Network Working Group J. Winterbottom

Network Working Group J. Winterbottom
Request for Comments: 5491 M. Thomson
Updates: 4119 Andrew Corporation
Category: Standards Track H. Tschofenig
Nokia Siemens Networks
March 2009

GEOPRIV Presence Information Data Format Location Object (PIDF-LO)
Usage Clarification, Considerations, and Recommendations

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.

Winterbottom, et al. Standards Track [Page 1]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

Abstract

The Presence Information Data Format Location Object (PIDF-LO)
specification provides a flexible and versatile means to represent
location information. There are, however, circumstances that arise
when information needs to be constrained in how it is represented.
In these circumstances, the range of options that need to be
implemented are reduced. There is growing interest in being able to
use location information contained in a PIDF-LO for routing
applications. To allow successful interoperability between
applications, location information needs to be normative and more
tightly constrained than is currently specified in RFC 4119 (PIDF-
LO). This document makes recommendations on how to constrain,
represent, and interpret locations in a PIDF-LO. It further
recommends a subset of Geography Markup Language (GML) 3.1.1 that is
mandatory to implement by applications involved in location-based
routing.

Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................3
3. Using Location Information ......................................4
3.1. Single Civic Location Information ..........................7
3.2. Civic and Geospatial Location Information ..................7
3.3. Manual/Automatic Configuration of Location Information .....8
3.4. Multiple Location Objects in a Single PIDF-LO ..............9
4. Geodetic Coordinate Representation .............................10
5. Geodetic Shape Representation ..................................10
5.1. Polygon Restrictions ......................................12
5.2. Shape Examples ............................................13
5.2.1. Point ..............................................13
5.2.2. Polygon ............................................14
5.2.3. Circle .............................................17
5.2.4. Ellipse ............................................17
5.2.5. Arc Band ...........................................19
5.2.6. Sphere .............................................21
5.2.7. Ellipsoid ..........................................22
5.2.8. Prism ..............................................24
6. Security Considerations ........................................26
7. Acknowledgments ................................................26
8. References .....................................................26
8.1. Normative References ......................................26
8.2. Informative References ....................................27

Winterbottom, et al. Standards Track [Page 2]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

1. Introduction

The Presence Information Data Format Location Object (PIDF-LO)
[RFC4119] is the recommended way of encoding location information and
associated privacy policies. Location information in a PIDF-LO may
be described in a geospatial manner based on a subset of Geography
Markup Language (GML) 3.1.1 [OGC-GML3.1.1] or as civic location
information [RFC5139]. A GML profile for expressing geodetic shapes
in a PIDF-LO is described in [GeoShape]. Uses for the PIDF-LO are
envisioned in the context of numerous location-based applications.
This document makes recommendations for formats and conventions to
make interoperability less problematic.

The PIDF-LO provides a general presence format for representing
location information, and permits specification of location
information relating to a whole range of aspects of a Target. The
general presence data model is described in [RFC4479] and caters to a
presence document to describe different aspects of the reachability
of a presentity. Continuing this approach, a presence document may
contain several GEOPRIV objects that specify different locations and
aspects of reachability relating to a presentity. This degree of
flexibility is important, and recommendations in this document make
no attempt to forbid the usage of a PIDF-LO in this manner. This
document provides a specific set of guidelines for building presence
documents when it is important to unambiguously convey exactly one
location.

2. Terminology

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].

The definition for "Target" is taken from [RFC3693].

In this document a "discrete location" is defined as a place, point,
area, or volume in which a Target can be found.

The term "compound location" is used to describe location information
represented by a composite of both civic and geodetic information.
An example of compound location might be a geodetic polygon
describing the perimeter of a building and a civic element
representing the floor in the building.

The term "method" in this document refers to the mechanism used to
determine the location of a Target. This may be something employed
by a location information server (LIS), or by the Target itself. It

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specifically does not refer to the location configuration protocol
(LCP) used to deliver location information either to the Target or
the Recipient.

The term "source" is used to refer to the LIS, node, or device from
which a Recipient (Target or Third-Party) obtains location
information.

3. Using Location Information

The PIDF format provides for an unbounded number of <tuple>,
<device>, and <person> elements. Each of these elements contains a
single <status> element that may contain more than one <geopriv>
element as a child. Each <geopriv> element must contain at least the
following two child elements: <location-info> element and <usage-
rules> element. One or more elements containing location information
are contained inside a <location-info> element.

Hence, a single PIDF document may contain an arbitrary number of
location objects, some or all of which may be contradictory or
complementary. Graphically, the structure of a PIDF-LO document can
be depicted as shown in Figure 1.

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<presence>
<tuple> -- #1
<status>
<geopriv> -- #1
<location-info>
location element #1
location element #2
...
location element #n
<usage-rules>
</geopriv>
<geopriv> -- #2
<geopriv> -- #3
...
<geopriv> -- #m
</status>
</tuple>
<device>
<geopriv> -- #1
<location-info>
location element(s)
<usage-rules>
</geopriv>
<geopriv> -- #2
...
<geopriv> -- #m
</device>
<person>
<geopriv> -- #1
<location-info>
location element(s)
<usage-rules>
</geopriv>
<geopriv> -- #2
...
<geopriv> -- #m
</person>
<tuple> -- #2
<device> -- #2
<person> -- #2
...
<tuple> -- #o
</presence>

Figure 1: Structure of a PIDF-LO Document

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All of these potential sources and storage places for location lead
to confusion for the generators, conveyors, and consumers of location
information. Practical experience within the United States National
Emergency Number Association (NENA) in trying to solve these
ambiguities led to a set of conventions being adopted. These rules
do not have any particular order, but should be followed by creators
and consumers of location information contained in a PIDF-LO to
ensure that a consistent interpretation of the data can be achieved.

Rule #1: A <geopriv> element MUST describe a discrete location.

Rule #2: Where a discrete location can be uniquely described in more
than one way, each location description SHOULD reside in a
separate <tuple>, <device>, or <person> element; only one geopriv
element per tuple.

Rule #3: Providing more than one <geopriv> element in a single
presence document (PIDF) MUST only be done if the locations refer
to the same place or are put into different element types. For
example, one location in a <tuple>, a second location in a
<device> element, and a third location in a <person> element.

This may occur if a Target's location is determined using a
series of different techniques or if the Target wishes to
represent her location as well as the location of her PC. In
general, avoid putting more than one location into a document
unless it makes sense to do so.

Rule #4: Providing more than one location chunk in a single
<location-info> element SHOULD be avoided where possible. Rule #5
and Rule #6 provide further refinement.

Rule #5: When providing more than one location chunk in a single
<location-info> element, the locations MUST be provided by a
common source at the same time and by the same location
determination method.

Rule #6: Providing more than one location chunk in a single
<location-info> element SHOULD only be used for representing
compound location referring to the same place.

For example, a geodetic location describing a point, and a
civic location indicating the floor in a building.

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Rule #7: Where the compound location is provided in a single
<location-info> element, the coarse location information MUST be
provided first.

For example, a geodetic location describing an area and a civic
location indicating the floor should be represented with the
area first followed by the civic location.

Rule #8: Where a PIDF document contains more than one <geopriv>
element, the priority of interpretation is given to the first
<device> element in the document containing a location. If no
<device> element containing a location is present in the document,
then priority is given to the first <tuple> element containing a
location. Locations contained in <person> tuples SHOULD only be
used as a last resort.

Rule #9: Where multiple PIDF documents can be sent or received
together, say in a multi-part MIME body, and current location
information is required by the recipient, then document selection
SHOULD be based on document order, with the first document
considered first.

The following examples illustrate the application of these rules.

3.1. Single Civic Location Information

Jane is at a coffee shop on the ground floor of a large shopping
mall. Jane turns on her laptop and connects to the coffee shop's
WiFi hotspot; Jane obtains a complete civic address for her current
location, for example, using the DHCP civic mechanism defined in
[RFC4776]. A Location Object is constructed consisting of a single
PIDF document, with a single <tuple> or <device> element, a single
<status> element, a single <geopriv> element, and a single location
chunk residing in the <location-info> element. This document is
unambiguous, and should be interpreted consistently by receiving
nodes if sent over the network.

3.2. Civic and Geospatial Location Information

Mike is visiting his Seattle office and connects his laptop into the
Ethernet port in a spare cube. In this case, location information is
geodetic location, with the altitude represented as a building floor
number. Mike's main location is the point specified by the geodetic
coordinates. Further, Mike is on the second floor of the building
located at these coordinates. Applying rules #6 and #7, the
resulting compound location information is shown in Figure 2.

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<presence xmlns="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:gml="http://www.opengis.net/gml"
xmlns:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
entity="pres:[email protected]">
<dm:device id="mikepc">
<gp:geopriv>
<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-43.5723 153.21760</gml:pos>
</gml:Point>
<cl:civicAddress>
<cl:FLR>2</cl:FLR>
</cl:civicAddress>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
<dm:deviceID>mac:8asd7d7d70cf</dm:deviceID>
<dm:timestamp>2007-06-22T20:57:29Z</dm:timestamp>
</dm:device>
</presence>

Figure 2: PIDF-LO Containing a Compound Location

3.3. Manual/Automatic Configuration of Location Information

Loraine has a predefined civic location stored in her laptop, since
she normally lives in Sydney, the address is for her Sydney-based
apartment. Loraine decides to visit sunny San Francisco, and when
she gets there, she plugs in her laptop and makes a call. Loraine's
laptop receives a new location from the visited network in San
Francisco. As this system cannot be sure that the preexisting and
new location both describe the same place, Loraine's computer
generates a new PIDF-LO and will use this to represent Loraine's
location. If Loraine's computer were to add the new location to her
existing PIDF location document (breaking rule #3), then the correct
information may still be interpreted by the Location Recipient
providing Loraine's system applies rule #9. In this case, the
resulting order of location information in the PIDF document should
be San Francisco first, followed by Sydney. Since the information is
provided by different sources, rule #8 should also be applied and the
information placed in different tuples with the tuple containing the
San Francisco location first.

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3.4. Multiple Location Objects in a Single PIDF-LO

Vanessa has her PC with her at the park, but due to a
misconfiguration, her PC reports her location as being in the office.
The resulting PIDF-LO will have a <device> element showing the
location of Vanessa's PC as the park, and a <person> element saying
that Vanessa is in her office.

<presence xmlns="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:ca="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<dm:device id="nesspc-1">
<gp:geopriv>
<gp:location-info>
<ca:civicAddress xml:lang="en-AU">
<ca:country>AU</ca:country>
<ca:A1>NSW</ca:A1>
<ca:A3> Wollongong
</ca:A3><ca:A4>North Wollongong
</ca:A4>
<ca:RD>Flinders</ca:RD><ca:STS>Street</ca:STS>
<ca:RDBR>Campbell Street</ca:RDBR>
<ca:LMK>
Gilligan's Island
</ca:LMK> <ca:LOC>Corner</ca:LOC>
<ca:NAM> Video Rental Store </ca:NAM>
<ca:PC>2500</ca:PC>
<ca:ROOM> Westerns and Classics </ca:ROOM>
<ca:PLC>store</ca:PLC>
<ca:POBOX>Private Box 15</ca:POBOX>
</ca:civicAddress>
</gp:location-info>
<gp:usage-rules/>
<gp:method>GPS</gp:method>
</gp:geopriv>
<dm:deviceID>mac:1234567890ab</dm:deviceID>
<dm:timestamp>2007-06-22T20:57:29Z</dm:timestamp>
</dm:device>
<dm:person id="ness">
<gp:geopriv>
<gp:location-info>
<gs:Circle srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-34.410649 150.87651</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">

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30
</gs:radius>
</gs:Circle>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Manual</gp:method>
</gp:geopriv>
<dm:timestamp>2007-06-24T12:28:04Z</dm:timestamp>
</dm:person>
</presence>

Figure 3: PIDF-LO Containing Multiple Location Objects

4. Geodetic Coordinate Representation

The geodetic examples provided in RFC 4119 [RFC4119] are illustrated
using the <gml:location> element, which uses the <gml:coordinates>
element inside the <gml:Point> element, and this representation has
several drawbacks. Firstly, it has been deprecated in later versions
of GML (3.1 and beyond) making it inadvisable to use for new
applications. Secondly, the format of the coordinates type is opaque
and so can be difficult to parse and interpret to ensure consistent
results, as the same geodetic location can be expressed in a variety
of ways. The PIDF-LO Geodetic Shapes specification [GeoShape]
provides a specific GML profile for expressing commonly used shapes
using simple GML representations. The shapes defined in [GeoShape]
are the recommended shapes to ensure interoperability.

5. Geodetic Shape Representation

The cellular mobile world today makes extensive use of geodetic-based
location information for emergency and other location-based
applications. Generally, these locations are expressed as a point
(either in two or three dimensions) and an area or volume of
uncertainty around the point. In theory, the area or volume
represents a coverage in which the user has a relatively high
probability of being found, and the point is a convenient means of
defining the centroid for the area or volume. In practice, most
systems use the point as an absolute value and ignore the
uncertainty. It is difficult to determine if systems have been
implemented in this manner for simplicity, and even more difficult to
predict if uncertainty will play a more important role in the future.
An important decision is whether an uncertainty area should be
specified.

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The PIDF-LO Geodetic Shapes specification [GeoShape] defines eight
shape types, most of which are easily translated into shape
definitions used in other applications and protocols, such as the
Open Mobile Alliance (OMA) Mobile Location Protocol (MLP). For
completeness, the shapes defined in [GeoShape] are listed below:

o Point (2d and 3d)

o Polygon (2d)

o Circle (2d)

o Ellipse (2d)

o Arc band (2d)

o Sphere (3d)

o Ellipsoid (3d)

o Prism (3d)

The above-listed shapes MUST be implemented.

The GeoShape specification [GeoShape] also describes a standard set
of coordinate reference systems (CRS), unit of measure (UoM) and
conventions relating to lines and distances. The use of the world
geodetic system 1984 (WGS84) [WGS84] coordinate reference system and
the usage of European petroleum survey group (EPSG) code 4326 (as
identified by the URN urn:ogc:def:crs:EPSG::4326, [CRS-URN]) for two-
dimensional (2d) shape representations and EPSG 4979 (as identified
by the URN urn:ogc:def:crs:EPSG::4979) for three-dimensional (3d)
volume representations is mandated. Distance and heights are
expressed in meters using EPSG 9001 (as identified by the URN
urn:ogc:def:uom:EPSG::9001). Angular measures MUST use either
degrees or radians. Measures in degrees MUST be identified by the
URN urn:ogc:def:uom:EPSG::9102, measures in radians MUST be
identified by the URN urn:ogc:def:uom:EPSG::9101. Angles
representing bearings are measured in a clockwise direction from
Northing, as defined by the WGS84 CRS, not magnetic north.

Implementations MUST specify the CRS using the srsName attribute on
the outermost geometry element. The CRS MUST NOT be respecified or
changed for any sub-elements. The srsDimension attribute SHOULD be
omitted, since the number of dimensions in these CRSs is known. A
CRS MUST be specified using the above URN notation only;
implementations do not need to support user-defined CRSs.

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Numerical values for coordinates and measures are expressed using the
lexical representation for "double" defined in
[W3C.REC-xmlschema-2-20041028]. Leading zeros and trailing zeros
past the decimal point are not significant; for instance "03.07500"
is equivalent to "3.075".

It is RECOMMENDED that uncertainty is expressed at a confidence of
95% or higher. Specifying a convention for confidence enables better
use of uncertainty values.

5.1. Polygon Restrictions

The polygon shape type defined in [GeoShape] intentionally does not
place any constraints on the number of vertices that may be included
to define the bounds of a polygon. This allows arbitrarily complex
shapes to be defined and conveyed in a PIDF-LO. However, where
location information is to be used in real-time processing
applications, such as location-dependent routing, having arbitrarily
complex shapes consisting of tens or even hundreds of points could
result in significant performance impacts. To mitigate this risk,
Polygon shapes SHOULD be restricted to a maximum of 15 points (16
including the repeated point) when the location information is
intended for use in real-time applications. This limit of 15 points
is chosen to allow moderately complex shape definitions while at the
same time enabling interoperation with other location transporting
protocols such as those defined in the 3rd Generation Partnership
Project (3GPP) (see [3GPP.23.032]) and OMA where the 15-point limit
is already imposed.

The edges of a polygon are defined by the shortest path between two
points in space (not a geodesic curve). Two-dimensional points MAY
be interpreted as having a zero value for their altitude component.
To avoid significant errors arising from potential geodesic
interpolation, the length between adjacent vertices SHOULD be
restricted to a maximum of 130 km. More information relating to this
restriction is provided in [GeoShape].

A connecting line SHALL NOT cross another connecting line of the same
Polygon.

Polygons MUST be defined with the upward normal pointing up. This is
accomplished by defining the vertices in a counter-clockwise
direction.

Points specified in a polygon using three-dimensional coordinates
MUST all have the same altitude.

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5.2. Shape Examples

This section provides some examples of where some of the more complex
shapes are used, how they are determined, and how they are
represented in a PIDF-LO. Complete details on all of the GeoShape
types are provided in [GeoShape].

5.2.1. Point

The point shape type is the simplest form of geodetic location
information (LI), which is natively supported by GML. The gml:Point
element is used when there is no known uncertainty. A point also
forms part of a number of other geometries. A point may be specified
using either WGS 84 (latitude, longitude) or WGS 84 (latitude,
longitude, altitude). Figure 4 shows a 2d point:

<presence xmlns="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:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:[email protected]">
<dm:device id="point2d">
<gp:geopriv>
<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-34.407 150.883</gml:pos>
</gml:Point>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
<dm:deviceID>mac:1234567890ab</dm:deviceID>
<dm:timestamp>2007-06-22T20:57:29Z</dm:timestamp>
</dm:device>
</presence>

Figure 4: PIDF-LO Containing a Two-Dimensional Point

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Figure 5 shows a 3d point:

<presence xmlns="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:gml="http://www.opengis.net/gml"
entity="pres:[email protected]">
<dm:device id="point3d">
<gp:geopriv>
<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4979"
xmlns:gml="http://www.opengis.net/gml">
<gml:pos>-34.407 150.883 24.8</gml:pos>
</gml:Point>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
<dm:deviceID>mac:1234567890ab</dm:deviceID>
<dm:timestamp>2007-06-22T20:57:29Z</dm:timestamp>
</dm:device>
</presence>

Figure 5: PIDF-LO Containing a Three-Dimensional Point

5.2.2. Polygon

The polygon shape type may be used to represent a building outline or
coverage area. The first and last points of the polygon have to be
the same. For example, looking at the hexagon in Figure 6 with
vertices, A, B, C, D, E, and F. The resulting polygon will be
defined with 7 points, with the first and last points both having the
coordinates of point A.

F--------------E
/ \
/ \
/ \
A D
\ /
\ /
\ /
B--------------C

Figure 6: Example of a Polygon

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<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:[email protected]">
<tuple id="polygon-pos">
<status>
<gp:geopriv>
<gp:location-info>
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:pos>43.311 -73.422</gml:pos> 
<gml:pos>43.111 -73.322</gml:pos> 
<gml:pos>43.111 -73.222</gml:pos> 
<gml:pos>43.311 -73.122</gml:pos> 
<gml:pos>43.411 -73.222</gml:pos> 
<gml:pos>43.411 -73.322</gml:pos> 
<gml:pos>43.311 -73.422</gml:pos> 
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 7: PIDF-LO Containing a Polygon

In addition to the form shown in Figure 7, GML supports a posList
that provides a more compact representation for the coordinates of
the Polygon vertices than the discrete pos elements. The more
compact form is shown in Figure 8. Both forms are permitted.

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<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:[email protected]">
<tuple id="polygon-poslist">
<status>
<gp:geopriv>
<gp:location-info>
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:posList>
43.311 -73.422 43.111 -73.322
43.111 -73.222 43.311 -73.122
43.411 -73.222 43.411 -73.322
43.311 -73.422
</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 8: Compact Form of a Polygon Expressed in a PIDF-LO

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5.2.3. Circle

The circular area is used for coordinates in two-dimensional CRSs to
describe uncertainty about a point. The definition is based on the
one-dimensional geometry in GML, gml:CircleByCenterPoint. The center
point of a circular area is specified by using a two-dimensional CRS;
in three dimensions, the orientation of the circle cannot be
specified correctly using this representation. A point with
uncertainty that is specified in three dimensions should use the
sphere shape type.

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="circle">
<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>
</gp:location-info>
<gp:usage-rules/>
<gp:method>OTDOA</gp:method>
</gp:geopriv>
</status>
</tuple>
</presence>

Figure 9: PIDF-LO Containing a Circle

5.2.4. Ellipse

An elliptical area describes an ellipse in two-dimensional space.
The ellipse is described by a center point, the length of its semi-
major and semi-minor axes, and the orientation of the semi-major
axis. Like the circular area (Circle), the ellipse MUST be specified
using the two-dimensional CRS.

Winterbottom, et al. Standards Track [Page 17]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="ellipse">
<status>
<gp:geopriv>
<gp:location-info>
<gs:Ellipse srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>42.5463 -73.2512</gml:pos>
<gs:semiMajorAxis uom="urn:ogc:def:uom:EPSG::9001">
1275
</gs:semiMajorAxis>
<gs:semiMinorAxis uom="urn:ogc:def:uom:EPSG::9001">
670
</gs:semiMinorAxis>
<gs:orientation uom="urn:ogc:def:uom:EPSG::9102">
43.2
</gs:orientation>
</gs:Ellipse>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Device-Assisted_A-GPS</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 10: PIDF-LO Containing an Ellipse

The gml:pos element indicates the position of the center, or origin,
of the ellipse. The gs:semiMajorAxis and gs:semiMinorAxis elements
are the length of the semi-major and semi-minor axes, respectively.
The gs:orientation element is the angle by which the semi-major axis
is rotated from the first axis of the CRS towards the second axis.
For WGS 84, the orientation indicates rotation from Northing to
Easting, which, if specified in degrees, is roughly equivalent to a
compass bearing (if magnetic north were the same as the WGS north
pole). Note: An ellipse with equal major and minor axis lengths is a
circle.

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5.2.5. Arc Band

The arc band shape type is commonly generated in wireless systems
where timing advance or code offsets sequences are used to compensate
for distances between handsets and the access point. The arc band is
represented as two radii emanating from a central point, and two
angles that represent the starting angle and the opening angle of the
arc. In a cellular environment, the central point is nominally the
location of the cell tower, the two radii are determined by the
extent of the timing advance, and the two angles are generally
provisioned information.

For example, Paul is using a cellular wireless device and is 7 timing
advance symbols away from the cell tower. For a GSM-based network,
this would place Paul roughly between 3,594 meters and 4,148 meters
from the cell tower, providing the inner and outer radius values. If
the start angle is 20 degrees from north, and the opening angle is
120 degrees, an arc band representing Paul's location would look
similar to Figure 11.

N ^ ,.__
| a(s) / `-.
| 20 / `-.
|--. / `.
| `/ \
| /__ \
| . `-. \
| . `. \
|. \ \ .
---c-- a(o) -- | | -->
|. / 120 ' | E
| . / '
| . / ;
.,' /
r(i)`. /
(3594m) `. /
`. ,'
`. ,'
r(o)`'
(4148m)

Figure 11: Example of an Arc Band

The resulting PIDF-LO is shown in Figure 12.

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RFC 5491 GEOPRIV PIDF-LO Usage March 2009

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="arcband">
<status>
<gp:geopriv>
<gp:location-info>
<gs:ArcBand srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>-43.5723 153.21760</gml:pos>
<gs:innerRadius uom="urn:ogc:def:uom:EPSG::9001">
3594
</gs:innerRadius>
<gs:outerRadius uom="urn:ogc:def:uom:EPSG::9001">
4148
</gs:outerRadius>
<gs:startAngle uom="urn:ogc:def:uom:EPSG::9102">
20
</gs:startAngle>
<gs:openingAngle uom="urn:ogc:def:uom:EPSG::9102">
20
</gs:openingAngle>
</gs:ArcBand>
</gp:location-info>
<gp:usage-rules/>
<gp:method>TA-NMR</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 12: PIDF-LO Containing an Arc Band

An important note to make on the arc band is that the center point
used in the definition of the shape is not included in resulting
enclosed area, and that Target may be anywhere in the defined area of
the arc band.

Winterbottom, et al. Standards Track [Page 20]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

5.2.6. Sphere

The sphere is a volume that provides the same information as a circle
in three dimensions. The sphere has to be specified using a three-
dimensional CRS. Figure 13 shows the sphere shape type, which is
identical to the circle example, except for the addition of an
altitude in the provided coordinates.

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="sphere">
<status>
<gp:geopriv>
<gp:location-info>
<gs:Sphere srsName="urn:ogc:def:crs:EPSG::4979">
<gml:pos>42.5463 -73.2512 26.3</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">
850.24
</gs:radius>
</gs:Sphere>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Device-Based_A-GPS</gp:method>
</gp:geopriv>
</status>
</tuple>
</presence>

Figure 13: PIDF-LO Containing a Sphere

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5.2.7. Ellipsoid

The ellipsoid is the volume most commonly produced by GPS systems.
It is used extensively in navigation systems and wireless location
networks. The ellipsoid is constructed around a central point
specified in three dimensions, and three axes perpendicular to one
another are extended outwards from this point. These axes are
defined as the semi-major (M) axis, the semi-minor (m) axis, and the
vertical (v) axis, respectively. An angle is used to express the
orientation of the ellipsoid. The orientation angle is measured in
degrees from north, and represents the direction of the semi-major
axis from the center point.

\
_.-\""""^"""""-._
.' \ | `.
/ v m \
| \ | |
| -c ----M---->|
| |
\ /
`._ _.'
`-...........-'

Figure 14: Example of an Ellipsoid

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RFC 5491 GEOPRIV PIDF-LO Usage March 2009

A PIDF-LO containing an ellipsoid appears as shown in Figure 15.

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="ellipsoid">
<status>
<gp:geopriv>
<gp:location-info>
<gs:Ellipsoid srsName="urn:ogc:def:crs:EPSG::4979">
<gml:pos>42.5463 -73.2512 26.3</gml:pos>
<gs:semiMajorAxis uom="urn:ogc:def:uom:EPSG::9001">
7.7156
</gs:semiMajorAxis>
<gs:semiMinorAxis uom="urn:ogc:def:uom:EPSG::9001">
3.31
</gs:semiMinorAxis>
<gs:verticalAxis uom="urn:ogc:def:uom:EPSG::9001">
28.7
</gs:verticalAxis>
<gs:orientation uom="urn:ogc:def:uom:EPSG::9102">
90
</gs:orientation>
</gs:Ellipsoid>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Hybrid_A-GPS</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 15: PIDF-LO Containing an Ellipsoid

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RFC 5491 GEOPRIV PIDF-LO Usage March 2009

5.2.8. Prism

A prism may be used to represent a section of a building or range of
floors of building. The prism extrudes a polygon by providing a
height element. It consists of a base made up of coplanar points
defined in 3 dimensions all at the same altitude. The prism is then
an extrusion from this base to the value specified in the height
element. The height of the Prism MUST be a positive value. The
first and last points of the polygon have to be the same.

For example, looking at the cube in Figure 16: if the prism is
extruded from the bottom up, then the polygon forming the base of the
prism is defined with the points A, B, C, D, A. The height of the
prism is the distance between point A and point E in meters.

G-----F
/| /|
/ | / |
H--+--E |
| C--|--B
| / | /
|/ |/
D-----A

Figure 16: Example of a Prism

Winterbottom, et al. Standards Track [Page 24]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

The resulting PIDF-LO is shown in Figure 17.

<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
entity="pres:[email protected]">
<tuple id="prism">
<status>
<gp:geopriv>
<gp:location-info>
<gs:Prism srsName="urn:ogc:def:crs:EPSG::4979">
<gs:base>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:posList>
42.556844 -73.248157 36.6 
42.656844 -73.248157 36.6 
42.656844 -73.348157 36.6 
42.556844 -73.348157 36.6 
42.556844 -73.248157 36.6 
</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gs:base>
<gs:height uom="urn:ogc:def:uom:EPSG::9001">
2.4
</gs:height>
</gs:Prism>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2007-06-22T20:57:29Z</timestamp>
</tuple>
</presence>

Figure 17: PIDF-LO Containing a Prism

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RFC 5491 GEOPRIV PIDF-LO Usage March 2009

6. Security Considerations

The primary security considerations relate to how location
information is conveyed and used, which are outside the scope of this
document. This document is intended to serve only as a set of
guidelines as to which elements MUST or SHOULD be implemented by
systems wishing to perform location dependent routing. The
ramification of such recommendations is that they extend to devices
and clients that wish to make use of such services.

7. Acknowledgments

The authors would like to thank the GEOPRIV working group for their
discussions in the context of PIDF-LO, in particular Carl Reed, Ron
Lake, James Polk, Henning Schulzrinne, Jerome Grenier, Roger Marshall
and Robert Sparks. Furthermore, we would like to thank Jon Peterson
as the author of PIDF-LO and Nadine Abbott for her constructive
comments in clarifying some aspects of the document.

Thanks to Karen Navas for pointing out some omissions in the
examples.

8. References

8.1. Normative References

[GeoShape] Thomson, M. and C. Reed, "GML 3.1.1 PIDF-LO Shape
Application Schema for use by the Internet Engineering
Task Force (IETF)", Candidate OpenGIS Implementation
Specification 06-142r1, Version: 1.0, April 2007.

[OGC-GML3.1.1]
Portele, C., Cox, S., Daisy, P., Lake, R., and A.
Whiteside, "Geography Markup Language (GML) 3.1.1",
OGC 03-105r1, July 2003.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.

[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479,
July 2006.

[RFC5139] Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for Presence Information Data Format Location
Object (PIDF-LO)", RFC 5139, February 2008.

Winterbottom, et al. Standards Track [Page 26]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

[W3C.REC-xmlschema-2-20041028]
Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
Second Edition", World Wide Web Consortium
Recommendation REC-xmlschema-2-20041028, October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

8.2. Informative References

[3GPP.23.032]
3rd Generation Partnership Project, "Universal
Geographical Area Description (GAD)", 3GPP TS 23.032
V6.0.0, January 2005,
<http://www.3gpp.org/ftp/Specs/html-info/23032.htm>.

[CRS-URN] Whiteside, A., "GML 3.1.1 Common CRSs Profile", OGC 03-
105r1, November 2005.

[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.

[RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", RFC 4776, November 2006.

[WGS84] US National Imagery and Mapping Agency, "Department of
Defense (DoD) World Geodetic System 1984 (WGS 84), Third
Edition", NIMA TR8350.2, January 2000.

Winterbottom, et al. Standards Track [Page 27]

RFC 5491 GEOPRIV PIDF-LO Usage March 2009

Authors' Addresses

James Winterbottom
Andrew Corporation
Wollongong
NSW Australia

EMail: [email protected]

Martin Thomson
Andrew Corporation
Wollongong
NSW Australia

EMail: [email protected]

Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland

Phone: +358 (50) 4871445
EMail: [email protected]
URI: http://www.tschofenig.priv.at

Winterbottom, et al. Standards Track [Page 28]