The Open GIS Consortium has released an approved version 3.0 for the OpenGIS Geography Markup Language (GML) Implementation Specification. GML "is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information; it provides a variety of kinds of objects for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values. New additions in GML 3.0 include support for complex geometries, spatial and temporal reference systems, topology, units of measure, metadata, gridded data, and default styles for feature and coverage visualization. The GML 3.0 modular structure means that developers choosing to use GML can literally pick out the schemas or schema components that apply to their work. GML 3.0 also includes a sample packaging tool that creates a tailored schema containing only the required components from the GML core schemas. The normative parts of the specification use the W3C XML Schema language to describe the grammar of conformant GML data instances. The specification also uses the Recommended XML encoding of Coordinate Reference System definitions prepared by the CRS SIG of OGC. OGC is an international industry consortium of more than 240 companies, government agencies and universities participating in a consensus process to develop publicly available interface specifications. OpenGIS Specifications support interoperable solutions that 'geo-enable' the Web, wireless and location-based services, and mainstream IT."

Bibliographic information: OpenGIS Geography Markup Language (GML) Implementation Specification. Edited by Simon Cox (CSIRO), Paul Daisey (U.S. Census Bureau, Geography Division), Ron Lake (Galdos Systems), Clemens Portele (Interactive Instruments), and Arliss Whiteside (BAE Systems Mission Solutions). From Open GIS Consortium, Inc. Submitting organizations: CSIRO Australia, Galdos Systems, Interactive Instruments, BAE Systems, US Census, POSC, MapInfo, Oracle Corp, NTT Data, and Laser-Scan Ltd. Reference number: OGC 02-023r4. Version 3.00. 2003-01-29. Normative annexes: (1) Annex A, Abstract Test Suite; (2) Annex B, Conformance; (3) Annex C, GML Schemas. Informative annexes: Annex D, CRS Schemas; Annex E, Tutorial; Annex F, Guidelines for Subsetting GML Schemas. 548 pages.

GML Overview

The Geography Markup Language (GML) is an XML encoding for the modeling, transport and storage of geographic information including both the spatial and non-spatial properties of geographic features. This specification defines the XML Schema syntax, mechanisms, and conventions that:

• Provide an open, vendor-neutral framework for the definition of geospatial application schemas and objects
• Allow profiles that support proper subsets of GML framework descriptive capabilities
• Support the description of geospatial application schemas for specialized domains and information communities
• Enable the creation and maintenance of linked geographic application schemas and datasets
• Support the storage and transport of application schemas and data sets
• Increase the ability of organizations to share geographic application schemas and the information they describe

Implementers may decide to store geographic application schemas and information in GML, or they may decide to convert from some other storage format on demand and use GML only for schema and data transport. [from the v3.00 Scope Statement]

GML Description

Geography Markup Language is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information. The key concepts used by Geography Markup Language (GML) to model the world are drawn from the OGC Abstract Specification. GML provides a variety of kinds of objects for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values.

A geographic feature is "an abstraction of a real world phenomenon; it is a geographic feature if it is associated with a location relative to the Earth". So a digital representation of the real world can be thought of as a set of features. The state of a feature is defined by a set of properties, where each property can be thought of as a {name, type, value} triple. The number of properties a feature may have, together with their names and types, are determined by its type definition. Geographic features with geometry are those with properties that may be geometry-valued. A feature collection is a collection of features that can itself be regarded as a feature; as a consequence a feature collection has a feature type and thus may have distinct properties of its own, in addition to the features it contains.

Geographic features in GML include coverages and observations as subtypes. A coverage is a sub-type of feature that has a coverage function with a spatial domain and a value set range of homogeneous 2 to n dimensional tuples. A coverage can represent one feature or a collection of features "to model and make visible spatial relationships between, and the spatial distribution of, earth phenomena."

An observation models the act of observing, often with a camera, a person or some form of instrument ("an act of recognizing and noting a fact or occurrence often involving measurement with instruments"). 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." A coordinate reference system consists of a set of coordinate system axes that is related to the earth through a datum that defines the size and shape of the earth. Geometries in GML indicate the coordinate reference system in which their measurements have been made. The "parent" geometry element of a geometric complex or geometric aggregate makes this indication for its constituent geometries.

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. [from the v3.0 Introduction]

GML 3 Application Schemas

"Designers of GML Application schemas may extend or restrict the types defined in the GML base schemas to define appropriate types for an application domain. They may also use non-abstract elements and attributes from the GML base schemas as-is in an application schema if no changes are required. GML Application schemas may be constructed by hand using a text editor or specialized XML/Schema editor, in effect using XML/Schema as a conceptual schema language. GML Application schemas may also be constructed as part of a model driven process by automated translation to XML/Schema from conceptual models defined in other conceptual schema languages such as UML..." Note: A UML diagram is used to show the hierarchy of GML objects defined in the XML schemas' GML 3.0 uses an object hierarchy to define the various entities such as features, geometries, and topologies.

New in GML Version 3.0

This version (3.0) of GML addresses the following needs that were not addressed or adequately met by the previous version:

• represent geospatial phenomena in addition to simple 2D linear features, including features with complex, non-linear, 3D geometry, features with 2D topology, features with temporal properties, dynamic features, coverages, and observations
• provide more explicit support for properties of features and other objects whose value is complex
• represent spatial and temporal reference systems, units of measure and standards information
• use reference system, units and standards information in the representation of geospatial phenomena, observations, and values
• represent default styles for feature and coverage visualization
• conform with other standards, including ISO DIS 19107 Geographic Information -- Spatial Schema, ISO DIS 19108 Geographic Information -- Temporal Schema, ISO DIS 19118 Geographic Information -- Encoding, and ISO DIS 19123 Geographic Information -- Coverages

The expansion of GML to meet these needs is reflected in base schemas for GML version 3.0 that are over eight times as large as the base schemas for GML version 2.1.2. However, few applications will use all of the definitions added to GML version 3. Implementers may use a selective subset of the GML version 3 schemas sufficient to support the definitions in their application schemas. Methods for modular use of GML are discussed in clause 7.13; examples are provided in Annex E; a schema subsetting tool is provided in Annex F.

From the Announcement

GML 3 defines a data encoding in XML that allows geographic data and its attributes to be moved between disparate systems with ease. The new release is modular, meaning that users can select out only the parts necessary for use, which simplifies and minimizes the size of implementations. New additions in GML 3 include support for complex geometries, spatial and temporal reference systems, topology, units of measure, metadata, gridded data, and default styles for feature and coverage visualization. GML 3 is almost entirely backwards compatible with GML 2, so that developers and users familiar with GML 2 can begin working immediately with GML 3. Like GML 2, GML 3 will play a key role in both spatial data encoding and transport, and in the description of geographic objects for geospatial Web services.

Kurt Buehler, OGC CTO, explains: "GML 3's modular structure means that those choosing to use GML can literally pick out the schemas or schema components that apply to their work. GML 3.0 also includes a sample packaging tool that creates a tailored schema containing only the required components from the GML core schemas." Buehler also points out that it is OGC's intention that this very broad specification, with more than 1000 new tags covering over 400 pages, remains relatively unchanged, save for minor bug fixes, over the next year or so. This stability will give potential implementers time to learn about and use GML 3 effectively.

Ron Lake of Galdos Systems, who pioneered GML 1.0 and led the subsequent growth of the specification, makes clear, "the new features are designed specifically to address real world problems." For example, new in GML 3 is the ability to store topology (the relationships between features), geometric curves, and time information. The storage of topology is a key requirement for organizations like the Ordnance Survey Great Britain, which uses GML to store its nationwide data layers. Topology is also required for routing applications popular in location-based services. Those working with highway designs and other transportation related data require accurate curve definitions. Time information is essential in tracking applications (like monitoring ambulance locations) and exploring the movement and growth of natural disasters such as floods and fires over hours, days or months.

The GML 3 specification was submitted by OGC member organizations CSIRO Australia, Galdos Systems (Canada), interactive instruments (Germany), BAE SYSTEMS Mission Solutions (U.S.), US Census Bureau, POSC (U.S.), MapInfo Corp. (U.S.), Oracle Corp. (U.S.), NTT Data Corp.(Japan), and Laser-Scan Ltd. [excerpted]

About The Open GIS Consortium

"OGC is an international industry consortium of more than 240 companies, government agencies and universities participating in a consensus process to develop publicly available geoprocessing specifications. Open interfaces and protocols defined by OpenGIS Specifications support interoperable solutions that 'geo-enable' the Web, wireless and location-based services, and mainstream IT, and empower technology developers to make complex spatial information and services accessible and useful with all kinds of applications."

OGC Vision and Mission

The core mission of the Open GIS Consortium "is to deliver spatial interface specifications that are openly available for global use. Its open interface specifications enable content providers, application developers and integrators to focus on delivering more capable products and services to consumers in less time, at less cost, and with more flexibility."

OGC executes this mission consistent with its vision: "A world in which everyone benefits from geographic information and services made available across any network, application, or platform. Approximately 80% of business and government information has some reference to location, but until recently the power of geographic or spatial information and location has been underutilized as a vital resource for improving economic productivity, decision-making, and delivery of services. We are an increasingly distributed and mobile society. Our technologies, services, and information resources must be able to leverage location, (i.e., my geographic position right now) and the spatial information that helps us visualize and analyze situations geographically. Products and services that conform to OGC's open interface specifications enable users to freely exchange and apply spatial information, applications and services across networks, different platforms and products."