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Understanding GeoRSS – GML – Geographic Markup Language

Sasirekha R — Thu, 17 Mar 2011 07:37:58 +0000

Understanding GeoRSS – GML – Geographic Markup Language

GeoRSS is a standard for providing location information as part of a data feed. Compared to GeoRSS-Simple, GeoRSS GML (or Pro GeoRSS) goes to the next level of complexity allowing different coordinate reference systems (CRS).

According to http://www.opengeospatial.org/projects/groups/gmlwg, GML, Geography Markup Language, is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information. GML is originally based on Resource Description Framework (RDF developed by W3C) and retains many features including intelligent agents and a standard syntax for describing and querying data. According to http://www.w3.org/Mobile/posdep/GMLIntroduction.html, for the first time spatial information will have a truly public encoding standard and the inherent transformability and accessibility of GML will open a whole new domain in geo-spatial information management.

GML provides a variety of objects including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values for describing geography. A geographic feature is associated with a location relative to the Earth and is “an abstraction of a real world phenomenon”. In GML, geographic features like lakes, rivers, mountains as well as buildings, parks and cities are described by using geometry (referred to as GeoTagging).

GeoRSS GML representatation are follows:

A point consists of a element with a child element. Within the latitude and longitude values are separated by a space.
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.
A polygon consists of a element with a child , and elements. specifies this shape as defining the outside of an area, and states that the coordinates should be connected with straight lines.
A bounding box defines a rectangular region 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. .

Of course, as in Simple, the default CRS is WGS84 with coordinates written in decimal degrees. From simplistic point of view, locations can be described using familiar features like postal codes, street names, address, landmarks and assigned points, lines or boundaries. Upper and lower elevation limits can be used if three-dimensional data is needed. Also while defining polygons, the edge may be a geodesic – a line representing the shortest distance between two points on the earth’s surface.

When inches or centimeter accuracy are required, then the latitude/longitude coordinates supported by WGS84 is not sufficient. Other coordinate reference systems – like UTM or Stateplane – are used especially by engineers, surveyors, scientists and by specific communities that share specialized information.

If your GeoRSS GML data is in a coordinate reference system other than WGS84, add in an srsName attribute to your geometry. To use 3D decimal degrees, specify the srsDimension attribute, and include a third number in your coordinate tuple.

…

srsName=”urn:ogc:def:crs:EPSG:6.6:4979″ srsDimension=”3″>

          42.3453 -156.2342 45





Some of the points worth noting about GML are:

The state of a feature is defined by a set of properties (number of properties determined by the feature), where each property is a {name, type, value} triple.

Geographic features with geometry are those with properties that may be geometry-valued.

Geographic features in GML include coverages and observations as subtypes.

1. A coverage has a coverage function with a spatial domain and a value set range of homogeneous two to n dimensional tuples. It can represent a collection of features “to model and make visible spatial relationships between, and the spatial distribution of, earth phenomena.”

2. An observation models the act of observing, often with a camera, a person or some form of instrument. An observation is considered to be a GML feature with a time at which the observation took place, and with a value for the observation.

A reference system provides a scale of measurement for assigning values “to a location, time or other descriptive quantity or quality”.

1. A coordinate reference system (CRS) is a set of coordinate system axes that is related to the earth through a datum that defines the size and shape of the earth. The “parent” geometry element of a geometric complex or geometric aggregate indicates the CRS in which the measurements are made for it as well as its constituent geometries.

2. A temporal reference system provides standard units for measuring time and describing temporal length or duration. Following ISO 8601, the Gregorian calendar with UTC is used in GML as the default temporal reference system.

A Units of Measure (UOM) dictionary provides definitions of numerical measures of physical quantities, such as length, temperature, and pressure, and of conversions between UOMs

GML is a large and complex language which is also defined by XML schema. The application schema defines as the tag that signals geographic content–either in GeoRSS Simple or GML. Refer http://www.georss.org/xml/1.1/geoss.xsd. Profiles that contain only those elements of GML needed (subsets) are defined for the encoding job at hand (e.g., http://www.georss.org/xml/1.1/gmlgeorss.xsd).

GeoRSS is a method of describing and pinpointing the physical locations of Internet content. GeoRSS is designed for use with Atom 1.0, RSS 2.0 and RSS 1.0 and can also be easily used in non-RSS XML encodings. The following are some of the examples of GeoRSS feed is used:

To describe places of interest in a boat trip or a hike, you can specify the trip’s path with a line and the specific locations of interest with points.

To create a calendar of events along with its specific geographic locations

To represent road hazards like Potholes, Traffic Light, Flood etc. in a city (ref. http://msdn.microsoft.com/en-us/library/aa907682.aspx)

Using GeoRSS, it is possible to search for Websites or items based on relevant geographic locations.

To pass on or search for information on the specific geographic areas, say veterinarians getting details of a spread of a wild life disease among a specific breed in a certain area.

GeoRSS facilitates the immediate dissemination of RSS feeds to specific users based on event type and location. For example, during an outbreak of severe weather in Minnesota, all tornado-related alerts and warnings can be delivered to computers, PDAs, mobiles or other communications devices within 100 kilometers of a specified location (say by using postal ZIP code).

To map your photos in MapQuest

GeoRSS GML is an evolving standard and is adopted by standard names like Google, IBM and Microsoft. The Folksonomy is emerging and in the current state quite flexible. GML forms a good backbone on which Mashups can be built easily.

GeoRSS Simple – standard for Geographic Information Feed

Sasirekha R — Wed, 16 Mar 2011 14:02:51 +0000

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. GeoRSS is designed for use with Atom 1.0, RSS 2.0 and RSS 1.0 and can also be easily used in non-RSS XML encodings.

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.

45.256 -71.92

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.

45.256 -110.45 46.46 -109.48 43.84 -109.86

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.

42.943 -71.032 43.039 -69.856

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).

45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45

5. A circle contains three coordinates with centerpoint latitude, centerpoint longitude in the WGS84 coordinate reference system and radius in meters.

42.943 -71.032 500

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”.

city

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.

is-centered-at

3. A single string containing a featureNametag which indicates the name or identifier of the referenced geographic entity.

Millis, MA

12 HURSTCOURT ROAD Sutton Surrey SM1 3JF

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.

313

“floor” is meant to contain the floor number of a building.

2

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:



      45.256 -71.92



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.