Expressing Confidence in a Location Object
draft-thomson-geopriv-confidence-03

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
    1.1.  Conventions used in this document
2.  Representation of Confidence in PIDF-LO
3.  Example
4.  Confidence Schema
5.  IANA Considerations
    5.1.  URN Sub-Namespace Registration for urn:ietf:params:xml:ns:geopriv:conf
    5.2.  XML Schema Registration
6.  Security Considerations
7.  References
    7.1.  Normative References
    7.2.  Informative References

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] (Thomson, M. and J. Winterbottom, “Representation of Uncertainty and Confidence in PIDF-LO,” May 2010.). Standard forms for the expression of uncertainty are included in [RFC5491] (Winterbottom, J., Thomson, M., and H. Tschofenig, “GEOPRIV Presence Information Data Format Location Object (PIDF-LO) Usage Clarification, Considerations, and Recommendations,” March 2009.), 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] (3GPP, “Universal Geographic Area Description (GAD),” December 2009.), 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:

• 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.
• The location server can ignore the confidence entirely, which results in giving the recipient 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.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

This document relies on the definitions in [I‑D.thomson‑geopriv‑uncertainty] (Thomson, M. and J. Winterbottom, “Representation of Uncertainty and Confidence in PIDF-LO,” May 2010.) and [RFC3693] (Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J. Polk, “Geopriv Requirements,” February 2004.).

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) (Peterson, J., “A Presence-based GEOPRIV Location Object Format,” December 2005.) [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] (Winterbottom, J., Thomson, M., and H. Tschofenig, “GEOPRIV Presence Information Data Format Location Object (PIDF-LO) Usage Clarification, Considerations, and Recommendations,” March 2009.). 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 (Example PIDF-LO with Confidence) 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>

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>

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] (Mealling, M., “The IETF XML Registry,” January 2004.).

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] (Mealling, M., “The IETF XML Registry,” January 2004.).

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 (Confidence Schema) of this document.

6.  Security Considerations

The security (and privacy) implications related to adding this information are not significant.

7.  References

7.1. Normative References

7.2. Informative References

Author's Address