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

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.

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

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
   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.

   <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

   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.

   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.

    <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:mike@seattle.example.com">
      <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.

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:ness@example.com">
      <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">
                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.

   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.

   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.

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:point2d@example.com">
      <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

   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:point3d@example.com">
       <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

     <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:hexagon@example.com">
       <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> <!--A-->
                     <gml:pos>43.111 -73.322</gml:pos> <!--F-->
                     <gml:pos>43.111 -73.222</gml:pos> <!--E-->
                     <gml:pos>43.311 -73.122</gml:pos> <!--D-->
                     <gml:pos>43.411 -73.222</gml:pos> <!--C-->
                     <gml:pos>43.411 -73.322</gml:pos> <!--B-->
                     <gml:pos>43.311 -73.422</gml:pos> <!--A-->
                   </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.

     <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:hexagon@example.com">
       <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

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:circle@example.com">
       <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.

     <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:Ellipse@somecell.example.com">
       <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.

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.

     <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:paul@somecell.example.com">
       <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.

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:sphere@example.com">
       <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

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

   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:somone@gpsreceiver.example.com">
       <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

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

   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:mike@someprism.example.com">
       <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 <!--A-->
                           42.656844 -73.248157 36.6 <!--B-->
                           42.656844 -73.348157 36.6 <!--C-->
                           42.556844 -73.348157 36.6 <!--D-->
                           42.556844 -73.248157 36.6 <!--A-->
                         </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

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.

   [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.

Authors' Addresses

   James Winterbottom
   Andrew Corporation
   Wollongong
   NSW Australia

   EMail: james.winterbottom@andrew.com


   Martin Thomson
   Andrew Corporation
   Wollongong
   NSW Australia

   EMail: martin.thomson@andrew.com


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at