Geography Markup Language

Geography Markup Language
Name	Geography Markup Language
Extension	.gml, .xml
Mime	application/gml+xml
Developer	Open Geospatial Consortium
Latest release version	3.3
Genre	GIS data format
Container for	Vector and coverage data
Extended from	XML
Standard	ISO 19136

Geography Markup Language. It is an XML grammar defined by the Open Geospatial Consortium for expressing geographical features. Serving as a fundamental modeling language for geographic systems, it provides a framework for describing a wide range of spatial objects and their properties. This encoding standard is pivotal for enabling data interoperability across diverse GIS platforms and services.

Overview

The primary role of this language is to facilitate the storage and transport of geographic information in a vendor-neutral, text-based format. It underpins many key Open Geospatial Consortium specifications, including the Web Feature Service and Sensor Observation Service. By providing a common schema, it allows disparate systems, from ESRI's ArcGIS to open-source projects like QGIS and GDAL, to exchange complex spatial data. Its adoption is widespread in national and international spatial data infrastructures, such as those governed by the International Organization for Standardization and the European Union's INSPIRE Directive.

Technical specifications

Built upon the XML Schema, the specification defines core geometry types like points, lines, and polygons using constructs such as `gml:Point` and `gml:Polygon`. It supports complex geometric constructs including curves, solids, and topological primitives. The language also provides a robust framework for temporal dimensions, coordinate reference systems, and metadata, often aligning with models from the International Hydrographic Organization. Its object-property model allows for rich feature descriptions, enabling the encoding of relationships between entities, which is critical for applications in urban planning and environmental science.

GML schemas and profiles

The base schema is extensive, so defined application schemas and profiles are created for specific domains to ensure practicality. Prominent profiles include the GML Simple Features Profile for basic vector data interchange and the CityGML profile for detailed three-dimensional city models. Other specialized schemas support domains like aeronautical information and hydrographic data exchange, often developed in collaboration with bodies like the International Civil Aviation Organization. These constrained implementations ensure the language can be efficiently used in focused contexts, such as within the United States Geological Survey or for transportation network modeling.

Applications and usage

This encoding is extensively used in web services, particularly the OGC Web Services suite, for serving dynamic map data. It forms the data payload for transactions in services like the Web Feature Service and is a common format for catalog entries in Catalogue Service for the Web implementations. National mapping agencies, including the Ordnance Survey and the National Geospatial-Intelligence Agency, utilize it for data distribution. Its application is also critical in sensor networks, disaster management systems, and climate modeling initiatives led by organizations like the World Meteorological Organization.

Standards and interoperability

The specification is formalized as ISO standard ISO 19136, ensuring its global recognition and stability. Its design is intrinsically linked to the broader ISO 19100 series of geographic information standards. This formal standardization is crucial for achieving legal interoperability in projects like the European Union's INSPIRE Directive, which mandates its use for data harmonization. Compliance with this standard is a key requirement for many government procurement contracts and international data-sharing agreements, fostering seamless collaboration between entities like NASA and the European Space Agency.

Development and versions

Development is managed by the Open Geospatial Consortium's technical committees, with significant input from members like Google, Oracle Corporation, and research institutions. Major versions include GML 3.0, which introduced complex geometries and temporal support, and the streamlined GML 3.2.1. The current version, GML 3.3, further refines the schema and aligns with updates in related ISO 19100 series standards. The evolution of the language is closely tied to advancements in web service technologies and the growing needs of the global spatial data infrastructure community.

Category:Geographic data formats Category:XML-based standards Category:Open Geospatial Consortium standards Category:ISO standards