Bibliographic Information

• OpenGIS Geography Markup Language (GML) Implementation Specification. Edited by Simon Cox, Paul Daisey, Ron Lake, Clemens Portele, and Arliss Whiteside. Version 3.1.0. Date: 2004-02-07. Category: OpenGIS Recommendation Paper. Copyright (c) 2004 Open GIS Consortium, Inc. and ISO. Reference: OGC 03-105r1. 601 pages. Balloted as OGC Document 03-105r1 and associated Schemas 03-107r1. Submitting organizations: [names omitted]. "This document is not an OGC Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an OGC Standard." Note: "The document is maturing, may undergo some more change before formal adoption, but the OGC membership wants to expose it for public review and comment."

  Note on Version 3.1.0 document status as an OpenGIS Recommendation Paper: Whereas version GML version 3.0.0 was designated an OpenGIS Implementation Specification, this version 3.1.0 is categorized as an OpenGIS Recommendation Paper, while bearing the [OGC] title OpenGIS Geography Markup Language (GML) Implementation Specification. It is projected that the specification will become ISO 19136 as well as being approved as an OGC Adopted Document. OGC project staff has clarified: (1) that the "Recommendation" category status is dictated by the requirements of keeping the OGC and ISO editorial processes in sync so that the final/authoritative version becomes an IS and an OGC Adopted Document; (2) that the GML specification which becomes ISO 19136 will also be freely and publicly available under OGC's royalty-free IPR policy. "OpenGIS Recommendation Papers are developed via OG's Technology Development Process and reflect an official position of OGC. Recommendation papers are not Adopted Specifications, but set direction for specification development. If a Recommendation Paper is focused on engineering specifications, then one may assume that the content is still under rapid change and that the reader should recognize the volatile nature of the specification when building implementations."

  OGC GML 3.1.0 has dual referencing as ISO/CD 19136, represented in bibliographic data from N005r3. Document title: Geography Markup Language (GML). ISO CD title: Geographic Information — Geography Markup Language (GML). Date: 2004-02-07. Produced by editorial teams of ISO/TC 211/WG 4/PT 19136 and OGC GML RWG. Document reference: ISO/TC 211/WG 4/PT 19136 N 005r3. ISO/CD 19136. Filename: GML-3.1.0.doc. Committee Draft for review by ISO/TC 211 and the OGC TC. International Standard ISO 19136 was prepared by the Technical Committee of the Open GIS Consortium and Technical Committee ISO/TC 211, Geographic Information/Geomatics. "This edition of GML supersedes version 3.0.0 (OGC document 02-023r4). GML is a living standard, originally developed within the Open GIS Consortium (OGC). This ISO/TC 211 Committee Draft is targeted to become GML version 3.1 within OGC. As GML is already used in several commercial data and software products worldwide, compatibility of this version with previous versions of GML is important. OGC has decided that the best way to introduce changes to existing structures within GML is to deprecate elements, types, attributes and groups that shall not be used in new applications. These deprecated structures will then be removed from GML in a future version — depending on the adoption of these changes by the market. Member bodies and liaisons are invited to address in their comments the issue whether this International Standard should contain the deprecated structures, too, or if they should be removed from the standard. In the latter case, ISO 19136 would be a profile of the GML Implementation Specification according to the rules of this standard."

  GML Version 3.1.0 XML Schemas. The GML v3.1.0 distribution package contains 33 XML Schemas (W3C XML Schema, xsd files) slightly revised/updated relative to the "gml/3.0.1/base" schemas released in June 2003. New XML Schema files in 3.1.0 include: (1) 'temporalReferenceSystems.xsd' — "The Temporal Reference Systems schema for GML 3.1 provides constructs for handling various styles of temporal reference system. This schema reflects a partial implementation of the model described in ISO 19108:2002." (2) 'temporalTopology.xsd' — "Temporal topology schema for ISO19136 provides constructs for handling topological complexes and temporal feature relationships. Temporal geometric characteristics of features are represented as instants and periods. While, temporal context of features that does not relate to the position of time is described as connectivity relationships among instants and periods. These relationships are called temporal topology as they do not change in time, as long as the direction of time does not change. It is used effectively in the case of describing a family tree expressing evolution of species, an ecological cycle, a lineage of lands or buildings, or a history of separation and merger of administrative boundaries. This schema reflects a partial yet consistent implementation of the model described in ISO 19108:2002."

  Normative references: specifications deemed indispensable for the application of the GML 3.0.1 specification

  • IETF RFC 2396, Uniform Resource Identifiers (URI): Generic Syntax. (August 1998)
  • IETF RFC 2732, Format for Literal IPv6 Addresses in URLs. (December 1999)
  • W3C XLink, XML Linking Language (XLink) Version 1.0. W3C Recommendation (27 June 2001)
  • W3C XMLName, Namespaces in XML. W3C Recommendation (14 January 1999)
  • W3C XMLSchema-1, XML Schema Part 1: Structures. W3C Recommendation (2 May 2001)
  • W3C XMLSchema-2, XML Schema Part 2: Datatypes. W3C Recommendation (2 May 2001)
  • W3C Xpointer, XML Pointer Language (XPointer) Version 1.0. W3C Working Draft (16 August 2002)
  • W3C XML Base, XML Base, W3C Recommendation (27 June 2001)
  • W3C XML, Extensible Markup Language (XML) 1.0 (Second Edition), W3C Recommendation 6 October 2000
  • W3C SVG, Scalable Vector Graphics (SVG) 1.0 Specification. W3C Recommendation (04 September 2001)
  • W3C SMIL, Synchronized Multimedia Integration Language (SMIL 2.0). W3C Recommendation (07 August 2001)
  • The Schematron Assertion Language 1.5. Rick Jelliffe 2002-10-01
  • ISO 8601:2000, Data elements and interchange formats — Information interchange Representation of dates and times
  • ISO/TS 19103:, Geographic Information — Conceptual Schema Language
  • ISO 19105:2000, Geographic information — Conformance and testing
  • ISO 19107:2003, Geographic Information — Spatial Schema
  • ISO 19108:2002, Geographic Information — Temporal Schema
  • ISO 19109, Geographic Information — Rules for Application Schemas
  • ISO 19115:2003, Geographic Information — Metadata
  • ISO 19117, Geographic Information — Portrayal
  • ISO 19118, Geographic Information — Encoding
  • ISO 19123, Geographic Information — Coverages
  • ISO/TS 19139: Geographic Information — Metadata — Implementation Specification
  • OpenGIS Abstract Specification Topic 0, Overview, OGC document 99-100r1
  • OpenGIS Abstract Specification Topic 1, Feature Geometry, OGC document 01-101
  • OpenGIS Abstract Specification Topic 2, Spatial referencing by coordinates, OGC document 03-071r1
  • OpenGIS Abstract Specification Topic 5, The OpenGIS Feature, OGC document 99-105r2
  • OpenGIS Abstract Specification Topic 8, Relations between Features, OGC document 99-108r2
  • OpenGIS Abstract Specification Topic 10, Feature Collections, OGC document 99-110

• OpenGIS Geography Markup Language (GML) Implementation Specification. Previous adopted specification. Edited by Simon Cox, Paul Daisey, Ron Lake, Clemens Portele, and Arliss Whiteside. Version 3.00. Date: 2003-01-29. Category: OpenGIS Implementation Specification. Copyright (c) 2003 Open GIS Consortium, Inc. Reference: OGC 02-023r4. 548 pages. Submitting organizations: CSIRO Australia, Galdos Systems, Interactive Instruments, BAE Systems, US Census, POSC, MapInfo, Oracle Corp, NTT Data, and Laser-Scan Ltd. [cache]

Introduction to Geography Markup Language (GML)

"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 OpenGIS Abstract Specification and the ISO 19100 series.

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 version 3.0.1 specification Introduction]

Geography Markup Language (GML) Specification Scope

"The Geography Markup Language (GML) is an XML encoding in compliance with ISO 19118 for the transport and storage of geographic information modelled according to the conceptual modelling framework used in the ISO 19100 series and 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 Scope Statement]

GML 3.1.0 Use of XLink and Schematron

Specification Clause 7.2 describes the Xlink schema: "this schema is an OGC implementation of the XLink specification using XML Schema; may be replaced in some future release by an equivalent schema from the W3C.

Xlink components are used in GML to implement associations between objects by reference. GML property elements (see clause 7.5.3) may carry xlink attributes, which support the encoding of an association relationship by reference, the name of the property element denoting the target role in the association. The most important xlink component is href: Identifier of the resource which is the target of the association, given as a URI The appearance of an xlink:href on a GML property indicates that the value of the property is to be found by traversing the link, that is the value is pointed to by the value of the xlink:href attribute. Following the terminology of Xlink, GML properties with xlink:href attributes are sometimes referred to as remote properties.

The other XLink components are used to indicate additional semantics of the relationship. The most useful of these are (1) role: description of the nature of the target resource, given as a URI; (2) arcrole: description of the role or purpose of the target resource in relation to the present resource, given as a URI; (3) title: description of the association or the target resource, given as text. For complete definitions of these and other xlink components, including their use in extended xlink association maps, refer to the xlink specification. In the GML core schemas (those defined in Clauses 7 to 20), simple xlinks are used exclusively to denote object, feature or geometry associations and to denote remotely referenced property values..." [from Clause 7.2]

Schematron (XML Structure Validation Language Using Patterns in Trees) is referenced normatively within GML version 3.1.0 [The Schematron Assertion Language 1.5]. The namespace is declared within many of the XML schema files (xmlns:sch="http://www.ascc.net/xml/schematron"). The 'gmlBase.xsd' schema file supplies the GML base schema for GML 3.1, defining components to support the GML encoding model. It describes Schematron validation: 'The abstract Schematron rules can be used by any schema that includes gmlBase'... In many cases it is desirable to impose the constraint prohibiting the occurence of both reference and value in the same instance, as that would be ambiguous. This is accomplished by adding a directive in the annotation element of the element declaration. This directive can be in the form of normative prose, or can use a Schematron pattern to automatically constrain co-occurrence; see the declaration for '_strictAssociation'. If co-occurence is not prohibited, then both a link and content may be present. If this occurs in an instance, then the rule for interpretation is that the instance found by traversing the href provides the normative value of the property, and should be used when possible. The value(s) included as content may be used if the remote instance cannot be resolved. This may be considered to be a 'cached' version of the value(s)... The gml:_strictAssociation constraint may be described precisely using an auxiliary constraint language Schematron. The abstract, global elements _association and _strictAssociation both use AssociationType, but the following schema fragments shows how an element declaration may include a Schematron constraint to limit the property to act in either by-value or by-reference mode, but not both. In the 'valueObjects.xsd' file a "Value Array is used for homogeneous arrays of primitive and aggregate values. The member values may be scalars, composites, arrays or lists. ValueArray has the same content model as CompositeValue, but the member values must be homogeneous. The element declaration contains a Schematron constraint which expresses this restriction precisely..."

Related News: OGC Web Services Initiative Phase 2 (OWS-2)

Open GIS Consortium Inc. (OGC) on March 19, 2004 announced the "successful March 8-12, 2004 kickoff of a major new Interoperability Initiative to develop and enhance OGC Web Services (OWS) standards that enable easy discovery, access and use of geographic data and geoprocessing services.

Building on work from previous OGC initiatives and technical working groups, OWS-2 participants are now working collaboratively to extend and "ruggedize" existing and draft OpenGIS standards. The goal is a robust and complete interoperability framework for implementation within a multi-vendor enterprise and to achieve interoperability between enterprises for geoprocessing solutions in government and business. The initiative's sponsors — BAE Systems, Collaxa, The Federal Geographic Data Committee (FGDC), Lockheed Martin, National Atmospheric and Space Administration (NASA), Questerra, Spot Image (France), Sun Microsystems, US Geological Survey (USGS), and other organizations — provided development requirements.

Requirements categories include:

• Common Architecture: Enabling OpenGIS Web Services utilizing W3C's WSDL and SOAP standards.

• Conformance & Interoperability Testing & Evaluation: Developing compliance tests for and improving the OpenGIS Specifications for Web Map Service (WMS), Web Feature Service (WFS), Web Coverage Service (WCS), and Catalog Services — Web (CS-W). A Reference Implementation for Web Coverage Server and for Catalog Services — Web will be created under this initiative.

• Image Handling and Decision Support Tools: Establishing a new benchmark for creating value-added products for geospatially enabled decisions. User-defined workflow is enabled by web service access to data and chaining of image processing services.

• Information Interoperability: Advancing information (semantic) interoperability of geospatial data sets using GML and advancing connections with Web Feature Server and GML 3. GML application schemas for civil and defense data products will be created using GML 3. Standards-based Commercial-of-the-shelf clients to OGC Web Services will be extended to meet the requirements of the government communities represented by the agencies sponsoring OWS-2.

• Location Based Services. New services for Navigation and Location Refinement will be added to OGC's existing Open Location Services Standard.

Technology provider participants in the initiative include: Autodesk, Collaxa, con terra, ESRI, Galdos Systems, George Mason University, interactive instruments, Intergraph, IONIC Software, Laser Scan, lat/lon, MapInfo, NASA, NAVTEQ, Northrop Grumman Information Technology, ObjectFX, PCI Geomatics, Polexis, Raytheon, Safe Software, SPOT Image, Tele Atlas, The Open Group, and University of Alabama, Huntsville.

The final demonstration of OWS-2 capabilities will take place in August 2004. The initiative manager is Chuck Heazel from Lockheed-Martin. For more information on the OWS-2 Initiative, please contact Kurt Buehler, +1 (812) 320-1423, email kbuehler@opengis.org.

Full text of announcements:

• "Major OGC Web Services Initiative Begins."
• "Intergraph Continues Interoperability Advancement through Participation in Interop Initiative. Development Begins for OGC Web Services Phase 2 (OWS-2) Initiative Aimed to Extend Enterprise Interoperability."

Completed OGC Web Services Initiatives

GML: The Book

Geography Mark-Up Language: Foundation for the Geo-Web. By Ron Lake, David Burggraf, Milan Trninic, and Laurie Rae. Wiley & Sons. ISBN: 0-470-87153-9. 432 pages. Available April 2004.

"The development of the Internet has changed the environment for Geographical Information Systems (GIS), with the emphasis shifting from analysis to the sharing of data and information over the Internet thus making GIS more mobile and powerful. The Geography Mark-Up Language (GML) was developed as the standard language and is emerging as the foundation for Internet GIS. Geography Mark-Up Language: Foundation for the Geo-Web provides a broad coverage of the use of GML in different application areas, along with the technical means for building these applications.

Starting from the basic concepts, this book works through all the important topics in both GML 2.0 and GML 3.0, with illustrations and worked examples to demonstrate its use. Organized into two sections, Volume I introduces readers to the new world of GML, and explains how it can be used across a broad range of GIS projects. It deals with the basic concepts of XML and GML, and enables readers to make decisions on the utility of GML in their projects and software acquisitions. Volume II is intended for the technical reader and answers questions on the meaning and structure of GML schema components, the development of GML application schemas, and the use of GML in connection with web services, legacy GIS and relational databases.

Contains worked examples: (1) Covers all aspects of GML 3.0 from geometry and topology to units of measure, default styling and coverages; (2) Explains the Geo-Web and its impact on vertical applications; (3) Authored by leading figures in GML development.

This book is a must have for GIS vendors, system integrators and data providers; local/state/provincial and national government agencies; utilities and telecommunication companies; location-based services companies; data distributors; software developers and technical managers. It would make an excellent reference for mid and upper-level undergraduate students and Masters students taking technical GIS modules as part of a GIS or Technical Geography programmes.

GML Conferences

The GeoTec Event 2004 and GML Days 2004 are two of several upcoming OGC/GML events.

• The GeoTec Event: Pathways to Integration. March 28 - 31, 2004. Metro Toronto Convention Centre, Toronto, Ontario, Canada. "Members of the Open GIS Consortium (OGC) will demonstrate how that can happen using OpenGIS Interface Specifications at the GeoTec Event 2004 in Toronto, Ontario, Canada March 28-31, 2004. OGC members will demonstrate how their products take advantage of OGC interfaces to optimize discovery, access, integration and application of geospatial information and applications on servers accessible via the Internet. Vendors will illustrate a range of OpenGIS Specifications, including Web Map Service (WMS), Web Feature Service (WFS), Coordinate Transformation, Grid Coverages, Simple Features, Catalog and Geography Markup Language (GML) Specifications. Cadcorp, Inc., Galdos Systems, GeoConnections, Intergraph Mapping and Geospatial Solutions, MapInfo, Safe Software and other vendors will illustrate the ability of users to share data and applications across distributed networks, varying processing platforms and vendor brands. Conference attendees are encouraged to visit each of the participating vendors for a chance to win software prizes including copies of Cadcorp SIS Map Manager V6, and MapInfo Professional..."

• GML And Geo-Spatial Web Services Conference 2004. July 25 - 29, 2004. Vancouver, British Columbia, Canada. "GML Days 2004 is the third annual conference on the OGC Geography Markup Language (GML) and Web Services for GIS. GML is rapidly emerging as the world standard for the XML encoding of geographic information and is the foundation for the Geo-Web. GML is being applied to a wide range of geographic applications including homeland security & critical infrastructure protection, integrated land and resource management, location-based services, telematics and intelligent transportation systems, and oceanography. If you are interested in GIS data interoperability or the application of web service technology for GIS this conference is not to be missed. At GML Days 2004 you can learn about new GML and Web Service Technologies, advances in OGC specifications, and see new products and services in action."

Principal References

• OpenGIS Geography Markup Language (GML) Implementation Specification. Version 3.1.0. Date: 2004-02-07. Extracted 2004-03-26 from the .ZIP archive. Unofficial display/cache copy.
• File listing for the GML version 3.1.0 distribution
• GML version 3.1.0 ZIP downloadable as portal artifact 4700
• OpenGIS Recommendation Papers. The GML version 3.1.0 official/canonical distribution is available [2004-03-26] from this location.
• Geography Markup Language (GML) Working Group (GML WG). Ron Lake (Galdos Systems Inc.) is the WG Lead.
• Geography Markup Language (GML) Revision Working Group. Simon Cox (CSIRO Exploration & Mining) is the WG Lead. The Geography Markup Language (GML) Revision Working Group maintains the OpenGIS GML Specification.
• XML schemas for GML version 3.0.1 [Check for currency]
• "Major OGC Web Services Initiative Begins."
• "OGC Members Bring Interoperability to GeoTec Event."
• OGC References:
  • Open GIS Consortium, Inc. (OGC) web site
  • OpenGIS FAQ document
  • OpenGIS Implementation Specifications
  • OpenGIS Abstract Specifications
  • OGC Adopted Document Baseline
  • OGC Reference Model
  • OGC Interoperability Program (IP)
  • XML Schemas
  • OGC IPR Policy. See the news story on OGC's Royalty-Free Intellectual Property Rights Policy.
  • Products Compliant to or Implementing OGC Specs or Interfaces. See also the testing program.
  • OGC Network. Development Resources.
  • OGC Cookbooks. Including Recipes, Implementations, Applications, and Implementor Experiences.
  • Open GIS Consortium mailing lists
  • See also: GML4J. "GML4J is a Java API for facilitating work with the Geography Markup Language (http://www.gmlcentral.com). GML is an XML-based framework for encoding geography information adopted as a recommendation paper by OGC.
  • See also: GML Central
• Other news from OGC:
  • "OGC Releases GML Simple Features Profile Specification for Review." News story 2005-07-07.
  • "OGC Interoperability Experiment for LandXML and Geography Markup Language (GML)." News story 2004-05-12.
  • "Open GIS Consortium Issues RFQ for OGC Web Services Phase 2 Interoperability Initiative."
  • "OpenGIS Consortium Adopts Revised Royalty-Free Intellectual Property Rights Policy."
  • "OpenGIS Consortium Publishes Web Map Server Cookbook."
  • "OGC Releases OpenGIS Location Services (OpenLS) Implementation Specification."
  • "OGC Announces OpenGIS Geography Markup Language Implementation Specification (GML 3)."
  • OGC Working Group Issues Draft Specification for Sensor Model Language (SensorML)."
• "Geography Markup Language (GML)" - Main reference page.