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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