GML (Geography Markup Language)

An XML grammar for expressing geographical features.

Geography Markup Language (GML) is a powerful, flexible, XML-based spatial data format developed by the Open Geospatial Consortium (OGC). Its primary function is to model geographical features represented in geographic information system (GIS) frameworks. It allows the transport and storage of geographical information, including both the geometry of spatial features and their properties.

GML specifies different kinds of geographic entities through the use of XML grammar. This makes it extensive and universal, able to express simple geographical features such as points, lines, and polygons. More complex structures like coverage, topology, and time-series can also be conveyed. Thus, allowing a standard way to express and transport geographical data through disparate systems and services.

One significant advantage of GML is its interoperability. By harnessing the universally accepted structure of XML, the language provides a standard way to express and exchange spatial data in a format that can be understood and utilized by any system that can process XML. This creates a common language for describing geospatial data and alleviates the complexities often associated with spatial data exchange.

GML can also store non-spatial attributes of geographical features. This makes the format capable of representing the richness and diversity of data associated with any given entity. For example, a GML document describing a road network can include non-spatial data such as speed limit, pavement type, or traffic volume data. Such holistic representation is often vital for geospatial analyses in various academic and industry applications.

The geospatial data expressed in GML can be used in diverse applications, including surveying, military and transport applications, spatial data infrastructures, and web-based Services. These include applications in environmental management, land information systems, and infrastructure protection.

Geography Markup Language supports all traditional geospatial processing functions including reference system transformations, spatial and nonspatial queries, spatial analyses and processing, filtering, and visualization. This makes Geography Markup Language an indispensable tool in the domain of GIS.

Spatial schemas in GML define the data structures for geographic features extensively. Each geographic feature in GML is defined by a collection of properties, which form related pairs of name-value, each associated with a type. The ability to extend schemas allows developers to represent business- and application-specific forms of these features.

GML encourages syntactic extensibility, supporting application schemas of one or more namespaces for defining the structure of an application's geographic features. This feature allows GML to accommodate new spatial data types that can be combined and nested to create complex spatial structures.

Nevertheless, despite its language complexity, users and developers can utilize GML subset profiles that provide common sets of GML components focusing on specific application areas and requirements. The standard comes with a suite of recommended profiles, although organizations can also define their custom profiles. This flexibility allows the data format to be optimized for efficiency, size, and data complexity for specific requirements by simplifying and stabilizing GML documents for use and reuse in particular contexts.

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