Geography Markup Language

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Geography Markup Language
File extension : .gml
Developed by: OGC , ISO TC 211
Type: Markup language
Extended by: XML
Website : OpenGIS Geography Markup Language (GML) Encoding Standard

Geography Markup Language , GML for short , is a markup language for the exchange of spatial objects ( features ). GML is an application of XML and is defined by schema descriptions (XML schema files * .xsd). GML allows the transmission of objects with attributes, relations and geometries in the area of spatial data , including non-conventional data such as sensor data.

GML is defined by the Open Geospatial Consortium (OGC) together with ISO TC 211 , the ISO technical committee for the definition of digital geospatial data. GML version 3.2.1 is now available. As with all results of the OGC process, the documentation is freely available to everyone.

Standard conformity

With version 3.2, GML is now also based on the international standards of the ISO 191xx series. The OGC specification for GML 3.2 is identical to ISO 19136 and therefore also an official international standard of ISO itself . In doing so, GML also takes into account, in particular, the specifications for basic spatial reference forms in ISO 19107 .

The GML model

The GML model was originally based on the Resource Description Framework (RDF) of the World Wide Web Consortium (W3C). Later, the OGC introduced XML Schemas in GML to facilitate the linking of the many different relational databases with geospatial data. The resulting XML schema-based GML retains many RDF elements including the concept of inheriting properties from the parent object and applying references to remote property references.

GML has a rich set of primitives that are used to build application-related schemes or application languages. The primitives include:

  • Object (feature)
  • Geometry
  • Coordinate reference system
  • Time
  • dynamic object
  • Coverage including geographic images (coverage)
  • Unit of measure
  • Design rules for the map display (map presentation styling rules)

Profiles

GML profiles are logical constraints of GML and can be expressed by a document, an XML schema, or both. These profiles serve to facilitate the introduction of the GML standard. The following profiles have been published or proposed for general use:

Profiles in GML are not application schemas: while profiles are an element of the GML namespaces and describe restricted subsets of GML, application schemas are XML vocabularies for using GML and are located in the target namespace specified by the application. Application schemes can use the full set of GML schemes or specific GML profiles. Profiles are often defined to implement GML-based languages ​​for specific application areas, such as: B. for air traffic, nautical mapping or the management of natural resources.

As of version 3 of GML, two XSLT scripts are included (usually referred to as a subset tool) which can be used to build GML profiles.

Application schemes

Application schemes are used for the suitable representation of geographic data for a special application area with the help of GML. This schema describes the types of objects whose data should be presented and which should be processed by the application.

GML geometries

GML encodes GML geometries (GML geometries or geometric characteristics) of geographic objects as elements within GML documents. The geometries of the objects can describe things like streets, rivers or bridges.

The most important object types in GML 1.0 and 2.0 are

  • Point
  • LineString
  • Polygon

This model is identical to that of KML .

Applications

GML is one of the XML standards for the GeoWeb infrastructure that enables Internet-enabled devices to access geographic data, such as: B. Traffic conditions and the location of shops. GML is also used in the standards-based exchange interface (NAS).

GML and KML

KML stands for Keyhole Markup Language and is distributed worldwide via Google . GML is suitable for opening up the content of geospatial documents by describing a spectrum of application objects and their properties (such as bridges, roads, buoys and vehicles). KML, on the other hand, stands for the visualization of geographic information and can be used to display GML content. On the other hand, GML can also be expanded in such a way that it can display content in the manner of KML.

See also

Web links

Individual evidence

  1. ^ GeoRSS GML. georss.org, June 1, 2009, accessed November 29, 2010 .