . . . . . . . . 4
3. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Confidence Schema . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:conf . . . . . . . . . . . . 7
5.2. XML Schema Registration . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
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1. Introduction
Location information is often less than perfect. Two measures are
used to quantify how imperfect the location information is:
uncertainty and confidence. These terms, and their relationship with
location information are explored in detail in
[I-D.thomson-geopriv-uncertainty]. Standard forms for the expression
of uncertainty are included in [RFC5491], but confidence is fixed to
a value of 95%.
On the whole, a fixed definition for confidence ensures consistency
between implementations. Location generators that are aware of this
constraint can generate location information at the required
confidence. Location recipients are able to make sensible
assumptions about the quality of the information that they receive.
In some circumstances - particularly with pre-existing systems -
location generators might provide location information with some
other confidence. Common values include 38%, 67% and 90%; all of
which are prevalent in current systems. Existing forms of expressing
location information, such as that defined in [3GPP-TS-23_032],
contain elements that express the confidence in the result.
The addition of a confidence element provides information that was
previously unavailable to recipients of location information.
Without this information, a location server or generator that has
access to location information with a confidence lower than 95% has
two options:
o The location server can scale regions of uncertainty in an attempt
to acheive 95% confidence. This scaling process significantly
degrades the quality of the information, because the location
server might not have the necessary information; the assumptions
that have to be made result in poor quality results.
o The location server can ignore the confidence entirely, which
results in giving the recipient of that information a false
impression of its quality.
Both of these choices degrade the quality of the information
provided.
The addition of a confidence element avoids this problem entirely if
a location recipient supports and understands the element. A
recipient that does not understand, and hence ignores, the confidence
element is in no worse a position than if the location server ignored
confidence.
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1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document relies on the definitions in
[I-D.thomson-geopriv-uncertainty] and [RFC3693].
2. Representation of Confidence in PIDF-LO
The confidence element MAY be added to the "location-info" element of
the Presence Information Data Format - Location Object (PIDF-LO)
[RFC4119] document. This element expresses the confidence in the
associated location information as a percentage.
The confidence element optionally includes an attribute that
indicates the shape of the probability density function (PDF) of the
associated region of uncertainty. Three values are possible:
unknown, normal and rectangular.
Indicating a particular PDF only indicates that the distribution
approximately fits the given shape based on the methods used to
generate the location information. The PDF is normal if there are a
large number of small, independent sources of error; rectangular if
all points within the area have roughly equal probability of being
the actual location of the Target; otherwise, the PDF MUST either be
set to unknown or omitted.
If a PIDF-LO does not include the confidence element, confidence is
95% [RFC5491]. A Point shape does not have uncertainty (or it has
infinite uncertainty), so confidence is meaningless for a point;
therefore, this element MUST be omitted if only a point is provided.
Location generators SHOULD attempt to ensure that confidence is equal
in each dimension when generating location information. This
restriction, while not always possible, allows for more accurate
scaling, if scaling is necessary.
Confidence SHOULD NOT be included unless location information cannot
be acquired with 95% confidence. Confidence SHOULD NOT be included
with civic address information; civic addresses are less subject to
variable errors than geodetic positions.
3. Example
The PIDF-LO document in Figure 1 includes a representation of
uncertainty as a circular area. The confidence element (on the line
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marked with a comment) indicates that the confidence is 67% and that
it follows a normal distribution.
See RFCXXXX.
END 5.2. XML Schema Registration This section registers an XML schema as per the guidelines in [RFC3688]. URI: urn:ietf:params:xml:schema:geopriv:conf Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com). Schema: The XML for this schema can be found as the entirety of Section 4 of this document. Thomson Expires July 23, 2010 [Page 7] Internet-Draft Confidence January 2010 6. Security Considerations The security (and privacy) implications related to adding this information are not significant. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004. [RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object Format", RFC 4119, December 2005. 7.2. Informative References [RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J. Polk, "Geopriv Requirements", RFC 3693, February 2004. [I-D.thomson-geopriv-uncertainty] Thomson, M. and J. Winterbottom, "Representation of Uncertainty and Confidence in PIDF-LO", draft- thomson-geopriv-uncertainty-04 (work in progress), November 2009. [RFC5491] Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV Presence Information Data Format Location Object (PIDF-LO) Usage Clarification, Considerations, and Recommendations", RFC 5491, March 2009. [3GPP-TS-23_032] "3GPP TS 23.032 V6.0.0 3rd Generation Partnership Project; Technical Specification Group Code Network; Universal Geographic Area Description (GAD)". Thomson Expires July 23, 2010 [Page 8] Internet-Draft Confidence January 2010 Author's Address Martin Thomson Andrew Corporation Andrew Building (39) Wollongong University Campus Northfields Avenue Wollongong, NSW 2522 AU EMail: martin.thomson@andrew.com Thomson Expires July 23, 2010 [Page 9]