Spec: GeoRSS (GML)
Geographically Encoded Objects for RSS feeds
_Geography Markup Language (GML) is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information...GML provides a variety of kinds of objects for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values. A geographic feature is "an abstraction of a real world phenomenon; it is a geographic feature if it is associated with a location relative to the Earth?. So a digital representation of the real world can be thought of as a set of features.
Here we present the encoding of GeoRSS' objects in a simple GML version 3.1.1 profile. Each section details the construction of GeoRSS' five objects, followed by some informative use cases. As with all GeoRSS encodings, if not specified, the implied coordinate reference system is WGS84 with coordinates written in decimal degrees.
Point Line Polygon Box Examples Coordinate Reference Systems GML Application Schema
A point consists of a element with a child element. Within the latitude and longitude values are separated by a space.
<title>Earthquakes</title>
International earthquake observation labs
2005-12-13T18:30:02Z
Dr. Thaddeus Remor
tremor@quakelab.edu
urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6
<title>M 3.2, Mona Passage</title>
urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a
2005-08-17T07:02:32Z
We just had a big one.
**
<**gml:**Point>
<**gml:**pos>45.256 -71.92**gml:**pos>
******gml****:**Point>
**georss:**where>**
A line consists of a element with a child element. Within the coordinates of the points on the line are entered as pairs of latitude and longitude values, separated by spaces. There must be at least two pairs. No two pairs may be separated by more than 179 degrees in either latitude or longitude.
...
**
** **
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** 45.256 -110.45 46.46 -109.48 43.84 -109.86**
**
** ****
A polygon consists of a element with a child , and elements. There must be at least four pairs with the last being identical to the first. (a boundary has a minimum of three actual points.) No two pairs may be separated by more than 179 degrees in either latitude or longitude.
...
****
**
**
**45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45**
****
**
** ****
****
specifies this shape as defining the outside of an area, and states that the coordinates should be connected with straight lines. Within the coordinates of the points are entered as pairs of latitude and longitude values, separated by spaces. There must be at least four pairs with the last being identical to the first. (a polygon has a minimum of three actual points.) No two pairs may be separated by more than 179 degrees in either latitude or longitude.
A bounding box defines a rectangular region. It is often used to define the extents of a map or define a rough area of interest. A GML box is called an Envelope. It consists of an element with a child element and a child element.
...
****
**
42.943 -71.032
43.039 -69.856
**
A good way to describe a trip that has many places of interest like a boat trip or a hike is to specify the overall trip's path with a line as a child of the . Then mark each location of interest with a point in the .
<title>Dino's Mt. Washington trip</title>
2005-12-13T18:30:02Z
Dino Bravo
dbv@example.org
http://www.myisp.com/dbv/
**
<**gml:LineString**>
<**gml:pos**List>
45.256 -110.45 46.46 -109.48 43.84 -109.86 45.8 -109.2
**gml:pos**List>
****gml:LineString****>
**georss:**where>**
<title>Setting off</title>
http://www.myisp.com/dbv/1
2005-08-17T07:02:32Z
getting ready to take the mountain!
**
<**gml:Point**>
<**gml:pos**>45.256 -110.45**gml:pos**>
****gml:Point****>
**georss:**where>**
<title>Crossing Muddy Creek</title>
http://www.myisp.com/dbv/2
2005-08-15T07:02:32Z
Check out the salamanders here
**
<****gml:Point****>
<**gml:pos**>45.94 -74.377**gml:pos**>
****gml:Point****>
**georss:**where>**
<title>Cambridge Calendar of Events</title>
Goings on around Cambridge
2005-12-13T18:30:02Z
Arty "the one man party" Collins
apcollins@cambridgema.gov
http://www.cambridgema.gov/calendar
**
<**gml:Polygon**>
45.256 -110.45 46.46 -109.48 43.84 -109.86
45.8 -109.2 45.256 -110.45
************
****gml:Polygon****>
**georss:**where>**
<title>City Council Meeting</title>
http://www.cambridgema.gov/calendar/2005/09/09/evt01
2005-08-17T07:02:32Z
Regular weekly meeting
**
<******gml:Point******>
<**gml:pos**>45.256 -110.45**gml:pos**>
******gml:Point******>
**georss:**where>**
<title>NoCa Arts Open House</title>
http://www.cambridgema.gov/calendar/2005/09/12/evt02
2005-08-15T07:02:32Z
North Cambridge arts festival featuring local artists
**
<******gml:Point******>
<**gml:pos**>45.94 -74.377**gml:pos**>
******gml:Point******>
**georss:**where>**
A lot of data out in the world doesn't come in latitude/longitude coordinates. It's in UTM or Stateplane or some other coordinate reference system. Why? Because lat/lon just isn't very good when you care about inches or centimeter accuracy. And thank goodness engineers building roads and bridges, and surveyors measuring your property boundaries care very much.
If your GeoRSS GML data is in a coordinate reference system other than lat/lon WGS84 (often technically referred to as EPSG:4236), add in an srsName attribute to your geometry.
The key thing here is to use CRS epsg:4979, specify the srsDimension attribute, and include a third number in your coordinate tuple.
...
42.3453 -156.2342 45
26986 is the Stateplane system for Massachusetts Mainland
...
****
**
**
**45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45**
****
**
** ****
****
All good information encodings should be backed by a formal definition. In this case, we use XML schema to do the job. This helps software developers--especially XML-savvy ones, know the full capabilities of the language, and program their software to handle any case, not just those they've come across in examples.
GML is a large and complex language which is also defined by XML schema. Since it is so large, a practice has emerged to define subsets of GML, called Profiles, that contain only those elements of GML needed for the encoding job at hand.
Here is our GeoRSS GML Profile [http://www.georss.org/xml/1.0/gmlgeorss.xsd].
We also have a rather large graphic of the UML model for this profile.
The application schema defines georss:where as the tag that signals geographic content--either in GeoRSS Simple or GML.
Here is our GeoRSS Application Schema [http://www.georss.org/xml/1.0/georss.xsd].
The UML model is shown below. Click on it for a larger, more readable version.
