One document matched: draft-thomson-geopriv-confidence-04.xml
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<rfc category="std" ipr="trust200902" docName="draft-thomson-geopriv-confidence-04">
<front>
<title abbrev="Confidence">
Expressing Confidence in a Location Object
</title>
<author initials="M." surname="Thomson" fullname="Martin Thomson">
<organization>Microsoft</organization>
<address>
<postal>
<street></street>
<city>Mountain View</city>
<region>VA</region>
<code>94043</code>
<country>US</country>
</postal>
<email>martin.thomson@gmail.com</email>
</address>
</author>
<date year="2013"/>
<area>RAI</area>
<workgroup>GEOPRIV</workgroup>
<keyword>Internet-Draft</keyword>
<keyword>Confidence</keyword>
<keyword>Location</keyword>
<keyword>GEOPRIV</keyword>
<abstract>
<t>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.
</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>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 <xref target="I-D.thomson-geopriv-uncertainty"/>. Standard forms for the expression of uncertainty are included in <xref target="RFC5491"/>, but confidence is fixed to a value of 95%.
</t>
<t>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.
</t>
<t>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 <xref target="TS-3GPP-23_032"/>, contain elements that express the confidence in the result.
</t>
<t>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:
<list style="symbols">
<t>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 to scale appropriately; the location server is forced to make assumptions that are likely result in either an overly conservative estimate with high uncertainty or a overestimate of confidence.
</t>
<t>The location server can ignore the confidence entirely, which results in giving the recipient a false impression of its quality.
</t>
</list>
Both of these choices degrade the quality of the information provided.
</t>
<t>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.
</t>
<section anchor="conventions" title="Conventions used in this document">
<t>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 <xref target="RFC2119"/>.
</t>
<t>This document relies on the definitions in <xref target="I-D.thomson-geopriv-uncertainty"/> and <xref target="RFC3693"/>.
</t>
</section>
</section>
<section anchor="conf" title="Representation of Confidence in PIDF-LO">
<t>The confidence element MAY be added to the <spanx style="verb">location-info</spanx> element of the <xref target="RFC4119">Presence Information Data Format - Location Object (PIDF-LO)</xref> document. This element expresses the confidence in the associated location information as a percentage.
</t>
<t>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.
</t>
<t>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.
</t>
<t>If a PIDF-LO does not include the confidence element, confidence is 95% <xref target="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.
</t>
<section title="Generating Locations with Confidence">
<t>Location generators SHOULD attempt to ensure that confidence is equal in each dimension when generating location information. This restriction, while not always practical, allows for more accurate scaling, if scaling is necessary.
</t>
<t>Confidence MUST NOT be included unless location information cannot be acquired with 95% confidence.
</t>
</section>
<section title="Consuming and Presenting Confidence">
<t>
The inclusion of confidence that is anything other than 95% presents a potentially difficult usability for applications that use location information. Effectively communicating the probability that a location is incorrect to a user can be difficult.
</t>
<t>
It is inadvisable to simply display locations of any confidence, or to display confidence in a separate or non-obvious fashion. If locations with different confidence levels are displayed such that the distinction is subtle or easy to overlook - such as using fine graduations of color or transparency for graphical uncertainty regions, or displaying uncertainty graphically, but providing confidence as supplementary text - a user could fail to notice a difference in the quality of the location information that might be significant.
</t>
<t>
Depending on the circumstances, different ways of handling confidence might be appropriate. <xref target="I-D.thomson-geopriv-uncertainty"/> describes techniques that could be appropriate for consumers that use automated processing as well as background on the issue.
</t>
<t>
Providing that the full implications of any choice for the application are understood, some amount of automated processing could be appropriate. In a simple example, applications could choose to discard or suppress the display of location information if confidence does not meet a pre-determined threshold.
</t>
<t>
In settings where there is an opportunity for user training, some of these problems might be mitigated by defining different operational procedures for handling location information at different confidence levels.
</t>
</section>
</section>
<section title="Example">
<t>The PIDF-LO document in <xref target="ex-conf"/> 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.
</t>
<figure anchor="ex-conf" title="Example PIDF-LO with Confidence">
<artwork><![CDATA[
<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>
]]></artwork>
</figure>
</section>
<section anchor="schema" title="Confidence Schema">
<figure><artwork><![CDATA[
<?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>
]]></artwork></figure>
</section>
<section anchor="iana" title="IANA Considerations">
<section title="URN Sub-Namespace Registration for urn:ietf:params:xml:ns:geopriv:conf">
<t>This section registers a new XML namespace, <spanx style="verb">urn:ietf:params:xml:ns:geopriv:conf</spanx>, as per the guidelines in <xref target="RFC3688"/>.
<list style="empty">
<t>URI: urn:ietf:params:xml:ns:geopriv:conf</t>
<t>Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).</t>
<t>XML:
<figure>
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<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>
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</figure>
</t>
</list>
</t>
</section>
<section title="XML Schema Registration">
<t>This section registers an XML schema as per the guidelines in <xref target="RFC3688"/>.
<list style="hanging">
<t hangText="URI:">urn:ietf:params:xml:schema:geopriv:conf</t>
<t hangText="Registrant Contact:">IETF, GEOPRIV working group, (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).</t>
<t hangText="Schema:">The XML for this schema can be found as the entirety of <xref target="schema"/> of this document.
</t>
</list>
</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>The security (and privacy) implications related to adding this information are not significant.
</t>
</section>
<!--
<section title="Acknowledgements">
<t>
</t>
</section>
<appendix title="Change Log">
<t>[[The RFC Editor is requested to remove this section at publication.]]</t>
<t>Changes since -0-1:
<list style="symbols">
<t>Document created.</t>
</list>
</t>
</appendix>
-->
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC3688;
&RFC4119;
</references>
<references title="Informative References">
&RFC3693;
&I-D.thomson-geopriv-uncertainty;
&RFC5491;
<reference anchor="TS-3GPP-23_032">
<front>
<title>Universal Geographic Area Description (GAD)</title>
<author><organization>3GPP</organization></author>
<date month="September" year="2012"/>
</front>
<seriesInfo name="3GPP TS" value="23.032 11.0.0"/>
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