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<html>
<head>
<meta http-equiv="Content-Type" content="text/html">
<title>GeoRSS Model</title>
<link href='http://fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'><link href="global.css" type="text/css" rel="stylesheet">
</head>
<body>
<div class="nav"><a href="index.html">home</a> | <a href="overview.html">overview</a> | <a href="model.html">model</a> | <a href="gml.html">gml</a> | <a href="simple.html">simple</a> | <a href="w3c.html">W3C Geo</a> | <a href="atom.html">atom</a> | <a href="rdf_rss1.html">rdf/rss1</a> | <a href="implementations.html">implementations</a> | <a href="proposals.html">proposals</a></div>
<h4>GeoRSS ::<br/>Geographically Encoded Objects for RSS feeds</h4>
<h2>GeoRSS Model</h2>
<a name="model"></a>
<p><img src="graphics/georss-model.png" width="587" height="346" />
<strong>
<p align="center">Figure 1. - GeoRSS Model</p>
</strong>
<p>Figure 1 shows the UML model for GeoRSS. The left side of Figure 1 represents
GeoRSS, the right side represents the "external" content that GeoRSS is being
used to describe. In the model, <strong>where</strong> is an association <em>of</em> a <strong>geometry</strong> to
some
<strong>content</strong>. GeoRSS places no constraint on the type of content,
nor on its format.
<p>GeoRSS itself is identified by a namespace, indicated by "georss:".
The model above does not show, but allows, the inclusion of any other elements
from other
namespaces.
<p>The model itself is an abstract concept. In order to use the model, it must
be expressed in a concrete form such as XML, RDF, etc. These expressions are
called <strong><a href="http://en.wikipedia.org/wiki/Serialization">serializations</a></strong>.
<p>By allowing for the definition of multiple serializations, all of which conform
to the model, GeoRSS will provide the means of relating one serialization to
another. The goal is to also map the model to existing serializations where
possible.
<p>The core concepts of GeoRSS are explained below. These concepts
are then encoded or serialized in
XML for use with <a href="#atomenc">Atom or RSS 2.0</a>. Alternate serializations
such as in RDF and XHTML are
also being developed.
<h3>Coordinate reference system (CRS)</h3>
<p>There aren't real grid lines or axes on the earth, so any GeoRSS coordinates
must refer to a <a href="https://portal.opengeospatial.org/files/?artifact_id=6716">coordinate
reference system</a> which has been defined and agreed to by geodesists and
software makers alike. The GeoRSS CRS is the global (and GPS-preferred) <strong>WGS84,
latitude, longitude</strong> (in that order), using <strong>decimal degrees</strong>.
Note that GeoRSS
lines and boundaries are allowed to do things like cross the dateline or circle
the poles.</p>
<h3>Geometry</h3>
<p>The geometric shapes which can be used to represent location in GeoRSS
are <strong>point</strong>, <strong>line</strong>,
and <strong>boundary</strong>. </p>
<p>A <strong>point</strong> contains a single coordinate pair. The coordinate pair contains a
latitude value and a longitude value<em> in that order</em>. The <em>preferred</em> serialization
of this uses a <em>space</em> to separate the two values.</p>
<p>A <strong>line</strong> contains two or more coordinate pairs. Each pair contains
a latitude value and a longitude value<em> in that order</em>. The <em>preferred</em> serialization
of this uses a <em>space</em> to separate the two values. Pairs are separated
from each other
by a <em>space</em>.</p>
<p>A <strong>box</strong> contains exactly two coordinate pairs. Each
pair contains a latitude value and a longitude value<em> in that order</em>.
The <em>preferred</em> serialization of this uses a <em>space</em> to separate
the two values. Pairs are separated from each other
by a <em>space</em>. The first coordinate pair (lower corner) must be a point
further
west and south of the second coordinate pair (upper corner) and the box is always
interpreted
as
not
containing the 180 (or -180) degree longitude line other than on its boundary
and
not
containing
the
North
or South pole other than on its boundary. A box <em>is generally</em> used to <em>roughly</em>demarcate
an area within which other data lie.</p>
<p align="center"><img src="graphics/gml_envelope.jpg" width="260" height="140" /></p>
<p>A <strong>polygon</strong> contains at least four coordinate pairs. Each
pair contains a latitude value and a longitude value<em> in that order</em>.
The <em>preferred</em> serialization
of this uses a <em>space</em> to separate the two values. Pairs are separated
from each other
by a <em>space</em>. The last coordinate pair must be identical to the first.</p>
<p align="center"><img src="graphics/179-rule.jpg" width="446" height="248" /></p>
<p align="center"><strong>Figure 2. - 179 degree rule</strong></p>
<p>Due to the fact that the Earth is round, we have to place some constraints
on the geometries to avoid ambiguous shapes. In order to be able to easily
represent the red box in Figure 2, where the left side of the box is at 160
degrees longitude and the right side of the box is at -175 degrees,<strong> no
two points in a line or polygon may have latitude values that are more than
179
degrees
apart</strong>. This constraint avoids other approaches that require either
additional information about the topology of the geometry or preclude the ability
to represen
geometries such as this. Since topology is beyond
the scope of this specification, it should not matter that you may have to
add
an extra point into your geometries. <strong>Line</strong> and <strong>polygon</strong> <strong>MUST</strong> follow
this rule. <strong>Box</strong> does not.</p>
<h3>Feature Type Tag</h3>
<p>GeoRSS geometry is meant to represent a real feature of the Earth's surface.
The GeoRSS model allows for a single <em>string</em> containing a <strong>featuretypetag</strong>.
No constraints are placed on this string. The intent is to allow a Feature
Type <a href="http://en.wikipedia.org/wiki/Folksonomy">folksonomy</a> to
emerge. The default is "<strong>location"</strong>. </p>
<h3>Relationship Tag</h3>
<p>GeoRSS is a way of <em>relating</em> Web <strong>content</strong> to Earth <strong>features</strong>.
The GeoRSS model allows for a single string containing a <strong>relationshiptag</strong>.
No constraints are placed on this string. The intent is to allow a relationship <a href="http://en.wikipedia.org/wiki/Folksonomy">folksonomy</a> to
emerge.The default
relationship, "<strong>is-located-at</strong>" simply indicates that
the subject of the content<strong> </strong>is located
at<strong> </strong> the
GeoRSS feature. </p>
<h3>Elevation</h3>
<p>In order to provide a means of expressing an elevation, the Simple form of
GeoRSS has two special tags. These tags are not meant to be used in the
GML version since elevation values would be properly expressed based in more
precise
terms. The tags are <strong>elev</strong> and <strong>floor</strong>.</p>
<p><strong>elev</strong> is meant to contain "common" GPS elevation readings,
i.e. height in meters from the WGS84 ellipsoid, which is a reading that should
be easy to get from any GPS device.</p>
<p><strong>floor</strong> is meant to contain the floor number of a building. In some countries
the numbering is different than in other countries, but since we'll know the
location of the building, it should be fairly unambiguous.</p>
<p> </p>
<!--<h2>Examples</h2>
<p>Default Example: “I go to the UPS Store {42.280297,-71.235639} a few times a week to pick up my mail.” This means “The subject of the text snippet (UPS Store) is located at a location represented by a position in WGS84 coordinates.”</p>
<p>Non-default Example: “I go to the UPS Store {point, UTM Zone 19 4683318.0N 315659.72E, mailbox, is located at} a few times a week to pick up my mail.” This means “The subject of the text snippet (UPS Store) is located at a location represented by a point in WGS84 coordinates.” </p>
<h2>Constraints<a name="constraints"></a></h2>
<p>When used in RSS, the <Item> tag can only contain one geo object. If your document has more than one geo object, then you should decide what the <Item> geo should be. Bounding box, center, etc.</p>
-->
</p>
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