One document matched: draft-thomson-geopriv-confidence-00.txt
GEOPRIV M. Thomson
Internet-Draft Andrew
Intended status: Standards Track December 18, 2008
Expires: June 21, 2009
Expressing Confidence in a Location Object
draft-thomson-geopriv-confidence-00
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Abstract
A confidence element is described that expresses the estimated
probability that the associated location information is correct.
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This element conveys information that might otherwise be lost about
the probability distribution represented by a region of uncertainty.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 4
2. Representation of Confidence in PIDF-LO . . . . . . . . . . . . 4
3. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
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 [I-D.ietf-geopriv-pdif-lo-profile],
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.
This element adds 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. Both of these
choices degrade the quality of the information provided:
o The location server is forced to either attempt to 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.
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 be set to
unknown or omitted.
If a PIDF-LO does not include the confidence element, confidence is
95% [I-D.ietf-geopriv-pdif-lo-profile]. 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.
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3. Example
The PIDF-LO document in Figure 1 includes a representation of
uncertainty as a circular area. The confidence element (on the line
marked with a comment) indicates that the confidence is 67% and that
it follows a normal distribution.
<pidf:presence
xmlns:pidf="urn:ietf:params:xml:ns:pidf"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
xmlns:gml="http://www.opengis.net/gml"
xmlns:con="urn:ietf:params:xml:ns:pidf:geopriv:conf"
entity="pres:alice@example.com">
<dm:device id="sg89ab">
<pidf:status>
<gp:geopriv>
<gp:location-info>
<gs:Circle srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>42.5463 -73.2512</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">
850.24
</gs:radius>
</gs:Circle>
<!-- c --> <con:confidence pdf="normal">67</con:confidence>
</gp:location-info>
<gp:usage-rules/>
</gp:geopriv>
</pidf:status>
<dm:deviceID>mac:010203040506</dm:deviceID>
</dm:device>
</pidf:presence>
Figure 1: Example PIDF-LO with Confidence
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4. Confidence Schema
<?xml version="1.0"?>
<xs:schema
xmlns:conf="urn:ietf:params:xml:ns:geopriv:conf"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="urn:ietf:params:xml:ns:geopriv:conf"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:annotation>
<xs:appinfo
source="urn:ietf:params:xml:schema:geopriv:conf">
PIDF-LO Confidence
</xs:appinfo>
<xs:documentation source="http://www.ietf.org/rfc/rfcXXXX.txt">
<!-- [[NOTE TO RFC-EDITOR: Please replace above URL with URL of
published RFC and remove this note.]] -->
This schema defines an element that is used for indicating
confidence in PIDF-LO documents.
</xs:documentation>
</xs:annotation>
<xs:element name="confidence" type="conf:confidenceType"/>
<xs:complexType name="confidenceType">
<xs:simpleContent>
<xs:extension base="conf:confidenceBase">
<xs:attribute name="pdf" type="conf:pdfType"
default="unknown"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:simpleType name="confidenceBase">
<xs:restriction base="xs:decimal">
<xs:minExclusive value="0.0"/>
<xs:maxExclusive value="100.0"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="pdfType">
<xs:restriction base="xs:token">
<xs:enumeration value="unknown"/>
<xs:enumeration value="normal"/>
<xs:enumeration value="rectangular"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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5. IANA Considerations
5.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:conf
This section registers a new XML namespace,
"urn:ietf:params:xml:ns:geopriv:conf", as per the guidelines in
[RFC3688].
URI: urn:ietf:params:xml:ns:geopriv:conf
Registrant Contact: IETF, GEOPRIV working group,
(geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>PIDF-LO Confidence Attribute</title>
</head>
<body>
<h1>Namespace for PIDF-LO Confidence Attribute</h1>
<h2>urn:ietf:params:xml:ns:geopriv:conf</h2>
[[NOTE TO IANA/RFC-EDITOR: Please update RFC URL and replace XXXX
with the RFC number for this specification.]]
<p>See <a href="[[RFC URL]]">RFCXXXX</a>.</p>
</body>
</html>
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.
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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-02
(work in progress),
November 2008.
[I-D.ietf-geopriv-pdif-lo-profile] Winterbottom, J., Thomson, M.,
and H. Tschofenig, "GEOPRIV
PIDF-LO Usage Clarification,
Considerations and
Recommendations", draft-ietf-
geopriv-pdif-lo-profile-14 (work
in progress), November 2008.
[3GPP-TS-23_032] "3GPP TS 23.032 V6.0.0 3rd
Generation Partnership Project;
Technical Specification Group
Code Network; Universal
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Geographic Area Description
(GAD)".
Author's Address
Martin Thomson
Andrew
PO Box U40
Wollongong University Campus, NSW 2500
AU
Phone: +61 2 4221 2915
EMail: martin.thomson@andrew.com
URI: http://www.andrew.com/
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