Geopriv J. Winterbottom
Internet-Draft M. Thomson
Intended status: Informational Andrew Corporation
Expires: April 26, 2007 H. Tschofenig
Siemens
October 23, 2006
GEOPRIV PIDF-LO Usage Clarification, Considerations and Recommendations
draft-ietf-geopriv-pdif-lo-profile-05.txt
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Copyright (C) The Internet Society (2006).
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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 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 routing
applications. To allow successfully interoperability between
applications, location information needs to be normative and more
tightly constrained than is currently specified in the PIDF-LO. This
document makes recommendations on how to constrain, represent and
interpret locations in a PIDF-LO. It further recommends a subset of
GML that MUST be implemented by applications involved in location
based routing.
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Table of Contents
1. CHANGES SINCE LAST TIME . . . . . . . . . . . . . . . . . . . 4
1.1. 05 changes . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. 04 changes . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. 03 changes . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. 01 changes . . . . . . . . . . . . . . . . . . . . . . . . 4
2. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Using Location Information . . . . . . . . . . . . . . . . . . 9
5.1. Single Civic Location Information . . . . . . . . . . . . 11
5.2. Civic and Geospatial Location Information . . . . . . . . 11
5.3. Manual/Automatic Configuration of Location Information . . 12
6. Geodetic Coordinate Representation . . . . . . . . . . . . . . 14
7. Geodetic Shape Representation . . . . . . . . . . . . . . . . 15
7.1. Polygon Restriction . . . . . . . . . . . . . . . . . . . 16
7.2. Emergency Shape Representations . . . . . . . . . . . . . 16
8. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative references . . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Uncertainty in The RFC-3825 LCI Representation . . . 22
A.1. Conversion From LCI Form . . . . . . . . . . . . . . . . . 22
A.2. Conversion To LCI Form . . . . . . . . . . . . . . . . . . 22
A.2.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . 23
A.2.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . 24
A.3. Problem . . . . . . . . . . . . . . . . . . . . . . . . . 24
A.4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 24
Appendix B. Creating a PIDF-LO from DHCP Geo Encoded Data . . . . 26
B.1. Latitude and Longitude . . . . . . . . . . . . . . . . . . 26
B.2. Altitude . . . . . . . . . . . . . . . . . . . . . . . . . 28
B.3. Generating the PIDF-LO . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. CHANGES SINCE LAST TIME
[[This section is informational only and will be removed before the
final version.]]
1.1. 05 changes
Clarified definitions more.
Clarified rules.
Clarified examples, and removed confusion caused by the illustration
of how not to represent location.
1.2. 04 changes
Added a section to recommend restricting Polygon to 16 points for
routing and other real-time applications.
Added section detailing caution when selecting shapes for emergency
routing.
Modified the recommendations section to include the two above
additions.
Added a second appendix detailing problems with expressing
uncertainty using LCI.
1.3. 03 changes
Removed some shape definitions, ellipses, arcbands.
Removed OMA shape definition comparisons.
Modified examples to use new civicAddr draft data.
Made extensive references to the GeoShape Draft.
1.4. 01 changes
minor changes to the abstract.
Minor changes to the introduction.
Added and appendix to take implementers through how to create a
PIDF-LO from data received using DHCP option 123 as defined in [3].
Rectified examples to use position and pos rather than location and
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point.
Corrected example 3 so that it does not violate SIP rules.
Added addition geopriv elements to the status component of the figure
in "Using Location Information" to more accurately reflect the
cardinality issues.
Revised text in section Geodetic Coordinate Representation. Removed
last example as this was addressed with the change to position and
pos in previous examples.
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2. To Do
Get Appendices moved into the RFC 3825 Biz document.
Get an OGC reference for the GeoShapes specification
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3. Introduction
The Presence Information Data Format Location Object (PIDF-LO) [2] is
the IETF 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 GMLv3, or as
civic location information [5]. A GML profile for expressing
geodetic shapes in a PIDF-LO is described in [7]. Uses for 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.
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4. 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 [1].
The definition for "Target" is taken from [6].
In this document a "discrete location" is defined as a place, point,
area or volume in which a Target can be found. It must be described
with sufficient precision to address the requirements of an intended
application.
The term "location complex" is used to describe location information
represented by a composite of both civic and geodetic information.
An example of a location complex might be a geodetic polygon
describing the perimeter of a building and a civic element
representing the floor in the building.
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5. 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
geopriv 2
geopriv 3
.
.
.
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 to a set of conventions being adopted. These rules
do not have any particular order, but should be followed by creators
and users of location information conatined 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.
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Rule #2: Where a discrete 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. This may occur if a Target's location is determined
using a series of different techniques.
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 the locations MUST be provided by a common
source.
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 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 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.
Rule #9: Where multiple PIDF documents can be sent of 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 be
considered first.
The following examples illustrate the application of these rules.
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5.1. 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 [4].
A Location Object is constructed consisting of a single PIDF
document, with a single geopriv tuple, and a single location residing
in the <location-info> element. This document is unambiguous, and
should be interpreted consitently by receiving nodes if sent over the
network.
5.2. 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 the location is a
geodetic location, with the altitude represented as a building floor
number. The main location of user is inside the rectangle bounded by
the geodetic coordinates specified. Further that the user is on the
second floor of the building located at these coordinates. Applying
rules #6 and #7 are applied, the PIDF-LO document creates a complex
as shown below.
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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:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gs="urn:ietf:params:xml:ns:pidf:geopriv10:geoShape"
entity="pres:mike@seattle.example.com">
<tuple id="sg89ab">
<status>
<gp:geopriv>
<gp:location-info>
<Polygon srsName="urn:ogc:def:crs:EPSG::4326"
xmlns="http://www.opengis.net/gml">
<exterior>
<LinearRing>
<pos>37.775 -122.4194</pos>
<pos>37.555 -122.4194</pos>
<pos>37.555 -122.4264</pos>
<pos>37.775 -122.4264</pos>
<pos>37.775 -122.4194</pos>
</LinearRing>
</exterior>
</Polygon>
<cl:civicAddress>
<cl:FLR>2</cl:FLR>
</cl:civicAddress>
</gp:location-info>
<gp:usage-rules/>
</gp:geopriv>
</status>
<timestamp>2003-06-22T20:57:29Z</timestamp>
</tuple>
</presence>
5.3. 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 her 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
pre-existing and new location 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 location
recipient providing Loraine's system applies rule #9. In this case
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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 San
Francisco first.
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6. Geodetic Coordinate Representation
The geodetic examples provided in RFC 4119 [2] are illustrated using
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. The
PIDF-LO Geodetic Shapes specification [7] provides a specific GML
profile for expressing commonly used shapes using simple GML
representations. The shapes defined in [7] are the recommended
shapes to ensure interoperability between location based
applications.
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7. 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
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.
The PIDF-LO Geodetic Shapes specification [7] defines eight shape
types most of which are easily translated in shapes definitions used
in other applications and protocol, such as Open Mobile Alliance
(OMA) Mobile Location Protocol (MLP). For completeness the shape
defined in [7] are listed below:
o Point (2d or 3d)
o Polygon (2d)
o Circle (2d)
o Ellipse (2d)
o Arc band (2d)
o Sphere (3d circle)
o Ellipsoid (3d)
o Prism (3d polygon)
The GeoShape specification [7] also describes a standard set of
coordinate reference systems (CRS), unit of measure and conventions
relating to lines and distances. GeoShape mandates the use the
WGS-84 Coordinate reference system and restricts usage to EPSG-4326
for two dimensional (2d) shape representations and EPSG-4979 for
three dimensional (3d) volume representations. Distance and heights
are expressed in meters using EPSG-9001.
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7.1. Polygon Restriction
The Polygon shape type defined in [7] intentionally does not place
any constraints on the number of vertices that may be included to
define the bounds of the 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 may
result in significant performance impacts. To mitigate this risk it
is recommended that Polygons be restricted to a maximum of 16 points
when the location information is intended for use in real-time
applications. This limit of 16 points is chosen to allow moderately
complex shape definitions while at the same time enabling
interworking with other location transporting protocols such as those
defined in 3GPP ([8]) and OMA where the 16 point limit is already
imposed.
Polygons are defined with the minimum distance between two adjacent
vertices (geodesic). A connecting line SHALL NOT cross another
connecting line of the same Polygon. Polygons SHOULD be defined with
the upward normal pointing up, this is accomplished by defining the
vertices in counter-clockwise direction.
7.2. Emergency Shape Representations
In some parts of the world cellular networks constraints are placed
on the shape types that can be used to represent the location of an
emergency caller. These restrictions, while to some extend are
artificial, may pose significant interoperability problems in
emergency networks were they to be unilaterally lifted. The largest
impact likely being on Public Safety Answer Point (PSAP) where
multiple communication networks report emergency data. Wholesale
swap-out or upgrading of this equipment is deemed to be complex and
costly and has resulted in a number of countries, most notably the
United States, to adopt migratory standards towards emergency IP
telephony support. Where these migratory standards are implemented
restrictions on acceptable geodetic shape types to represent the
location of an emergency caller may exist. Conversion from one shape
type to another should be avoided to eliminate the introduction of
errors in reported location.
In North America the migratory VoIP emergency services standard (i2)
[11] reuses the NENA E2 interface [12] which restriction geodetic
shape representation to a point, a point with an uncertain circle, a
point with an altitude and an uncertainty circle. The NENA
recommended shapes can be represented in a PIDF-LO using the GeoShape
Point, GeoShape Circle, and GeoShape Sphere definitions respectively.
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8. Recommendations
As a summary, this document gives a few recommendations on the usage
of location information in PIDF-LO. Nine rules specified in
Section 5 give guidelines on avoiding ambiguity in PIDF-LO
interpretations when multiple locations may be provided to a Target
or location recipient.
It is recommend that only the shape types and shape representations
described in [7] be used to express geodetic locations for exchange
between general applications. By standardizing geodetic data
representation interoperability issues are mitigated.
It is recommended that GML Polygons be restricted to a maximum of 16
points when used in location-dependent routing and other real-time
applications to mitigate possible performance issues. This allows
for interoperability with other location protocols where this
restriction applies.
Geodetic location may require restricted shape definitions in regions
where migratory emergency IP telephony implementations are deployed.
Where the acceptable shape types are not understood restrictions to
Point, Circle and Sphere representations should be used to
accommodate most existing deployments.
Conversions from one geodetic shape type to another should be avoided
where data is considered critical and the introduction of errors
considered unacceptable.
If geodetic information is to be provided via DHCP, then a minimum
resolution of 20 bits SHOULD be specified for both the Latitude and
Longitude fields to achieve sub 100 meter precision.
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9. 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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10. IANA Considerations
This document does not introduce any IANA considerations.
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11. 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 and Henning Schulzrinne. 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.
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12. References
12.1. Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[2] Peterson, J., "A Presence-based GEOPRIV Location Object Format",
RFC 4119, December 2005.
[3] Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based Location
Configuration Information", RFC 3825, July 2004.
[4] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv4
and DHCPv6) Option for Civic Addresses Configuration
Information", draft-ietf-geopriv-dhcp-civil-09 (work in
progress), January 2006.
[5] Thomson, M. and J. Winterbottom, "Revised Civic Location Format
for PIDF-LO", draft-ietf-geopriv-revised-civic-lo-04 (work in
progress), September 2006.
[6] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
[7] Thomson, M., "draft-thomson-geopriv-geo-shape, Geodetic Shapes
for the Representation of Uncertainty in PIDF-LO", January 2006.
12.2. Informative References
[8] "3GPP TS 23.032 V6.0.0 3rd Generation Partnership Project;
Technical Specification Group Code Network; Universal
Geographic Area Description (GAD)".
[9] Schulzrinne, H., "Common Policy: A Document Format for
Expressing Privacy Preferences",
draft-ietf-geopriv-common-policy-11 (work in progress),
August 2006.
[10] "TR-45 J-STD-036-AD-2 Enhanced Wireless 9-1-1 Phase 2".
[11] "abbrev"i2">NENA VoIP-Packet Technical Committee, Interim VoIP
Architecture for Enhanced 9-1-1 Services (i2), NENA 08-001, Dec
2005".
[12] "NENA Standard for the Implementation of the Wireless Emergency
Service Protocol E2 Interface, NENA 05-001, Dec 2003".
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Appendix A. Uncertainty in The RFC-3825 LCI Representation
RFC-3825 [3] defines a binary geodetic representation referred to as
Location Configuration Information LCI. The way that LCI represents
uncertainty is through a resolution parameter that indicates how many
binary digits of each axis are significant or accurate. This is
explained in detail in [3] with a series of examples, with a further
example provided in Appendix B of this document. In short LCI
describes a rectangular prism that is aligned along the north-south/
east-west/up-down axes.
This appendix should be regarded as informative only and provides
guidance on aspects concerning the interpretation of uncertainty as
it applies to the binary geodetic LCI representation defined in RFC-
3825 [3].
A.1. Conversion From LCI Form
From the example in RFC 3825, 38.89868 degrees is encoded into a
34bit twos-complement number:
000100110.1110011000001111111001000
The resolution value for this axis indicates how many of thess bits
are actually significant. A resolution of 18 indicates that the last
16 bits of the number could be either 1 or zero:
000100110.111001100xxxxxxxxxxxxxxxx
To determine the uncertainty assume a range from the minimum possible
value (all zeros for the last 16 bits) to the maximum (all ones):
000100110.1110011000000000000000000 to
000100110.1110011001111111111111111
This yields the range in the example to be between 38.8984375 degrees
and 38.9003906 degrees (rounded to 7 decimal places).
A.2. Conversion To LCI Form
Converting location information into the LCI format involves
converting the original shape to a rectangular prism. To do this
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determine the minimum and maximum values for each of the axes:
latitude, longitude and altitude. This results in a slightly
increased area, but the overall effect is minimal.
+----------.....----------+
| _d^^^^^^^^^b_ |
| .d''yyyyyyyyyyy``b. |
| .p'yyyyyyyyyyyyyyyyy`q. |
|.d'yyyyyyyyyyyyyyyyyyy`b.|
.d'yyyyyyyyyyyyyyyyyyyyy`b.
::yyyyyyyyyyyyyyyyyyyyyyy::
:: ................... ::
::vvvvvvvvvvvvvvvvvvvvvvv::
`p.vvvvvvvvvvvvvvvvvvvvv.q'
|`p.vvvvvvvvvvvvvvvvvvv.q'|
| `b.vvvvvvvvvvvvvvvvv.d' |
| `q..vvvvvvvvvv..p' <-+----Area Increase
| ^q........p^ |
+---------''''------------+
It's important to note the resulting area cannot be less that the
starting area. This is because the starting area represents a set of
points and the Target may reside at anyone of these points with equal
probability. If the area is cropped there is a risk that the
Target's position will be one of the discarded points yielding an
incorrect result. In general the increases in area are minimal, for
a circular area, as shown, the increase ratio is 4:pi; a square
building will at most double the size of the area.
A.2.1. Example 1
Looking at a random example from 32.98004 degrees to 32.98054397
degrees the approximate distance is 56 meters. Converting each value
into a 34-bit twos-complement number yields the following:
000100000.1111101011100011111001110 to
000100000.1111101100000100111011100
^^^^^^^^^^^^^^^^^
To ensure that the encoded value represents the full range from the
lowest to highest value, take the common stem as marked this above.
There are 16 common bits between low and high. To check, convert the
value back by making the last 18 bits either 0 or 1 as described
earlier. This leads to a range from 32.9765625 degrees to 32.9843745
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degrees, which is approximately 870 meters a significant increase
over the original 56 meters.
A.2.2. Example 2
Take the range from 31.9999985 degrees to 32.00000274 degrees, which
is about 0.5 meters in distances ranging around 32 degrees. This
results in the following binary values:
000011111.1111111111111111111001110 to
000100000.0000000000000000001011100
^^^
Only 3 bits are common to both values which yields an encoded range
from 0 to 64 degrees, or a distance of 3,500 kilometers.
A.3. Problem
The LCI encoding breaks when the uncertainty that is being
represented causes a change in a relatively significant binary digit.
This results in an expanded uncertainty, possibly very large,
depending on which binary digit changes. In many cases the change
will be in lower-order digits, which will result in a relatively
small increase in uncertainty, but certain values will yield an
almost useless location see Appendix A.2.2.
This problem is exacerbated at the three zero points - the Greenwich
Meridian, Equator and at the surface of the geoid (altitude). In
these cases, if the input uncertainty spans the zero point, the
resolution value ends up as zero; that is, it indicates that there is
no useful information for that parameter.
The original uncertainty has very little bearing on this problem - a
small value can be increased to any value. More precise location
determination technologies only reduce the probability of large
problems occurring, although the nature of the encoding is such that
any uncertainty can be greatly increased.
A.4. Conclusion
Uncertainty is a reality of location and important for a number of
applications. LCI's limited form means that adapting existing
uncertainty information, for example a circle as in Appendix A.2.2,
results in a small error. The introduction of this small encoding
error is, however, insignificant when compared to the error that can
be introduced by the way that the resolution parameter is
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interpreted.
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Appendix B. Creating a PIDF-LO from DHCP Geo Encoded Data
This appendix is informative only.
RFC 3825 [3] describes a means by which an end point may learn it
location from information encoded into DHCP option 123. The
following section describes how and end point can take this
information and represent it in a well formed PIDF-LO describing this
geodetic location.
The location information described in RFC 3825 consists of a
latitude, longitude, altitude and datum.
B.1. Latitude and Longitude
The latitude and longitude values are represented in degrees and
decimal degrees. Latitude values are positive if north of the
equator, and negative if south of the equator. Similarly
longitudinal values are positive if east of the Greenwich meridian,
and negative if west of the Greenwich meridian.
The latitude and longitude values are each 34 bit long fields
consisting of a 9 bit integer component and a 25 bit fraction
component, with negative numbers being represented in 2s complement
notation. The latitude and longitude fields are each proceeded by a
6 bit resolution field, the LaRes for latitude, and the LoRes for
longitude. The value in the LaRes field indicates the number of
significant bits to interpret in the Latitude field, while the value
in the LoRes field indicates the number of significant bits to
interpret in the Longitude field.
For example, if you are in Wollongong Australia which is located at
34 Degrees 25 minutes South and 150 degrees 32 minutes East, this
would translate to -34.41667, 150.53333 in decimal degrees. If these
numbers are translated to their full 34 bit representations, then we
arrive the following:
Latitude = 111011101.1001010101010101000111010
Longitude = 0100101101000100010001000010100001
RFC 3825, uses the LaRes and LoRes values to specify a lower and
upper boundary for location thereby specifying an area. The size of
the area specified is directly related to the value specified in the
LaRes and LoRes fields.
Using the previous example, if LaRes is set 7, then lower latitude
boundary can be calculated as -256+128+64+16+8+4, which is -36
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degrees, the upper boundary then becomes -256+128+64+16+8+4+2+1 which
is -35 degrees. LoRes may be used similarly for Longitude.
So what level of precision is useful? Well, certain types of
applications and regulations call for different levels of precision,
and the required precision may vary depending on how the location was
determined. For cellular 911 calls in the United States, for
example, if the network measures the location then the caller should
be within 100 meters, while if the handset does the measurement then
the location should be within 50 meters. Since DHCP is a network
based mechanism we will benchmark off 100 meters (approximately 330
ft) which is still a large area.
For simplicity we shall assume that we are defining a square, in
which we are equally to appear anywhere. The greatest distance
through this square is across the diagonal, so we make this 100
meters.
+----------------------+
| _/|
| _/ |
| _/ |
| _/ |
| _/ |
| 100_/ metres |
| _/ |
| _/ |
| _/ |
| _/ |
|_/ |
+----------------------+
The distance between the top and the bottom and the left and the
right is the same, the area being a square, and this works out to be
70.7 meters. When expressed in decimal degrees, the third point
after the decimal place represents about 100 meter precision, this
equates to 10 binary places of fractional part. A 70 meter distance
is required, so 11 fractional binary digits are necessary resulting
in a total of 20 bits of precision.
With -34.4167, 150.5333 encoded with 20 bits of precision for the
LaRes and LoRes, the corners of the enclosing square are:
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Point 1 (-34.4170, 150.5332)
Point 2 (-34.4170, 150.5337)
Point 3 (-34.4165, 150.5332)
Point 4 (-34.4165, 150.5337)
B.2. Altitude
The altitude elements define how the altitude is encoded and to what
level of precision. The units for altitude are either meters, or
floors, with the actual measurement being encoded in a similar manner
to those for latitude and longitude, but with 22 bit integer, and 8
bit fractional components.
B.3. Generating the PIDF-LO
If altitude is not required, or is expressed in floors then a
geodetic location expressed by a polygon SHOULD be used, with points
expressed in a counter-clockwise direction. If the altitude is
expressed in floors and is required, the altitude SHOULD be expressed
as a civic floor number as part of the same <location-info> element.
In the example above the GML for the location would be expressed as
follows:
<Polygon srsName="urn:ogc:def:crs:EPSG::4326"
xmlns="http://www.opengis.net/gml">
<exterior>
<LinearRing>
<pos>-34.4165 150.5332</pos>
<pos>-34.4170 150.5532</pos>
<pos>-34.4170 150.5537</pos>
<pos>-34.4165 150.5337</pos>
<pos>-34.4165 150.5332</pos>
</LinearRing>
</exterior>
</Polygon>
No Altitude
If a floor number of say 3 were included, then the location-info
element would contain the above information and the following:
<civicAddress
xlmns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<FLR>2</FLR>
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</civicAddress>
Civic Altitude
When altitude is expressed as an integer and fractional component, as
with the latitude and longitude, it expresses a range which requires
the prism form to be used. Care must be taken to ensure that the
points are defined in a counter-clockwise direction to ensure that
the upward normal points up.
Extending the previous example to include an altitude expressed in
metres rather than floors. AltRes is set to a value of 19, and the
Altitude value is set to 34. Using similar techniques as shown in
the latitude and longitude section, a range of altitudes between 32
meters and 40 meters is described. The prism would therefore be
defined as follows:
<Prism srsName="urn:ogc:def:crs:EPSG::4976"
xmlns:gs="urn:ietf:params:xml:ns:pidf:geopriv10:geoShape"
xmlns:gml="http://www.opengis.net/gml">
<base>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:pos>-34.4165 150.5332 32</gml:pos>
<gml:pos>-34.4170 150.5532 32</gml:pos>
<gml:pos>-34.4170 150.5537 32</gml:pos>
<gml:pos>-34.4165 150.5337 32</gml:pos>
<gml:pos>-34.4165 150.5332 32</gml:pos>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</base>
<height uom="urn:ogc:def:uom:EPSG::9001">
8
</height>
</Prism>
The Method value SHOULD be set to Wiremap.
The timestamp value SHOULD be set to the time that location was
retrieved from the DHCP server.
The client application MAY insert any usage rules that are pertinent
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to the user of the device and that comply with [9]. A guideline is
that the any retention-expiry value SHOULD NOT exceed the current
lease time.
The Provided-By element SHOULD NOT be populated as this is not
provided by the source of the location information.
The 3 completed PIDF-LO representations are provided below, and
represent a location without altitude, a location with a civic
altitude, and a location represented as a 3 dimensional rectangular
prism.
<?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:gs="urn:ietf:params:xml:ns:pidf:geopriv10:geoShape"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:user@example.com">
<tuple id="a6fea09">
<status>
<gp:geopriv>
<gp:location-info>
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:pos>-34.4165 150.5332</gml:pos>
<gml:pos>-34.4170 150.5532</gml:pos>
<gml:pos>-34.4170 150.5537</gml:pos>
<gml:pos>-34.4165 150.5337</gml:pos>
<gml:pos>-34.4165 150.5332</gml:pos>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2005-07-05T14:49:53+10:00</timestamp>
</tuple>
</presence>
Geodetic Location Only PIDF-LO
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<?xml version="1.0"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:pidf="urn:ietf:params:xml:ns:pidf"
xmlns:cl=" urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gs="urn:ietf:params:xml:ns:pidf:geopriv10:geoShape"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:user@example.com">
<tuple id="a6fea09">
<status>
<gp:geopriv>
<gp:location-info>
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:pos>-34.4165 150.5332</gml:pos>
<gml:pos>-34.4170 150.5532</gml:pos>
<gml:pos>-34.4170 150.5537</gml:pos>
<gml:pos>-34.4165 150.5337</gml:pos>
<gml:pos>-34.4165 150.5332</gml:pos>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
<cl:civilAddress>
<cl:FLR>2</cl:FLR>
</cl:civilAddress>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2005-07-05T14:49:53+10:00</timestamp>
</tuple>
</presence>
Geodetic Location with Civic Floor PIDF-LO
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<?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:gs="urn:ietf:params:xml:ns:pidf:geopriv10:geoShape"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:user@example.com">
<tuple id="a6fea09">
<status>
<gp:geopriv>
<gp:location-info>
<gs:Prism srsName="urn:ogc:def:crs:EPSG::4976">
<gs:base>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:pos>-34.4165 150.5332 32</gml:pos>
<gml:pos>-34.4170 150.5532 32</gml:pos>
<gml:pos>-34.4170 150.5537 32</gml:pos>
<gml:pos>-34.4165 150.5337 32</gml:pos>
<gml:pos>-34.4165 150.5332 32</gml:pos>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gs:base>
<gs:height uom="urn:ogc:def:uom:EPSG::9001">
8
</gs:height>
</gs:Prism>
</gp:location-info>
<gp:usage-rules/>
<gp:method>Wiremap</gp:method>
</gp:geopriv>
</status>
<timestamp>2005-07-05T14:49:53+10:00</timestamp>
</tuple>
</presence>
Rectangular Prism PIDF-LO
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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
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
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