One document matched: draft-polk-geopriv-centerpoint-element-00.txt
Geopriv WG James Polk
Internet-Draft Allan Thomson
Expires: September 25, 2009 Marc Linsner
Intended Status: Standards Track (PS) Cisco Systems
Updates: RFC 4119 (if published as an RFC) March 25, 2009
Defining a centerpoint element for use in the Presence
Information Data Format - Location Object (PIDF-LO)
draft-polk-geopriv-centerpoint-element-00
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Abstract
This document creates a centerpoint element for use in the Presence
Information Data Format - Location Object (PIDF-LO).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Centerpoint Coordinate Element in PIDF-LO . . . . . . . . . . 4
3. Unit of Measurement Altitude Element in PIDF-LO . . . . . . . 5
4. Centerpoint Schema . . . . . . . . . . . . . . . . . . . . . 6
5. Security considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 7
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 RFC 2119 [RFC2119].
1. Introduction
In the Presence Information Data Format - Location Object (PIDF-LO),
as defined in RFC 4119 [RFC4119], location can currently be
expressed in 2 forms, geo-coordinates and civic (i.e., country,
state, county, street, landmark, etc - which is similar to postal
addresses). Within the geo-coordinate realm, GML [OpenGIS] is the
borrowed XML for expression of these coordinates. GML can describe
dozens shapes as well as points/lines/arcs/etc in many ways.
One shape in particular is a polygon. Polygons are quite useful
representations of areas when using coordinates. A polygon is a
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series of points connected by straight or arced lines, usually
polygon is what GML calls a Linear Ring, in which the first
coordinate point and the last coordinate point are the same. This
effectively completes the circular pattern, indicating the machine
or observer that the polygon is now closed, with no more additional
points in the polygon. For example, there needs to be at least 4
points to express a polygon, which is 3 unique points and the 4th
point, which is a repeat of the first point.
point point
B C
+-------------------+
| |
| |
| |
| |
| |
| |
| |
+-------------------+
A D
point point
Figure 1. Minimum Points in a Polygon
In Figure 1., there are 4 points in this polygon, however, there
needs to be 5 points in the linear ring, shown this way in XML
(where each point is in a <gml:pos> element. These are represented
in a linear ring in this order:
point A,
point B,
point C,
point D,
point A
Something that GML does not express within a polygon is its
calculated centerpoint, shown as point E in figure 2. Many
applications that use location information may only represent
location of an entity based on a point in space rather than the
generally accepted view that location results are represented better
as a polygon due to the errors and inaccuracies in determining the
entity's location.
Another aspect of concern is how location information is shown to
the user of location applications. Some applications may provide
location based on a textual representation rather than a graphical
form. Therefore in textual applications showing the user a list of
points that represent location can be unwieldy and unfriendly.
Generally, the ability to represent the location of an entity using
a single point is more useful than the more technically correct
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polygon representation. For example, when drawing the position of an
entity on a floorplan, the entity image will be drawn around the
centerpoint of the polygon rather than the upper left corner of the
polygon.
By having the supplier of the location object, containing the
polygon definition calculate the centerpoint, the recipient of the
location information can avoid a possibly computationally intensive
operation and potentially use the centerpoint directly in textual
outputs.
point point
B C
+-------------------+
| |
| centerpoint |
| E |
| + |
| |
| |
| |
+-------------------+
A D
point point
Figure 2. A Polygon with a Centerpoint
This document creates the centerpoint XML element <gml:centerpos>.
2. Centerpoint Coordinate Element in PIDF-LO
This document creates the XML element <gml:centerpos> to indicate
the centerpoint of a polygon.
Polygons are not supported by the mandatory to implement schema
contained within RFC 4119, only the feature.xsd is [RFC4119], shown
here
urn:opengis:specification:gml:schema-xsd:feature:v3.0
In order to support a polygon, the geometryPrimatives.xsd MUST be
supported from GML3.1.1 or later, which is here
urn:opengis:specification:gml:schema-xsd:geometryprimatives:v3.1.1
The European Petroleum Steering Group (EPSG) [Geoshape] provides the
datum to be used in the urn in XML. Specifically, EPSG::4326
indicates the WGS84 datum in 2 dimensions, and EPSG::4979 indicates
the WGS84 datum in 3 dimensions. An example urn for WGS84 in 2D is
here
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srsName="urn:ogc:def:crs:EPSG::4326"
For 2D, here an example of the just the centerpoint contained within
the <location-info> element of a PIDF-LO.
<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
<gml:centerpos>32.6 -86.6</gml:centerpos>
</gml:Point>
</gp:location-info>
A more complete XML example for a 2D polygon with centerpoint is
shown here
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326"
xmlns:gml="http://www.opengis.net/gml">
<gp:location-info>
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:posList>
<gml:pos>32.1 -86.1</gml:pos> <!--A-->
<gml:pos>32.1 -87.1</gml:pos> <!--B-->
<gml:pos>33.1 -87.1</gml:pos> <!--C-->
<gml:pos>33.1 -86.1</gml:pos> <!--D-->
<gml:pos>32.1 -86.1</gml:pos> <!--A-->
</gml:posList>
</gml:LinearRing>
</gml:exterior>
<gml:centerpos>
<gml:centerpos>32.6 -86.6</gml:centerpos>
</gml:centerpos>
</gml:Polygon>
</gp:location-info>
3. Unit of Measurement Altitude Element in PIDF-LO
The unit of measurement for altitude of a centerpoint, if present is
expressed in two ways,
- either in the <gml:centerpos> element as the 3rd number, where the
X and Y coordinates are the first 2 numbers. The unit of
measurement in this case MUST be in meters only.
or
- in the civic format as floors of a building <cl:FLR>.
For 3D, here an example of altitude (Z-Coordinate) in meters:
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<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4979">
<gml:centerpos>32.6 -86.6 10</gml:centerpos>
</gml:Point>
</gp:location-info>
For 3D, here an example of altitude (Z-Coordinate) in civic floors:
<gp:location-info>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4979">
<gml:centerpos>32.6 -86.6 10</gml:centerpos>
</gml:Point>
<cl:civicAddress>
<cl:FLR>2</cl:FLR>
</cl:civicAddress>
</gp:location-info>
Caution SHOULD be used if this new centerpoint element has altitude
included. If altitude is included in a centerpoint element, a much
less complex scenario has either the centerpoint being the only
element with an altitude, or the altitude in the centerpoint element
is the same altitude as all the other points in the polygon (that
include altitude).
It is possible, configurations MAY want the centerpoint altitude to
be the average (or mean) of all the points in the polygon, even when
no points of the polygon are at that exact altitude.
4. Centerpoint Schema
TBD
5. Security considerations
The security considerations are the same as is in RFC 4119
[RFC4119].
6. IANA considerations
This document IANA registers the following... tbd
7. Acknowledgments
Your name here... or if you contribute a fair amount of text, you
can be a co-author.
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8. References
8.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
[RFC4119] J. Peterson, "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005
[OpenGIS] - http://www.opengeospatial.org/standards/gml
[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
8.2. Informative References
There currently are no informative references within this document.
Authors' Addresses
James Polk
3913 Treemont Circle
Colleyville, Texas, USA
+1.817.271.3552
mailto: jmpolk@cisco.com
Allan Thomson
Cisco Systems, Inc.
San Jose, California, USA
Email: althomso@cisco.com
Marc Linsner
Cisco Systems, Inc.
Marco Island, Florida, USA
Email: mlinsner@cisco.com
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