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Geopriv                                                  J. Winterbottom
Internet-Draft                                                M. Thomson
Expires: August 18, 2005                                          Nortel
                                                           H. Tschofenig
                                                                 Siemens
                                                       February 14, 2005


        GEOPRIV PIDF-LO Usage Clarification, Considerations and
                            Recommendations
           draft-winterbottom-geopriv-pdif-lo-profile-00.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on August 18, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The GeoPriv 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



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   how it is represented so that the number of options that need to be
   implemented in order to make use of it are reduced.  There is growing
   interest in being able to use location information contained in a
   PIDF-LO for message and call routing applications.  For such
   applications to interoperate successfully location information will
   need to be normative and more constrained than is currently described
   in the PIDF-LO specification.  This paper makes recommendations on
   how to constrain, represent and interpret locations in a PIDF-LO.  It
   further looks at existing communications standards that make use of
   geodetic information for routing purposes and recommends a subset of
   GML that MUST be implemented by applications to allow message routing
   to occur.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Using Location Information . . . . . . . . . . . . . . . . .   5
     3.1  Single Civic Location Information  . . . . . . . . . . . .   6
     3.2  Civic and Geospatial Location Information  . . . . . . . .   6
     3.3  Manual/Automatic Configuration of Location Information . .   8
   4.   Geodetic Coordinate Representation . . . . . . . . . . . . .   9
   5.   Uncertainty in Location Representation . . . . . . . . . . .  11
     5.1  Arc band . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.2  Ellipsoid Point With Uncertainty Circle  . . . . . . . . .  15
     5.3  Polygon  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.   Baseline Geometry  . . . . . . . . . . . . . . . . . . . . .  20
     6.1  Zero Dimensions  . . . . . . . . . . . . . . . . . . . . .  20
     6.2  One Dimensions . . . . . . . . . . . . . . . . . . . . . .  20
     6.3  Two Dimensions . . . . . . . . . . . . . . . . . . . . . .  21
     6.4  Three Dimensions . . . . . . . . . . . . . . . . . . . . .  21
     6.5  Envelopes  . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.6  Temporal Dimensions  . . . . . . . . . . . . . . . . . . .  22
     6.7  Units of Measure . . . . . . . . . . . . . . . . . . . . .  22
     6.8  Coordinate Reference System (CRS)  . . . . . . . . . . . .  22
   7.   Recommendations  . . . . . . . . . . . . . . . . . . . . . .  23
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  24
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  25
   10.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  26
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  27
     11.1   Normative references . . . . . . . . . . . . . . . . . .  27
     11.2   Informative References . . . . . . . . . . . . . . . . .  27
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  27
        Intellectual Property and Copyright Statements . . . . . . .  29







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1.  Introduction

   PIDF-LO [1] which was developed by the GEOPRIV working group, is the
   IETF recommended way encoding location information and privacy
   policies.  Location information in PIDF-LO may be described in a
   geospatial manner based on a subset of GMLv3, or as civic location
   information.  PIDF-LO may be used in a variety of ways.  For example,
   [3] motivates the usage of PIDF-LO in presence based systems.
   Further usages are envisioned in the context of emergency services
   and other location based routing applications.  This document details
   the usage of GMLv3 in PIDF-LO by incorporating implementation
   experience.  Recommendations for formats and conventions are provided
   where interoperability might be problematic.  For the sake of
   compatibility, ease of use and removal of inherent ambiguity apparent
   in GML the functionality of other geospatial systems, such as the
   3GPP MLP standard [4], are examined.



































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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 [2].














































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3.  Using Location Information

   The PIDF format provides for an unbounded number of tuples.  The
   geopriv element resides inside the status component of a tuple, hence
   a single PIDF document may contain an arbitrary number of location
   objects some or all of which may be contradictory or complementary.
   The actual location information is contained inside a <location-info>
   element, and there may be one or more actual locations described
   inside the <location-info> element.

   Graphically, the structure of the PIDF/PIDF-LO can be depicted as
   follows:


   PIDF document
      tuple 1
          status
               geopriv
                   location-info
                       civicAddress
                        location
                    usage-rules
      tuple 2
      tuple 3

   All of these potential sources and storage places for location lead
   to confusion for the generators, conveyors and users of location
   information.  Practical experience within the United States National
   Emergency Number Association (NENA) in trying to solve these
   ambiguities led the following conventions being adopted:

   Rule #1: A GeoPriv tuple MUST completely define a specific location.

   Rule #2: Where a location can be uniquely described in more than one
      way, each location description SHOULD reside in a separate tuple.

   Rule #3: Providing more than one location in a single presence
      document (PIDF) MUST only be done if all objects describe the same
      location.

   Rule #4: Providing more than one location in a single <location-info>
      element SHOULD be avoided where possible.

   Rule #5: When providing more than one location in a single
      <location-info> element they MUST be provided by a common source.
      If you have more than one location in the <location-info> element,
      then the combination (complex of) these elements defines the
      complete location.



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   Rule #6: Providing more than one location in a single <location-info>
      element SHOULD only be done if they form a complex to describe the
      same location.  For example, a geodetic location describing a
      point, and a civic location indicating the floor in a building.

   Rule #7: Where a location complex is provided in a single
      <location-info> element, the higher precision locations MUST be
      provided first.  For example, a geodetic location describing a
      point, and a civic location indicating the floor MUST be
      represented with the point first followed by the civic location.

   Rule #8: Where a PIDF document contains more than one tuple
      containing a status element with a geopriv location element , the
      priority of tuples SHOULD be based on tuple position within the
      PIDF document.  That is to say, the tuple with the highest
      priority location occurs earliest in the PIDF document.  Initial
      priority SHOULD be determined by the originating UA, the final
      priority MAY be determined by a proxy along the way.

   Rule #9: Where multiple PIDF documents are contained within a single
      request, document selection SHOULD be based on document order.

   The following examples illustrate the useful 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 [5].  She constructs a Location Object
   which consists of a single PIDF document, with a single geopriv
   tuple, with a single location residing in the <location-info>
   element.  This is largely unambiguous, and if this location is sent
   over the network, providing it understands civic addresses, correct
   handling of any request should be possible.

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.  Mike's computer receives a location
   over DHCP as defined in [6].  In this case the location is a geodetic
   location, with the altitude represented as a building floor number.
   This is constructed by Mike's computer into the following PIDF
   document:









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   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
      xmlns:gml="urn:opengis:specification:gml:schema-xsd:feature:v3.0"
      entity="pres:mike@seattle.example.com">
      <tuple id="sg89ab">
        <status>
         <gp:geopriv>
           <gp:location-info>
             <cl:civilAddress>
                <cl:country>US</cl:country>
                <cl:FLR>2</cl:FLR>
             </cl:civilAddress>
            </gp:location-info>
            <gp:usage-rules>
            </gp:usage-rules>
          </gp:geopriv>
         </status>
        <timestamp>2003-06-22T20:57:29Z</timestamp>
      </tuple>
      <tuple id="sg89ae">
        <status>
         <gp:geopriv>
           <gp:location-info>
             <gml:location>
               <gml:Point gml:id="point1" srsName="epsg:4326">
                 <gml:coordinates>37:46:30N 122:25:10W</gml:coordinates>
               </gml:Point>
              </gml:location>
           </gp:location-info>
           <gp:usage-rules>
           </gp:usage-rules>
         </gp:geopriv>
        </status>
       <timestamp>2003-06-22T20:57:29Z</timestamp>
      </tuple>
   </presence>

   So the resulting PIDF document contains two geopriv elements each in
   a separate PIDF tuple element, the first being a civic address made
   up of only country and floor, the second containing the received
   geodetic information.  If the location is required for emergency
   routing purposes, which information does a SIP proxy use? Applying
   rule #8, we will likely fail, or at a minimum need to fall back to
   the second tuple describing the geodetic location, a routing by the
   second floor somewhere in the US is not particularly descriptive.  If
   rule #6 and #7 are applied, then the PIDF-LO document would look
   like:



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   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
      xmlns:gml="urn:opengis:specification:gml:schema-xsd:feature:v3.0"
      entity="pres:mike@seattle.example.com">
      <tuple id="sg89ab">
        <status>
         <gp:geopriv>

           <gp:location-info>
             <gml:location>
               <gml:Point gml:id="point1" srsName="epsg:4326">
                 <gml:coordinates>37:46:30N 122:25:10W</gml:coordinates>
               </gml:Point>
             </gml:location>
             <cl:civilAddress>
                <cl:country>US</cl:country>
                <cl:FLR>2</cl:FLR>
             </cl:civilAddress>
            </gp:location-info>
            <gp:usage-rules>
            </gp:usage-rules>
          </gp:geopriv>
         </status>
        <timestamp>2003-06-22T20:57:29Z</timestamp>
      </tuple>
   </presence>

   It is now clear that the main location of user is a geodetic location
   at latitude 37:46:30 North and longitude 122:25:10 West.  Further
   that the user is on the second floor of the building located at those
   coordinates.

3.3  Manual/Automatic Configuration of Location Information

   Erin has a predefined civic location stored in her laptop, since she
   normally lives in Sydney, the address in her address is for her
   Sydney-based apartment.  Erin decides to visit sunny San Francisco,
   and when she gets there she plugs in her laptop and makes a call.
   The location sent to the local proxy is her Sydney address, her local
   outbound SIP proxy inserts a new PIDF document asserting her new
   location (or returns an error message with the current location
   information).  Using rule #9, the resulting PIDF order should be San
   Francisco document first, followed by Sydney document.  If the San
   Francisco proxy were to add the location to Erin's existing PIDF
   document, then the San Francisco tuple SHOULD be placed ahead of the
   Sydney tuple following rule #8.




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4.  Geodetic Coordinate Representation

   The geodetic examples provided previously were all based around the
   gml:location element which uses the gml:coordinates elements (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.  An
   alternative is to use the gml:position and gml:pos elements.  These
   elements have a structured format, in that each field is represented
   as a double, and a single space exists between each field.  Such a
   format does not introduce the same degree of misinterpretation.  A
   suggested representation therefore for expressing geodetic
   coordinates for emergency call routing would be:


    <gml:position>
      <gml:Point gml:id="point1" srsName="urn:EPSG:geographicCRS:4326">
        <gml:pos>37.775 -122.422</gml:pos>
      </gml:Point>
    </gml:position>

   The coordinate reference system (CRS) indicates which numbers in the
   sequence equate to latitude, longitude etc, and in addition to this
   the CRS also provides an indication of direction represented by the
   sign of the number.  For example, in WGS-84 (represented as
   CRS:4326), as shown in the code snippet above, the format is latitude
   followed by longitude.  A positive value for latitude represents a
   location north of the equator while a negative value represents a
   location south of the equator.  Similarly for longitude, a positive
   value represents a location east of Greenwich, while a negative value
   represents a location west of Greenwich.

   EPSG 4326 is the two dimensional WGS-84 representation, if we wanted
   to represented this in three dimensions, that is with an altitude as
   well, then we would use EPSG 4979 and the format would be as follows:


    <gml:position>
      <gml:Point gml:id="point1" srsName="urn:EPSG:geographicCRS:4979">
        <gml:pos>37.775 -122.422 22</gml:pos>
      </gml:Point>
    </gml:position>

   The format using CRS:4979 is similar to CRS:4326, though we now have
   an altitude value.  Specifically the altitude is provided in metres



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   above the geoid, which will not be useful for general routing
   applications since the geoid is generally neither ground-level nor
   sea-level.  However, for more specialized geographic applications it
   may be useful.

   Revisiting the final version of Section 3.2, but using gml:position
   and gml:pos, has the following structure:


   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
      xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
      xmlns:gml="urn:opengis:specification:gml:schema-xsd:feature:v3.0"
      entity="pres:mike@seattle.example.com">
      <tuple id="sg89ab">
        <status>
         <gp:geopriv>
           <gp:location-info>
             <gml:position>
               <gml:Point gml:id="point1" srsName="epsg:4326">
                 <gml:pos>37.775 -122.422</gml:pos>
               </gml:Point>
             </gml:position>
             <cl:civilAddress>
                <cl:country>US</cl:country>
                <cl:FLR>2</cl:FLR>
             </cl:civilAddress>
            </gp:location-info>
            <gp:usage-rules>
            </gp:usage-rules>
          </gp:geopriv>
         </status>
        <timestamp>2003-06-22T20:57:29Z</timestamp>
      </tuple>
   </presence>
















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5.  Uncertainty in Location 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 today use the point as an absolute value and ignore the
   uncertainty.  It is difficult to determine if systems have been
   implement 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.

   There are six common ways to represent location and uncertainty, but
   are listed below for completeness:
   o  Arc band
   o  Ellipsoid point with uncertainty circle
   o  Polygon
   o  Ellipsoid point with altitude
   o  Ellipsoid point with uncertainty ellipse
   o  Ellipsoid point with altitude and uncertainty ellipsoid

   GML was designed to provide a very flexible abstraction on which
   specific representations of geometric and geographic schemes could be
   extended.  Representing some of the above shapes is difficult if not
   impossible using base GML.  However, only a subset of GML, namely
   feature.xsd, is mandatory for a PIDF-LO implementation.  Extending
   GML to easily represent these shapes may lead to interoperability
   issues and so is not recommended.  The authors of this document were
   unable to find a means to express either an ellipse or and ellipsoid
   using only the elements defined in feature.xsd.

   The following sections describe four shapes that can be defined in
   GML, and show the equivalent representation in 3GPP MLP [4].

5.1  Arc band

   Arc band is used primarily where timing advance (TA) information is
   known.  Timing advance is a mechanism used in wireless communications
   to help ensure that handsets and base-stations remained synchronized.
   Timing advance is stepped based on signal propagation and is fairly
   deterministic, for GSM each increase in TA value represents 553.85
   metres.




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   The arc band type was developed to represent the area between two
   successive TA values and an antenna opening.  This is presented in
   3GPP as a point, two radii, and two angles representing the start and
   the stop of the angles for the opening.


                 ,..__
                /     `-.
               /         `-.
              /             `.
             /                \
            /__                \
           .   `-.              \
     start.       `.             \
     angle          \             .
         .          |             |
        o           '             |
          .        /              '
           .      /              ;
        stop . .,'              /
        angle   `.             /
                  `.          /
                    `.      ,'
                      `.  ,'
                        `'


   <pd>
     <time utc_off="+1000">20041201092843</time>
     <shape>
       <CircularArcArea>
         <coord>
           <X>42.5463</X>
           <Y>-73.2512</Y>
         </coord>
         <inRadius>1938.5</inRadius>
         <outRadius>2492.3</outRadius>
         <startAngle>63.7</startAngle>
         <stopAngle>118.4</stopAngle>
       </CircularArcArea>
     </shape>
   </pd>

   The GML representation of this is below:


   <?xml version="1.0"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"



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             xmlns:pidf="urn:ietf:params:xml:ns:pidf"
             xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
             xmlns:gml="http://opengis.net/gml"
             entity="pres:user@example.com">
     <tuple id="a6fea09">
       <status>
         <gp:geopriv>
           <gp:location-info>
             <gml:extentOf>
               <gml:Polygon>
                 <gml:exterior>
                   <gml:Ring>
                     <gml:curveMember>
                       <gml:Curve>
                         <gml:segments>
                           <gml:ArcByCenterPoint >
                             <gml:pos
                                 srsName="urn:EPSG:geographicCRS:4326">
                                 42.5463 -73.2512
                             </gml:pos>
                             <gml:radius uom="urn:EPSG:uom:9001">
                              2492.3
                             </gml:radius>
                      <!--
                        It is difficult to determine the correct
                        interpretation of GML and EPSG #4326 to
                        determine how these angles are to be
                        interpreted.
                        Neither specification specifies how the values
                        are to be interpolated.  That is, the direction
                        of rotation from start angle to end angle.  It
                        is therefore assumed that a "clockwise"
                        (Northing to Easting) direction is chosen to
                        link the two points on the arc.
                        It is also assumed that a value of 0 degrees
                        indicates Northing and 90 degrees indicates
                        Easting.
                       -->
                             <gml:startAngle uom="urn:EPSG:uom:9102">
                               63.7
                             </gml:startAngle>
                             <gml:endAngle uom="urn:EPSG:uom:9102">
                               118.4
                             </gml:endAngle>
                           </gml:ArcByCenterPoint>
                           <gml:LineStringSegment>
                             <gml:posList
                               srsName="urn:EPSG:geographicCRS:4326">



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                               42.535651 -73.224473 42.538018 -73.230411
                             </gml:posList>
                           </gml:LineStringSegment>
                           <gml:ArcByCenterPoint >
                             <gml:pos
                                srsName="urn:EPSG:geographicCRS:4326">
                                42.5463 -73.2512
                             </gml:pos>
                      <!--
                        Note that the decision to go with a "clockwise"
                        pass means that the start position of this
                        second arc is not contiguous with the end of
                        the last line.
                      -->
                             <gml:radius uom="urn:EPSG:uom:9001">
                               1938.5
                             </gml:radius>
                             <gml:startAngle uom="urn:EPSG:uom:9102">
                               63.7
                             </gml:startAngle>
                             <gml:endAngle uom="urn:EPSG:uom:9102">
                               118.4
                             </gml:endAngle>
                           </gml:ArcByCenterPoint>
                           <gml:LineStringSegment>
                             <gml:posList
                              srsName="urn:EPSG:geographicCRS:4326">
                              42.554016 -73.230007 42.556220 -73.223952
                             </gml:posList>
                           </gml:LineStringSegment>
                         </gml:segments>
                       </gml:Curve>
                     </gml:curveMember>
                   </gml:Ring>
                 </gml:exterior>
               </gml:Polygon>
             </gml:extentOf>
           </gp:location-info>
           <gp:usage-rules>
           </gp:usage-rules>
         </gp:geopriv>
       </status>
       <timestamp>2004-12-01T09:28:43+10:00</timestamp>
     </tuple>
   </presence>

   This representation poses a few potential problems over the 3GPP
   representation .  In the 3GPP representation the point is absolute,



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   and everything else is defined relative to this point, ensuring that
   the band is indeed bounded.  The representation of arc band above
   does not share all of these properties.  In the GML arc band
   representation above, the point and radii are relative, but the
   bounding lines of the starting and finishing angles are not, these
   are necessarily defined as independent line segments.  By having to
   define the arc enclosures as individual line segments it is possible
   to define an unbounded arc band which would consist of two arcs some
   arbitrary distance apart with two lines that may or may not intersect
   them.

   A second concern with this representing uncertainty using this
   method, is that there is no explicit statement or way of indicating
   to the receiving application what type of uncertainty is being
   represented.  Today several different representations of uncertainty
   are valid with in the same application, so knowing which type is
   being used, and how to interpret it is important, and this is
   particularly true if the shape must also be validated as is the case
   above.

   Ensuring the legality of this shape type when represented in GML is
   more complex than in MLP as the type must first be determined before
   its validity can be assessed.  Users of this shape type may be better
   served by a formal shape definition being introduced into GeoPriv so
   that these problems can be more readily overcome.

5.2  Ellipsoid Point With Uncertainty Circle

   This shape type is used extensively over the North American NENA
   defined E2 interface for transporting mobile geodetic location from
   the MPC/GMLC to the ALI and subsequently the PSAPs.  In 3GPP this is
   defined as a WGS-84 point (ellipsoid point), and a radius or
   uncertainty around that point, specified in metres.  The 3GPP MLP
   representation for an ellipsoid point with uncertainty is defined as
   follows:


   <pd>
     <time utc_off="+1000">20041201092843</time>
     <shape>
       <CircularArea>
         <coord>
           <X>42.5463</X>
           <Y>-73.2512</Y>
         </coord>
         <radius>850.24</radius>
       </CircularArea>
     </shape>



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   </pd>

   This shape is similarly defined in GML below:


   <?xml version="1.0"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
             xmlns:pidf="urn:ietf:params:xml:ns:pidf"
             xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
             xmlns:gml="http://opengis.net/gml"
             entity="pres:user@example.com">
     <tuple id="a6fea09">
       <status>
         <gp:geopriv>
           <gp:location-info>
             <gml:extentOf>
               <gml:Polygon>
                 <gml:exterior>
                   <gml:Ring>
                     <gml:curveMember>
                       <gml:Curve>
                         <gml:segments>
                           <gml:CircleByCenterPoint >
                             <gml:pos
                               srsName="urn:EPSG:geographicCRS:4326">
                               42.5463 -73.2512
                             </gml:pos>
                             <gml:radius uom="urn:EPSG:uom:9001">
                               850.24
                             </gml:radius>
                           </gml:CircleByCenterPoint>
                         </gml:segments>
                       </gml:Curve>
                     </gml:curveMember>
                   </gml:Ring>
                 </gml:exterior>
               </gml:Polygon>
             </gml:extentOf>
           </gp:location-info>
           <gp:usage-rules>
           </gp:usage-rules>
         </gp:geopriv>
       </status>
       <timestamp>2004-12-01T09:28:43+10:00</timestamp>
     </tuple>
   </presence>

   This type does not have all of the problems associated with the arc



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   band representation, in that the radius of the circle is relative to
   the centre, and so the validation is unnecessary.  However it does
   suffer from the potential problem that the application still needs to
   determine the type of uncertainty being represented, though this
   maybe made more clear through the explicit use of the
   gml:CircleByCenterPoint element.

5.3  Polygon

   A polygon is defined as a set of points to form an enclosed bounded
   shape.  It is here that GML and the 3GPP shapes are most similar.
   The representation for a polygon in GML is given first:


   <?xml version="1.0"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
             xmlns:pidf="urn:ietf:params:xml:ns:pidf"
             xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
             xmlns:gml="http://opengis.net/gml"
             entity="pres:user@example.com">
     <tuple id="a6fea09">
       <status>
         <gp:geopriv>
           <gp:location-info>
             <gml:extentOf>
               <gml:Polygon>
                 <gml:exterior>
                   <gml:LinearRing>
                     <gml:posList
                       srsName="urn:EPSG:geographicCRS:4326">
                       42.556844 -73.248157
                       42.549631 -73.237283
                       42.539087 -73.240328
                       42.535756 -73.254242
                       42.542969 -73.265115
                       42.553513 -73.262075
                       42.556844 -73.248157
                     </gml:posList>
                   </gml:LinearRing>
                 </gml:exterior>
               </gml:Polygon>
             </gml:extentOf>
           </gp:location-info>
           <gp:usage-rules>
           </gp:usage-rules>
         </gp:geopriv>
       </status>
       <timestamp>2004-12-13T14:49:53+10:00</timestamp>



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     </tuple>
   </presence>

   The GML object here is clear in its definition.  A gml:LinearRing
   MUST have a minimum of four points, with the first and last points
   being the same.  The 3GPP MLP representation for a polygon is
   provided below.


   <pd>
     <time utc_off="+1000">20041201092843</time>
     <shape>
       <Polygon>
         <outerBoundaryIs>
           <LinearRing>
             <coord>
               <X>42.556844</X>
               <Y>-73.248157</Y>
             </coord>
             <coord>
               <X>42.549631</X>
               <Y>-73.237283</Y>
             </coord>
             <coord>
               <X>42.539087</X>
               <Y>-73.240328</Y>
             </coord>
             <coord>
               <X>42.535756</X>
               <Y>-73.254242</Y>
             </coord>
             <coord>
               <X>42.542969</X>
               <Y>-73.265115</Y>
             </coord>
             <coord>
               <X>42.553513</X>
               <Y>-73.262075</Y>
             </coord>
             <coord>
               <X>42.556844</X>
               <Y>-73.248157</Y>
             </coord>
           </LinearRing>
         </outerBoundaryIs>
       </Polygon>
     </shape>
   </pd>



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   While these two representations are very similar and precise, they
   are not widely used at present.  If only a coverage area is required
   without a nominal central point requiring specification, then this
   form is ideal for representation using GML.















































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6.  Baseline Geometry

   PIDF-LO suggests to use GMLv3 feature.xsd, which provides a subset of
   the available GML functionality.  As a consequence a number of
   further XML files are implicitly included, namely
   geometryBasic0d1d.xsd, geometryBasic2d.xsd, temporal.xsd,
   measure.xsd, units.xsd, gmlBase.xsd, dictionary.xsd, xLinks.xsd and
   basicTypes.xsd, as being necessary to support.  This provides for a
   vast range of possibilities which would pose significant
   complications to implementors wish to develop location dependent
   routing applications.  By agreeing to a minimal set of data
   appropriate for routing, a minimum set of GML that MUST be
   implemented by a given application type can also be set.  This does
   not preclude the additional functionality from being implemented,
   merely that it may not be understood by some nodes.

6.1  Zero Dimensions

   The minimum supported set of elements is position/Point/pos provided
   by geometryBasic0d1d.xsd.

   Thus a point location has only one representation as follows:


   <gml:position xmlns:gml="http://www.opengis.net/gml">
        <gml:Point srsName="urn:ogc:def:crs:EPSG:4326">
                <gml:pos>4.5 -36.2</gml:pos>
        </gml:Point>
   </gml:position>

   The <location> and <coord> objects MUST NOT be used since they are
   deprecated in GML 3.1 and their functionality can be substituted with
   the above-described elements.

   Note that pos allows altitude to be expressed based on the selected
   Coordinate Reference Systems (e.g., EPSG:4979 or EPSG:4326).  Most
   Coordinate Reference Systems use altitude above the geoid and not
   altitude above the ground.

6.2  One Dimensions

   Support for one dimensional shapes (such as the LineString or the
   posList object) is not required except as a part of two dimensional
   shapes.

   geometryBasic0d1d.xsd provides these geometric properties and
   objects.




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6.3  Two Dimensions

   The examples previously used were all contructed using elements from
   this schema which reuse functionality from geometryBasic2d.xsd.  As
   was described earlier the arcband definition in GML is problematic
   for producing a closed solid and SHOULD consequently be avoided.  As
   a result of this, elements required exclusively for representing the
   arcband shape have not been included in the minimum supported element
   set.  The minimum element set is therefore restricted to circle and
   polygon.

   Circle:

   extentOf/
      Polygon/
         exterior/
            Ring/
               curveMember/
                  Curve/
                     segments/
                        CircleByCentrePoint/   -> Circle
                           pos
                           radius

    Alternatively it would be possible to use the following structure to
   express a circle using the <gml:Circle> element with three pos
   elements as well.  However, the usage of pos and radius, as shown
   above, is inline with the model used by the 3GPP.

   Polygon:

   extentOf/
      Polygon/
         exterior/
            LinearRing/
               pos or posList   -> Polygon


6.4  Three Dimensions

   Support for three dimensions is not required

6.5  Envelopes

   The Envelope element is a representation of a bounding box and can be
   expressed in two or three dimensions.  Defining a space using the
   Envelope element should be done with extreme caution due to
   continuity problems at the extremities of the CRS.  In WGS-84, two



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   envelopes are required at the 180th meridian.  The minimum set of
   elements required to support an Envelope are:


   boundBy/
      Envelope/
         upperCorner/
            Point/
               Pos
         lowerCorner/
             Point/
                Pos/


6.6  Temporal Dimensions

   Support for temporal elements is not required

6.7  Units of Measure

   The base SI units as a minimum MUST be supported.  For measures of
   distance this is metres.  The EPSG URN for metres is:


   metres = urn:ogc:def:uom:EPSG:9001:6.6

   Angles are frequently expressed in terms of both degrees and radians,
   consequently both MUST be implemented.


   degrees = urn:ogc:def:uom:EPSG:9102:6.6
   radians = urn:ogc:def:uom:EPSG:9101:6.6

   Further units of measurement are not required.

6.8  Coordinate Reference System (CRS)

   There are a very large number of coordinate reference systems in
   existence today, but many are, however, not in widespread use.
   Existing communications protocols such as those used in both the
   ANSI, 3GPP and NENA standards (see [7], [8], [9]) have standardized
   on WGS-84.  It is recommended for routing purpose that only WGS-84
   coordinate types MUST be implemented and further that this set be
   resatricted to the following:


   WGS84(2D) = urn:ogc:def:crs:EPSG:4326:6.6
   WGS84(3D) = urn:ogc:def:crs:EPSG:4979:6.6



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7.  Recommendations

   As a summary this document gives a few recommendations on the usage
   of location information in PIDF-LO.  Nine rules specified in
   Section 3 give guidelines on the ambiguity of PIDF-LO with regard to
   the occurrence of multiple location information.  It is recommend
   that gml:position, gml:pos types be used to specify locations when
   locations are needed for routing and specifically emergency routing.
   Enhancements to GMLv3 feature.xsd may need to be defined to allow
   complex shapes types to be specified in a way that makes them easy to
   distinguish and validate.  This is particularly important if the data
   is to be used during the decision making process of routing signaling
   messages.

   Only a limited subset of GML functionality from the feature.xsd
   schema is necessary to describe a geodetic location with sufficient
   precision to allow a routing decision to be made.  Restricting both
   the amount of GML that MUST be implemented, and the number of
   variations in which this data can be expressed significantly reduces
   the likelihood of interoperability issues in the future.  Precedents
   exist in the other communications protocols for restricting CRS types
   and representations for the sake of simplicity and interoperability,
   and the recommendation is made to adopt similar restrictions for
   mandatory implementable components of GeoPriv.



























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8.  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.










































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9.  IANA Considerations

   This document does not introduce any IANA considerations.
















































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10.  Acknowledgments

   The authors would like to thank the GEOPRIV working group for their
   discussions in the context of PIDF-LO.  Furthermore, we would like to
   thanks Jon Peterson as the author of PIDF-LO and Nadine Abbott for
   her constructive comments in clarifying some aspects of the document.













































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11.  References

11.1  Normative references

   [1]  Peterson, J., "A Presence-based GEOPRIV Location Object Format",
        Internet-Draft draft-ietf-geopriv-pidf-lo-03, September 2004.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", March 1997.

11.2  Informative References

   [3]  Peterson, J., "A Presence Architecture for the Distribution of
        GEOPRIV Location Objects",
        Internet-Draft draft-ietf-geopriv-pres-02, September 2004.

   [4]  "Mobile Location Protocol (MLP), OMA, Candidate Version 3.1",
        March 2004.

   [5]  Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv4
        and DHCPv6) Option for Civic  Addresses Configuration
        Information", Internet-Draft draft-ietf-geopriv-dhcp-civil-04,
        October 2004.

   [6]  Polk, J., Schnizlein, J. and M. Linsner, "Dynamic Host
        Configuration Protocol Option for Coordinate-based Location
        Configuration Information", RFC 3825, July 2004.

   [7]  "TR-45 J-STD-036-AD-2 Enhanced Wireless 9-1-1 Phase 2".

   [8]  "3GPP TS 23.032 V6.0.0 3rd Generation Partnership Project;
        Technical Specification Group Code Network; Universal Geographic
        Area Description (GAD)".

   [9]  "NENA Standard for the Implementation of the Wireless Emergency
        Service Protocol E2 Interface".


Authors' Addresses

   James Winterbottom
   Nortel
   Wollongong
   NSW Australia

   Email: winterb@nortel.com





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   Martin Thomson
   Nortel
   Wollongong
   NSW Australia

   Email: martin.thomson@nortel.com


   Hannes Tschofenig
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com




































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