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2 GEOPRIV M. Thomson
3 Internet-Draft Microsoft
4 Intended status: Standards Track August 27, 2013
5 Expires: February 28, 2014
7 Expressing Confidence in a Location Object
8 draft-thomson-geopriv-confidence-04
10 Abstract
12 A confidence element is described that expresses the estimated
13 probability that the associated location information is correct.
14 This element conveys information that might otherwise be lost about
15 the probability distribution represented by a region of uncertainty.
17 Status of This Memo
19 This Internet-Draft is submitted in full conformance with the
20 provisions of BCP 78 and BCP 79.
22 Internet-Drafts are working documents of the Internet Engineering
23 Task Force (IETF). Note that other groups may also distribute
24 working documents as Internet-Drafts. The list of current Internet-
25 Drafts is at http://datatracker.ietf.org/drafts/current/.
27 Internet-Drafts are draft documents valid for a maximum of six months
28 and may be updated, replaced, or obsoleted by other documents at any
29 time. It is inappropriate to use Internet-Drafts as reference
30 material or to cite them other than as "work in progress."
32 This Internet-Draft will expire on February 28, 2014.
34 Copyright Notice
36 Copyright (c) 2013 IETF Trust and the persons identified as the
37 document authors. All rights reserved.
39 This document is subject to BCP 78 and the IETF Trust's Legal
40 Provisions Relating to IETF Documents
41 (http://trustee.ietf.org/license-info) in effect on the date of
42 publication of this document. Please review these documents
43 carefully, as they describe your rights and restrictions with respect
44 to this document. Code Components extracted from this document must
45 include Simplified BSD License text as described in Section 4.e of
46 the Trust Legal Provisions and are provided without warranty as
47 described in the Simplified BSD License.
49 Table of Contents
51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
52 1.1. Conventions used in this document . . . . . . . . . . . . 3
53 2. Representation of Confidence in PIDF-LO . . . . . . . . . . . 3
54 2.1. Generating Locations with Confidence . . . . . . . . . . 4
55 2.2. Consuming and Presenting Confidence . . . . . . . . . . . 4
56 3. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
57 4. Confidence Schema . . . . . . . . . . . . . . . . . . . . . . 5
58 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
59 5.1. URN Sub-Namespace Registration for
60 urn:ietf:params:xml:ns:geopriv:conf . . . . . . . . . . . 6
61 5.2. XML Schema Registration . . . . . . . . . . . . . . . . . 7
62 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
63 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
64 7.1. Normative References . . . . . . . . . . . . . . . . . . 7
65 7.2. Informative References . . . . . . . . . . . . . . . . . 8
67 1. Introduction
69 Location information is often less than perfect. Two measures are
70 used to quantify how imperfect the location information is:
71 uncertainty and confidence. These terms, and their relationship with
72 location information are explored in detail in
73 [I-D.thomson-geopriv-uncertainty]. Standard forms for the expression
74 of uncertainty are included in [RFC5491], but confidence is fixed to
75 a value of 95%.
77 On the whole, a fixed definition for confidence ensures consistency
78 between implementations. Location generators that are aware of this
79 constraint can generate location information at the required
80 confidence. Location recipients are able to make sensible
81 assumptions about the quality of the information that they receive.
83 In some circumstances - particularly with pre-existing systems -
84 location generators might provide location information with some
85 other confidence. Common values include 38%, 67% and 90%; all of
86 which are prevalent in current systems. Existing forms of expressing
87 location information, such as that defined in [TS-3GPP-23_032],
88 contain elements that express the confidence in the result.
90 The addition of a confidence element provides information that was
91 previously unavailable to recipients of location information.
92 Without this information, a location server or generator that has
93 access to location information with a confidence lower than 95% has
94 two options:
96 o The location server can scale regions of uncertainty in an attempt
97 to acheive 95% confidence. This scaling process significantly
98 degrades the quality of the information, because the location
99 server might not have the necessary information to scale
100 appropriately; the location server is forced to make assumptions
101 that are likely result in either an overly conservative estimate
102 with high uncertainty or a overestimate of confidence.
104 o The location server can ignore the confidence entirely, which
105 results in giving the recipient a false impression of its quality.
107 Both of these choices degrade the quality of the information
108 provided.
110 The addition of a confidence element avoids this problem entirely if
111 a location recipient supports and understands the element. A
112 recipient that does not understand, and hence ignores, the confidence
113 element is in no worse a position than if the location server ignored
114 confidence.
116 1.1. Conventions used in this document
118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
120 document are to be interpreted as described in [RFC2119].
122 This document relies on the definitions in
123 [I-D.thomson-geopriv-uncertainty] and [RFC3693].
125 2. Representation of Confidence in PIDF-LO
127 The confidence element MAY be added to the "location-info" element of
128 the Presence Information Data Format - Location Object (PIDF-LO)
129 [RFC4119] document. This element expresses the confidence in the
130 associated location information as a percentage.
132 The confidence element optionally includes an attribute that
133 indicates the shape of the probability density function (PDF) of the
134 associated region of uncertainty. Three values are possible:
135 unknown, normal and rectangular.
137 Indicating a particular PDF only indicates that the distribution
138 approximately fits the given shape based on the methods used to
139 generate the location information. The PDF is normal if there are a
140 large number of small, independent sources of error; rectangular if
141 all points within the area have roughly equal probability of being
142 the actual location of the Target; otherwise, the PDF MUST either be
143 set to unknown or omitted.
145 If a PIDF-LO does not include the confidence element, confidence is
146 95% [RFC5491]. A Point shape does not have uncertainty (or it has
147 infinite uncertainty), so confidence is meaningless for a point;
148 therefore, this element MUST be omitted if only a point is provided.
150 2.1. Generating Locations with Confidence
152 Location generators SHOULD attempt to ensure that confidence is equal
153 in each dimension when generating location information. This
154 restriction, while not always practical, allows for more accurate
155 scaling, if scaling is necessary.
157 Confidence MUST NOT be included unless location information cannot be
158 acquired with 95% confidence.
160 2.2. Consuming and Presenting Confidence
162 The inclusion of confidence that is anything other than 95% presents
163 a potentially difficult usability for applications that use location
164 information. Effectively communicating the probability that a
165 location is incorrect to a user can be difficult.
167 It is inadvisable to simply display locations of any confidence, or
168 to display confidence in a separate or non-obvious fashion. If
169 locations with different confidence levels are displayed such that
170 the distinction is subtle or easy to overlook - such as using fine
171 graduations of color or transparency for graphical uncertainty
172 regions, or displaying uncertainty graphically, but providing
173 confidence as supplementary text - a user could fail to notice a
174 difference in the quality of the location information that might be
175 significant.
177 Depending on the circumstances, different ways of handling confidence
178 might be appropriate. [I-D.thomson-geopriv-uncertainty] describes
179 techniques that could be appropriate for consumers that use automated
180 processing as well as background on the issue.
182 Providing that the full implications of any choice for the
183 application are understood, some amount of automated processing could
184 be appropriate. In a simple example, applications could choose to
185 discard or suppress the display of location information if confidence
186 does not meet a pre-determined threshold.
188 In settings where there is an opportunity for user training, some of
189 these problems might be mitigated by defining different operational
190 procedures for handling location information at different confidence
191 levels.
193 3. Example
195 The PIDF-LO document in Figure 1 includes a representation of
196 uncertainty as a circular area. The confidence element (on the line
197 marked with a comment) indicates that the confidence is 67% and that
198 it follows a normal distribution.
200
See RFCXXXX.
305 306 307 END 309 5.2. XML Schema Registration 311 This section registers an XML schema as per the guidelines in 312 [RFC3688]. 314 URI: urn:ietf:params:xml:schema:geopriv:conf 316 Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org), 317 Martin Thomson (martin.thomson@andrew.com). 319 Schema: The XML for this schema can be found as the entirety of 320 Section 4 of this document. 322 6. Security Considerations 324 The security (and privacy) implications related to adding this 325 information are not significant. 327 7. References 329 7.1. Normative References 331 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 332 Requirement Levels", BCP 14, RFC 2119, March 1997. 334 [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, 335 January 2004. 337 [RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object 338 Format", RFC 4119, December 2005. 340 7.2. Informative References 342 [RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and 343 J. Polk, "Geopriv Requirements", RFC 3693, February 2004. 345 [I-D.thomson-geopriv-uncertainty] 346 Thomson, M. and J. Winterbottom, "Representation of 347 Uncertainty and Confidence in PIDF-LO", draft-thomson- 348 geopriv-uncertainty-07 (work in progress), March 2012. 350 [RFC5491] Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV 351 Presence Information Data Format Location Object (PIDF-LO) 352 Usage Clarification, Considerations, and Recommendations", 353 RFC 5491, March 2009. 355 [TS-3GPP-23_032] 356 3GPP, "Universal Geographic Area Description (GAD)", 3GPP 357 TS 23.032 11.0.0, September 2012. 359 Author's Address 361 Martin Thomson 362 Microsoft 363 Mountain View, VA 94043 364 US 366 EMail: martin.thomson@gmail.com