Understanding Geographic Feed standard – GeoRSS Simple
GeoRSS is an open and emerging standard aimed at describing geographic information (location) in an interoperable manner so that applications can request, aggregate, share and map geographically tagged feeds. GeoRSS, derived from RSS, extends existing web feeds – rendered by aggregators and web browsers – with geographic information.
The GeoRSS collaboration has completed work on two primary encodings namely:
1. GeoRSS-Simple – Lightweight format that supports basic geometries – point, line, box, polygon – and useful for encoding locations.
2. GeoRSS GML (Geography Markup Language) – Formal Open Geospatial Consortium (OGC) GML application profile supporting a greater range of features.
GeoRSS coordinates refers to World Geodetic System (WGS84) that defines a reference frame for the earth which is the accepted standard for use in geodesy and navigation. WGS84 comprises a standard coordinate frame for the Earth, a standard spheroidal reference surface for raw altitude data and a gravitational equipotential surface that defines the nominal sea level. It is to be noted that GeoRSS lines and boundaries are allowed to cross the dateline or circle the poles.
GeoRSS Simple is concise in both representation and conception and quite sufficient for many needs. The four GeoRSS objects require only a single tag.
1. A point contains a single latitude-longitude pair, separated by whitespace.
2. A line contains a space separated list of latitude-longitude pairs (minimum two pairs) in WGS84 coordinate reference system, with each pair separated by whitespace.
<georss:line>45.256 -110.45 46.46 -109.48 43.84 -109.86</georss:line>
3. A box contains two space separate latitude-longitude pairs. The first pair is the lower corner and the second pair is the upper corner. A bounding box is a rectangular region used to define the extents of a map or a rough area of interest.
<georss:box>42.943 -71.032 43.039 -69.856</georss:box>
4. A polygon contains a space separated list of latitude-longitude pairs. There must be at least four pairs, with the last being identical to the first (so a polygon has a minimum of three actual points).
<georss:polygon>45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45</georss:polygon>
5. A circle contains three coordinates with centerpoint latitude, centerpoint longitude in the WGS84 coordinate reference system and radius in meters.
<georss:circle>42.943 -71.032 500</georss:circle>
As Earth is a spheroid, GeoRSS enforces some constraints on the geometries to avoid ambiguous shapes. For example, it requires that no two points in a line or polygon may have latitude values that are more than 179 degrees apart. This constraint allows the use of the simple convention with just asking the users to add an extra point in the geometries to avoid ambiguity.
GeoRSS geometry is meant to represent a real feature of the Earth’s surface. It has these additional case-sensitive properties with no constraints (intent being the folksonomy to emerge).
1. A single string containing a featuretypetag. The default is “location”.
2. A single string containing a relationshiptag. The default relationship, “is-located-at” simply indicates that the subject of the content is located at the GeoRSS feature.
3. A single string containing a featureNametag which indicates the name or identifier of the referenced geographic entity.
<georss:featurename>12 HURSTCOURT ROAD Sutton Surrey SM1 3JF</georss: featurename>
In GeoRSS, Elevation can be expressed as “elev” or “floor”.
- “elev” is meant to contain height in meters from the WGS84 ellipsoid, easily obtainable from any GPS device.
- “floor” is meant to contain the floor number of a building.
GeoRSS Simple permits the uses of comma instead of whitespace and GeoRSS parsers should just treat commas as whitespace. Refer to http://www.windsorsolutions.biz/xsd/ENGeoTF/georss11.htm for details on the GeoRSS schema properties.
The downside to the GeoRSS Simple’s simplicity is the cost of direct upward compatibility with GML which represents a point as follows:
GeoRSS Simple meets the need for many applications and the transformation from Simple serialization to the GML serialization is straightforward.
GeoRSS is used in many sites and one such example is http://mapufacture.com/about/buildingmaps. Multiple GeoRSS-Simple Readers which will take the geographic data and display maps – say in Google Maps or Virtual Earth – are available in the web. GeoRSS using the fixed coordinate system of decimal longitude, latitude degrees (global WGS84) that is used by most online map services, makes it easy for “mashups” without complex coordinate transformations.