--- 1/draft-ietf-geopriv-uncertainty-02.txt 2014-09-16 14:14:36.277107249 -0700
+++ 2/draft-ietf-geopriv-uncertainty-03.txt 2014-09-16 14:14:36.345108907 -0700
@@ -1,42 +1,46 @@
GEOPRIV M. Thomson
Internet-Draft Mozilla
-Intended status: Standards Track J. Winterbottom
-Expires: February 15, 2015 Unaffiliated
- August 14, 2014
+Updates: 3693,4119,5491 (if approved) J. Winterbottom
+Intended status: Standards Track Unaffiliated
+Expires: March 20, 2015 September 16, 2014
Representation of Uncertainty and Confidence in PIDF-LO
- draft-ietf-geopriv-uncertainty-02
+ draft-ietf-geopriv-uncertainty-03
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 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.
+
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 February 15, 2015.
+ This Internet-Draft will expire on March 20, 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
@@ -112,21 +116,21 @@
protocols. This document outlines how uncertainty, and its
associated datum, confidence, are expressed and interpreted.
This document provides a common nomenclature for discussing
uncertainty and confidence as they relate to location information.
This document also provides guidance on how to manage location
information that includes uncertainty. Methods for expanding or
reducing uncertainty to obtain a required level of confidence are
described. Methods for determining the probability that a Target is
- within a specified region based on their location estimate are
+ within a specified region based on its location estimate are
described. These methods are simplified by making certain
assumptions about the location estimate and are designed to be
applicable to location estimates in a relatively small geographic
area.
A confidence extension for the Presence Information Data Format -
Location Object (PIDF-LO) [RFC4119] is described.
This document describes methods that can be used in combination with
automatically determined location information. These are
@@ -223,22 +227,22 @@
o A rectangular PDF is used where the errors are known to be
consistent across a limited range. A rectangular PDF can occur
where a single error source, such as a rounding error, is
significantly larger than other errors. A rectangular PDF is
often described by the half-width of the distribution; that is,
half the width of the distribution.
Each of these probability density functions can be characterized by
its center point, or mean, and its width. For a normal distribution,
uncertainty and confidence together are related to the standard
- deviation (see Section 5.4). For a rectangular distribution, half of
- the width of the distribution is used.
+ deviation of the function (see Section 5.4). For a rectangular
+ distribution, the half-width of the distribution is used.
Figure 1 shows a normal and rectangular probability density function
with the mean (m) and standard deviation (s) labelled. The half-
width (h) of the rectangular distribution is also indicated.
***** *** Normal PDF
** : ** --- Rectangular PDF
** : **
** : **
.---------*---------------*---------.
@@ -394,22 +398,22 @@
If the Point shape is used, confidence and uncertainty are unknown; a
receiver can either assume a confidence of 0% or infinite
uncertainty. The same principle applies on the altitude axis for
two-dimension shapes like the Circle.
3.3. Uncertainty and Confidence for Civic Addresses
Automatically determined civic addresses [RFC5139] inherently include
uncertainty, based on the area of the most precise element that is
specified. In this case, uncertainty is effectively described by the
- presence or absence of elements -- elements that are not present are
- deemed to be uncertain.
+ presence or absence of elements. To the recipient of location
+ information, elements that are not present are uncertain.
To apply the concept of uncertainty to civic addresses, it is helpful
to unify the conceptual models of civic address with geodetic
location information. This is particularly useful when considering
civic addresses that are determined using reverse geocoding (that is,
the process of translating geodetic information into civic
addresses).
In the unified view, a civic address defines a series of (sometimes
non-orthogonal) spatial partitions. The first is the implicit
@@ -445,29 +449,30 @@
3.4. DHCP Location Configuration Information and Uncertainty
Location information is often measured in two or three dimensions;
expressions of uncertainty in one dimension only are rare. The
"resolution" parameters in [RFC6225] provide an indication of how
many bits of a number are valid, which could be interpreted as an
expression of uncertainty in one dimension.
[RFC6225] defines a means for representing uncertainty, but a value
for confidence is not specified. A default value of 95% confidence
- is assumed for the combination of the uncertainty on each axis. This
- is consistent with the transformation of those forms into the
- uncertainty representations from [RFC5491]. That is, the confidence
- of the resultant rectangular polygon or prism is assumed to be 95%.
+ should be assumed for the combination of the uncertainty on each
+ axis. This is consistent with the transformation of those forms into
+ the uncertainty representations from [RFC5491]. That is, the
+ confidence of the resultant rectangular polygon or prism is assumed
+ to be 95%.
4. Representation of Confidence in PIDF-LO
- On the whole, a fixed definition for confidence is preferable.
- Primarily because it ensures consistency between implementations.
+ 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
information, such as that defined in [TS-3GPP-23_032], contain
@@ -890,21 +895,21 @@
Uncertainty that follows a rectangular distribution can only be
decreased in size. Increasing uncertainty has no value, since it has
no effect on confidence. Since the PDF is constant over the region
of uncertainty, the resulting confidence is determined by the
following formula:
Cr = Co * Ur / Uo
Where "Uo" and "Ur" are the sizes of the original and reduced regions
of uncertainty (either the area or the volume of the region); "Co"
- and "Cb" are the confidence values associated with each region.
+ and "Cr" are the confidence values associated with each region.
Information is lost by decreasing the region of uncertainty for a
rectangular distribution. Once reduced in size, the uncertainty
region cannot subsequently be increased in size.
5.4.2. Normal Distributions
Uncertainty and confidence can be both increased and decreased for a
normal distribution. This calculation depends on the number of
dimensions of the uncertainty region.
@@ -1080,40 +1085,42 @@
3.31
28.7
43
+ 95
Figure 8
This information can be reduced to a point simply by extracting the
center point, that is [-34.407242, 150.882518, 34].
If some limited uncertainty were required, the estimate could be
converted into a circle or sphere. To convert to a sphere, the
radius is the largest of the semi-major, semi-minor and vertical
axes; in this case, 28.7 meters.
However, if only a circle is required, the altitude can be dropped as
can the altitude uncertainty (the vertical axis of the ellipsoid),
resulting in a circle at [-34.407242, 150.882518] of radius 7.7156
meters.
- Bob receives a location estimate with a Polygon shape. This
+ Bob receives a location estimate with a Polygon shape (which roughly
+ corresponds to the location of the Sydney Opera House). This
information is shown in Figure 9.
-33.856625 151.215906 -33.856299 151.215343
-33.856326 151.214731 -33.857533 151.214495
-33.857720 151.214613 -33.857369 151.215375
-33.856625 151.215906
@@ -1121,22 +1128,23 @@
Figure 9
To convert this to a polygon, each point is firstly assigned an
altitude of zero and converted to ECEF coordinates (see Appendix A).
Then a normal vector for this polygon is found (see Appendix B). The
result of each of these stages is shown in Figure 10. Note that the
- numbers shown are all rounded; no rounding is possible during this
- process since rounding would contribute significant errors.
+ numbers shown in this document are rounded only for formatting
+ reasons; the actual calculations do not include rounding, which would
+ generate significant errors in the final values.
Polygon in ECEF coordinate space
(repeated point omitted and transposed to fit):
[ -4.6470e+06 2.5530e+06 -3.5333e+06 ]
[ -4.6470e+06 2.5531e+06 -3.5332e+06 ]
pecef = [ -4.6470e+06 2.5531e+06 -3.5332e+06 ]
[ -4.6469e+06 2.5531e+06 -3.5333e+06 ]
[ -4.6469e+06 2.5531e+06 -3.5334e+06 ]
[ -4.6469e+06 2.5531e+06 -3.5333e+06 ]
@@ -1173,46 +1181,46 @@
The point conversion for the polygon uses the final result, "Cg",
ignoring the altitude since the original shape did not include
altitude.
To convert this to a circle, take the maximum distance in ECEF
coordinates from the center point to each of the points. This
results in a radius of 99.1 meters. Confidence is unchanged.
6.2. Increasing and Decreasing Confidence
- Assuming that confidence is known to be 19% for Alice's location
+ Assume that confidence is known to be 19% for Alice's location
information. This is a typical value for a three-dimensional
ellipsoid uncertainty of normal distribution where the standard
deviation is used directly for uncertainty in each dimension. The
confidence associated with Alice's location estimate is quite low for
many applications. Since the estimate is known to follow a normal
distribution, the method in Section 5.4.2 can be used. Each axis can
be scaled by:
scale = erfinv(0.95^(1/3)) / erfinv(0.19^(1/3)) = 2.9937
Ensuring that rounding always increases uncertainty, the location
estimate at 95% includes a semi-major axis of 23.1, a semi-minor axis
of 10 and a vertical axis of 86.
Bob's location estimate (from the previous example) covers an area of
approximately 12600 square meters. If the estimate follows a
rectangular distribution, the region of uncertainty can be reduced in
size. Here we find the confidence that Bob is within the smaller
- area of the concert hall. For the concert hall, the polygon
+ area of the Concert Hall. For the Concert Hall, the polygon
[-33.856473, 151.215257; -33.856322, 151.214973;
-33.856424, 151.21471; -33.857248, 151.214753;
-33.857413, 151.214941; -33.857311, 151.215128] is used. To use this
new region of uncertainty, find its area using the same translation
method described in Section 5.1.1.2, which produces 4566.2 square
- meters. Given that the concert hall is entirely within Bob's
+ meters. Given that the Concert Hall is entirely within Bob's
original location estimate, the confidence associated with the
smaller area is therefore 95% * 4566.2 / 12600 = 34%.
6.3. Matching Location Estimates to Regions of Interest
Suppose that a circular area is defined centered at
[-33.872754, 151.20683] with a radius of 1950 meters. To determine
whether Bob is found within this area - given that Bob is at
[-34.407242, 150.882518] with an uncertainty radius 7.7156 meters -
we apply the method in Section 5.5. Using the converted Circle shape
@@ -1369,21 +1377,33 @@
Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org),
Martin Thomson (martin.thomson@gmail.com).
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.
+ most of the information provided. The considerations of [RFC4119]
+ all apply.
+
+ 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].
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