--- 1/draft-ietf-geopriv-uncertainty-03.txt 2014-10-23 21:14:47.004954403 -0700
+++ 2/draft-ietf-geopriv-uncertainty-04.txt 2014-10-23 21:14:47.072956069 -0700
@@ -1,46 +1,47 @@
GEOPRIV M. Thomson
Internet-Draft Mozilla
Updates: 3693,4119,5491 (if approved) J. Winterbottom
Intended status: Standards Track Unaffiliated
-Expires: March 20, 2015 September 16, 2014
+Expires: April 25, 2015 October 22, 2014
Representation of Uncertainty and Confidence in PIDF-LO
- draft-ietf-geopriv-uncertainty-03
+ draft-ietf-geopriv-uncertainty-04
Abstract
- The key concepts of uncertainty and confidence as they pertain to
- location information are defined. Methods for the manipulation of
- location estimates that include uncertainty information are outlined.
+ This document defines key concepts of uncertainty and confidence as
+ they pertain to location information. Methods for the manipulation
+ of location estimates that include uncertainty information are
+ outlined.
- This draft normatively updates the definition of location information
- representations defined in RFC 4119 and RFC 5491. It also deprecates
- related terminology defined in RFC 3693.
+ This document normatively updates the definition of location
+ information representations defined in RFC 4119 and RFC 5491. It
+ also deprecates related terminology defined in RFC 3693.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
- This Internet-Draft will expire on March 20, 2015.
+ This Internet-Draft will expire on April 25, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
@@ -92,21 +93,21 @@
8.2. XML Schema Registration . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
11.1. Normative References . . . . . . . . . . . . . . . . . . 31
11.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Conversion Between Cartesian and Geodetic
Coordinates in WGS84 . . . . . . . . . . . . . . . . 33
Appendix B. Calculating the Upward Normal of a Polygon . . . . . 34
B.1. Checking that a Polygon Upward Normal Points Up . . . . . 35
- Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
Location information represents an estimation of the position of a
Target [RFC6280]. Under ideal circumstances, a location estimate
precisely reflects the actual location of the Target. For automated
systems that determine location, there are many factors that
introduce errors into the measurements that are used to determine
location estimates.
@@ -143,26 +144,28 @@
document are to be interpreted as described in [RFC2119].
This document assumes a basic understanding of the principles of
mathematics, particularly statistics and geometry.
Some terminology is borrowed from [RFC3693] and [RFC6280], in
particular Target.
Mathematical formulae are presented using the following notation: add
"+", subtract "-", multiply "*", divide "/", power "^" and absolute
- value "|x|". Precedence is indicated using parentheses.
- Mathematical functions are represented by common abbreviations:
- square root "sqrt(x)", sine "sin(x)", cosine "cos(x)", inverse cosine
- "acos(x)", tangent "tan(x)", inverse tangent "atan(x)", two-argument
- inverse tangent "atan2(y,x)", error function "erf(x)", and inverse
- error function "erfinv(x)".
+ value "|x|". Precedence follows established conventions: power
+ operations precede multiply and divide, multiply and divide precede
+ add and subtract, and parentheses are used to indicate operations
+ that are applied together. Mathematical functions are represented by
+ common abbreviations: square root "sqrt(x)", sine "sin(x)", cosine
+ "cos(x)", inverse cosine "acos(x)", tangent "tan(x)", inverse tangent
+ "atan(x)", two-argument inverse tangent "atan2(y,x)", error function
+ "erf(x)", and inverse error function "erfinv(x)".
2. A General Definition of Uncertainty
Uncertainty results from the limitations of measurement. In
measuring any observable quantity, errors from a range of sources
affect the result. Uncertainty is a quantification of what is known
about the observed quantity, either through the limitations of
measurement or through inherent variability of the quantity.
Uncertainty is most completely described by a probability
@@ -363,25 +366,25 @@
region. When locating a personal device using contemporary location
determination techniques, the space the device occupies relative to
the uncertainty is proportionally quite small. Even where that
device is used as a proxy for a person, the proportions change
little.
This assumption is less useful as uncertainty becomes small relative
to the size of the Target of the PIDF-LO (or conversely, as
uncertainty becomes small relative to the Target). For instance,
describing the location of a football stadium or small country would
- include a region of uncertainty that is infinitesimally larger than
- the Target itself. In these cases, much of the guidance in this
- document is not applicable. Indeed, as the accuracy of location
- determination technology improves, it could be that the advice this
- document contains becomes less relevant by the same measure.
+ include a region of uncertainty that is only slightly larger than the
+ Target itself. In these cases, much of the guidance in this document
+ is not applicable. Indeed, as the accuracy of location determination
+ technology improves, it could be that the advice this document
+ contains becomes less relevant by the same measure.
3.2. Representation of Uncertainty and Confidence in PIDF-LO
A set of shapes suitable for the expression of uncertainty in
location estimates in the Presence Information Data Format - Location
Object (PIDF-LO) are described in [GeoShape]. These shapes are the
recommended form for the representation of uncertainty in PIDF-LO
[RFC4119] documents.
The PIDF-LO can contain uncertainty, but does not include an
@@ -465,23 +468,23 @@
4. Representation of Confidence in PIDF-LO
On the whole, a fixed definition for confidence is preferable,
primarily because it 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 unable to provide location information with
- consistent confidence. Existing systems sometimes specify confidence
- at 38%, 67% or 90%. Existing forms of expressing location
+ location generators might be unable to provide location information
+ with consistent confidence. Existing systems sometimes specify
+ confidence at 38%, 67% or 90%. Existing forms of expressing location
information, such as that defined in [TS-3GPP-23_032], 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:
o The location server can scale regions of uncertainty in an attempt
@@ -535,21 +538,21 @@
4.2. Generating Locations with Confidence
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.
A confidence element MUST be included with all location information
that includes uncertainty (that is, all forms other than a point). A
- special "unknown" MAY be used if confidence is not known.
+ special "unknown" is used if confidence is not known.
4.3. Consuming and Presenting Confidence
The inclusion of confidence that is anything other than 95% presents
a potentially difficult usability problem for applications that use
location information. Effectively communicating the probability that
a location is incorrect to a user can be difficult.
It is inadvisable to simply display locations of any confidence, or
to display confidence in a separate or non-obvious fashion. If
@@ -700,22 +703,22 @@
later in this section for cases where "Nx" or "Ny" are zero.
[ -Ny/p Nx/p 0 ] [ -Ny/p -Nx*Nz/p Nx ]
T = [ -Nx*Nz/p -Ny*Nz/p p ] T' = [ Nx/p -Ny*Nz/p Ny ]
[ Nx Ny Nz ] [ 0 p Nz ]
(Transform) (Reverse Transform)
Figure 3: Recommended Transformation Matrices
To apply a transform to each point in the polygon, form a matrix from
- the ECEF coordinates and use matrix multiplication to determine the
- translated coordinates.
+ the Cartesian Earth-Centered, Earth-Fixed (ECEF) coordinates and use
+ matrix multiplication to determine the translated coordinates.
[ -Ny/p Nx/p 0 ] [ x[1] x[2] x[3] ... x[n] ]
[ -Nx*Nz/p -Ny*Nz/p p ] * [ y[1] y[2] y[3] ... y[n] ]
[ Nx Ny Nz ] [ z[1] z[2] z[3] ... z[n] ]
[ x'[1] x'[2] x'[3] ... x'[n] ]
= [ y'[1] y'[2] y'[3] ... y'[n] ]
[ z'[1] z'[2] z'[3] ... z'[n] ]
Figure 4: Transformation
@@ -1005,25 +1008,25 @@
Target is within the region of interest, "Pi", is:
Pi = Co * Ao / Au
Given that the area of the region of uncertainty is "Au" and the
confidence is "Co".
This probability is often input to a decision process that has a
limited set of outcomes; therefore, a threshold value needs to be
selected. Depending on the application, different threshold
- probabilities might be selected. In the absence of specific
- recommendations, this document suggests that the probability be
- greater than 50% before a decision is made. If the decision process
- selects between two or more regions, as is required by [RFC5222],
- then the region with the highest probability can be selected.
+ probabilities might be selected. A probability of 50% or greater is
+ recommended before deciding that an uncertain value is within a given
+ region. If the decision process selects between two or more regions,
+ as is required by [RFC5222], then the region with the highest
+ probability can be selected.
5.5.1. Determining the Area of Overlap for Two Circles
Determining the area of overlap between two arbitrary shapes is a
non-trivial process. Reducing areas to circles (see Section 5.2)
enables the application of the following process.
Given the radius of the first circle "r", the radius of the second
circle "R" and the distance between their center points "d", the
following set of formulas provide the area of overlap "Ao".
@@ -1243,21 +1246,21 @@
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.
42.5463 -73.2512
850.24
@@ -1380,30 +1383,38 @@
Schema: The XML for this schema can be found as the entirety of
Section 7 of this document.
9. Security Considerations
This document describes methods for managing and manipulating
uncertainty in location. No specific security concerns arise from
most of the information provided. The considerations of [RFC4119]
all apply.
+ A thorough treatment of the privacy implications of describing
+ location information are discussed in [RFC6280]. Including
+ uncertainty information increases the amount of information
+ available; and altering uncertainty is not an effective privacy
+ mechanism.
+
Providing uncertainty and confidence information can reveal
information about the process by which location information is
generated. For instance, it might reveal information that could be
used to infer that a user is using a mobile device with a GPS, or
that a user is acquiring location information from a particular
network-based service. A Rule Maker might choose to remove
uncertainty-related fields from a location object in order to protect
- this information; though it is noted that this information might not
- be perfectly protected due to difficulties associated with location
- obfuscation, as described in Section 13.5 of [RFC6772].
+ this information. Note however that information might not be
+ perfectly protected due to difficulties associated with location
+ obfuscation, as described in Section 13.5 of [RFC6772]. In
+ particular, increasing uncertainty does not necessarily result in a
+ reduction of the information conveyed by the location object.
Adding confidence to location information risks misinterpretation by
consumers of location that do not understand the element. This could
be exploited, particularly when reducing confidence, since the
resulting uncertainty region might include locations that are less
likely to contain the target than the recipient expects. Since this
sort of error is always a possibility, the impact of this is low.
10. Acknowledgements