Contents

• Summary
• Bibliographic Information
• From the Announcement
• About GML
• About the Open Geospatial Consortium (OGC)
• Principal References

The Open Geospatial Consortium Inc. has issued an invitation for public review of a GML Simple Features Profile specification. OGC's Geography Markup Language (GML), now being prepared for publication as ISO/IEC 19136 Geographic Information — Geography Markup Language by ISO/TC 211/WG 4 (Geographic Information/Geomatics). OGC Specification Profiles are subsets of existing OpenGIS Specifications. GML is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information.

This GML profile is a product of OGC's Interoperability Program: "a global, collaborative, hands-on engineering and testing program designed to deliver prototype technologies and proven candidate specifications into the OGC's Specification Development Program. In OGC Interoperability Initiatives, international teams of technology providers work together to solve specific geo-processing interoperability problems posed by Initiative."

The OGC Interoperability Program "uses member approved processes for organizing and facilitating several types of Interoperability Initiatives, including: (1) Test beds, which are fast-paced, multi-vendor collaborative efforts to define, design, develop, and test candidate interface and encoding specifications; (2) Pilot Projects, which apply and test OpenGIS specifications in real world applications using standards based commercial off-the-shelf (SCOTS) products that implement OpenGIS Specifications; (3) Interoperability Support Services, designed to help organizations with open, standards based architecture; (4) Interoperability Experiments, designed as brief, low-overhead, formally structured and approved initiatives led and executed by OGC members to achieve specific technical objectives that further the OGC Technical Baseline."

The new GML Simple Features Profile defines a restricted but useful subset of XML-Schema and GML. The Geography Markup Language (GML) full specification defines "an XML grammar for the encoding of geographic information including geographic features, coverages, observations, topology, geometry, coordinate reference systems, units of measure, time, and value objects. The GML Simple Feature Profile candidate specification defines a set of schema encoding rules that allow simple features, such as points, lines, and polygons, to be described using GML application schemas."

The Profile's restricted subset GML is designed to "lower the implementation bar of time and resources required for an organization to commit for developing software that supports GML. It is hoped that by lowering the effort required to manipulate XML encoded feature data, organizations will be encouraged to invest more time and effort to take greater advantage of GML's rich functionality."

The GML Simple Features Profile defines a "rigid coding patterns for the use of a subset of XML Schema constructs (XML Schema profile) and also rigid coding patterns for the use of a subset of GML constructs (GML profile). It prescribes the encoding of GML application schemas in sufficient detail that client applications do not need to deal with the entire scope of XML-Schema and GML but only need to accommodate a restricted subset of both specifications in order to be able to interpret schema documents generated by servers offering data encoded in GML. Easier interpretation of GML application schemas will enhance interoperability between clients and servers and make the task of implementing client applications easier."

According to the specification Introduction, in both GML and previous Simple Features (SF) specifications for OGC, such as Simple Features for SQL, "features are considered to be objects which can have geometry and other properties. The SF specifications are more restrictive than GML, however, in that geometry is limited to points, lines, and polygons (and collections of these), with linear interpolation between vertices of lines, and planar (flat) surfaces within polygons. The new GML Simple Features (GML-SF) profile has a similar understanding of the structure and geometry of features as SF-SQL. However, GML-SF goes beyond SF-SQL in some important ways: GML-SF supports three-dimensional coordinates (location and elevation) on feature geometry, where SF-SQL just supports two dimensions (location). GML-SF also goes beyond SF-SQL by supporting metadata, a means of referencing local or remote resources which could be used for primary/foreign key references, and dynamic codelists. These extensions have been incorporated into GML-SF based on their usefulness across a large base of applications."

Normative Annex D of the Profile presents the XML schema document, called gmlSimpleFeaturesProfile.xsd, which "contains only those element declarations from GML that are required to validate a GML application schema that complies with this profile. GML application schemas can ensure compliance with this profile by importing this schema document rather schema documents from the standard GML 3.1.1 distribution."

It is expected that the GML Simple Features Profile will be made available for use on a royalty-free basis, as with other OGC specifications. In June 2003 the Consortium announced the adoption of a revised IPR policy which requires all contributors to license technology on a royalty-free basis. OGC's published definition of an open standard asserts that an open standard "has free rights of distribution: an 'open' license shall not restrict any party from selling or giving away the specification as part of a software distribution, and the 'open' license shall not require a royalty or other fee."

Bibliographic Information

• GML Simple Features Profile. Edited by Panagiotis (Peter) A. Vretanos. Co-authored by Aleksander Milanovic. Open Geospatial Consortium Inc. Date: 05-July-2005 Reference number of this OGC document: 'OGC 05-033r9'. Version: 0.0.19. Category: OGC Implementation Specification Profile. 61 pages. Draft distributed for review and comment. Namespace: "http://www.opengis.net/gml/profile/sfgml/1.0". [cache]

  Contributors: Craig Bruce (CubeWerx Inc), Simon Cox (CSIRO), Edric Keighan (CubeWerx Inc), Paul Daisey (Census), David Arctur (ESRI), David Burggraf (Galdos Inc), Martin Kyle (Galdos Inc), Ron Lake (Galdos Inc), Martin Daly (Cadcorp Ltd), David Danko (ESRI), Sylvain Lemieux (PCI Ltd), Clemens Portele (Interactive Instrument GmbH), Jerome Sonnet (IONIC), and Arliss Whiteside (BAE SYSTEMS NSS).

• 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, the GML spec version 3.1.0 was 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) 'temporalReferenceps. 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."

From the Announcement

OGC issued an announcement for the profile on 2005-07-05 ("OGC Invites Comment on GML Simple Features Specification Profile"):

"The Open Geospatial Consortium Inc. (OGC) invites public comment on a candidate specification that will soon be presented for approval by OGC members as an OpenGIS Implementation Specification Profile.

The OGC candidate specification, "GML simple features profile" is available for downloading [and] comments can be submitted to gmlsf-rfc@opengeospatial.org for a thirty day period ending August 4th, 2005. Comments received will be consolidated and reviewed by OGC members for incorporation into the proposed specification.

The Geography Markup Language (GML) is an XML grammar for the encoding of geographic information including geographic features, coverages, observations, topology, geometry, coordinate reference systems, units of measure, time, and value objects. The GML Simple Feature Profile candidate specification defines a set of schema encoding rules that allow simple features, such as points, lines, and polygons, to be described using GML application schemas.

This specification defines: rigid coding patterns for the use of a subset of XML Schema constructs (XML Schema profile) and also rigid coding patterns for the use of a subset of GML constructs (GML profile). It prescribes the encoding of GML application schemas in sufficient detail that client applications do not need to deal with the entire scope of XML-Schema and GML but only need to accommodate a restricted subset of both specifications in order to be able to interpret schema documents generated by servers offering data encoded in GML. Easier interpretation of GML application schemas will enhance interoperability between clients and servers and make the task of implementing client applications easier.

The following organizations submitted the draft specification for consideration: Cadcorp Ltd. (UK), CubeWerx Inc. (Canada), ESRI (USA), Galdos (Canada), Interactive Instruments (Germany) and PCI Geomatics (Canada). BAE Systems (USA), CSIRO (Australia), and the US Census Bureau also contributed to the effort..."

About GML

The Geography Markup Language (GML) is the most widely supported open specification for representation of geographic (spatial and location) information. It defines XML encoding for the transport and storage of geographic information, including both the geometry and properties of geographic features. In keeping with OGC's IPR policies for Open GIS standards, GML is freely available for use on royalty-free terms. The GML Specifiction Version 3.1.0 has been edited by Simon Cox (CSIRO), Paul Daisey (U.S. Census Bureau), Ron Lake (Galdos Systems), Clemens Portele (Interactive Instruments), and Arliss Whiteside (BAE Systems). The 601-page prose document is supported by thirty-three (33) separate XML Schema files. The specification is based upon a large number of other W3C, IETF, ISO, and OpenGIS standards; it normatively references the XML Linking Language (XLink) Version 1.0 and The Schematron Assertion Language 1.5. GML Version 3.1.0 adds new geometries, is more compliant with the ISO/TC 211 family of specifications, and contains some items for increased efficiency and simplicity. It "maintains backward compatibility for GML version 3.0.0 and 2.1.2 instance documents by preserving, but deprecating, some schema components that have been replaced by different constructs in the current version."

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]

About the Open Geospatial Consortium (OGC)

"OGC develops, releases and promotes open standards for spatial processing through its Specification Program, Interoperability Program, and Outreach and Community Adoption Program. In the OGC Specification Program, the Technical Committee and Planning Committee work in a formal consensus process to arrive at approved (adopted) OpenGIS/Open Geospatial specifications. A large number of OGC specificatios has been approved so far, and numerous software products implement these specifications. The OGC Interoperability Program is a series of hands-on engineering initiatives to accelerate the development and acceptance of OGC Specifications. OGC and its members offer resources to help technology developers and users take advantage of OGC's open standards. Technical documents, training materials, test suites, reference implementations and other interoperability resources developed in OGC's Interoperability Initiatives are available on the OGCNetwork. In addition, OGC and its members support publications, workshops, seminars and conferences to help technology developers, integrators and procurement managers introduce OGC plug and play capabilities into their architectures..."

The core mission of the OGC is to develop spatial interface specifications that are openly available and royalty free. Products and services that conform to OGC interface specifications enable users to freely exchange and process spatial information across networks, computing platforms, and products. Interoperability in such an environment is facilitated by the use of a system of persistent identifiers that are global in scope. [URN document]

Principal References

• GML Simple Features Profile:
  • Announcement 2005-07-05: "OGC Invites Comment on GML Simple Features Specification Profile."
  • GML Simple Features Profile. 05-July-2005. OGC Implementation Specification Profile. [cache]
  • XML Schema for GML Simple Features Profile. From Normative Annex D of the 05-July-2005 specification.
  • Feedback: Send comments by email to gmlsf-rfc@opengeospatial.org.
  • GMLsf-rfc list archives. GMLsf-rfc is the repository for RFC comments to the GML simple features profile. The comment period closes on 4-August-2005.
• OCG GML specifications [select]:
  • Geography Markup Language. GML v3.1 as an OGC Recommendation Paper. See the PDF and specification overview.
  • Geography Markup Language. Version 3.0. 2003-01-29. "The Geography Markup Language (GML) is an XML encoding for the transport and storage of geographic information, including both the geometry and properties of geographic features."
  • GML XML Schemas
  • Recommended XML/GML 3.1.1 Encoding of Common CRS Definitions (XML for CRS). Version 1.0. January 2005. "This document recommends standard XML encodings of data defining some commonly-used coordinate reference systems, including geographic, projected, and vertical CRSs. These recommended encodings are based on GML 3.1.1. These common CRS definitions will often be referenced in data transferred between client and server software that implements various standardised interfaces. This specified definition data encoding is expected to be used by multiple OGC Implementation Specifications..."
  • Recommended XML/GML 3.1.1 Encoding of Image CRS Definitions (ImageCRS). April 2005. "This document recommends standard XML encodings of data defining monoscopic image coordinate reference systems. The scope of this encoding now includes unrectified and georectified images. The recommended CRSs for georectified images are recommended for multiple georectified images that are ready to be mosaicked together. These recommended encodings are based on GML 3.1.1 and use XML Schemas. These image CRS definitions will often be referenced in data transferred between client and server software that implements various standardised interfaces..."
  • Geographic Information — Geography Markup Language (GML). Published as ISO/TC 211 N 1576 (replacing N 1432) as ISO/CD 19136 Geographic Information — Geography Markup Language. OGC Reference (OpenGIS Recommendation Paper, Version 3.1.0): 03-105r1. Date: 2004-02-11. 580 pages. Edited (as OGC 03-105r1) by Simon Cox, Paul Daisey, Ron Lake, Clemens Portele, and Arliss Whiteside. ISO 19136 is a work product of ISO/TC 211 "Geographic Information/Geomatics". A working draft (WD 19136, Geographic information — Geography Markup Language) was produced in April 2003; a Committee Draft (CD 19136 Geographic information — Geography Markup Language) was issued in February 2004; the text for a final CD was completed in April 2005. [source PDF]
• OGC general references:
  • Open Geospatial Consortium (OGC) web site
  • OGC Implementation Specifications
  • OGC Adopted Baseline Specifications
  • OGC Recommendation Papers
  • OGC Interoperability Program
  • OGC Interoperablity Experiment Summary
  • OGC Interoperability Experiment Policies and Procedures. OGC Project Document Number: 04-029.
• OGC and Open Standards:
  • Open Standards and Open Geospatial Consortium (OGC)
  • "The Importance of Going 'Open': An Open Geospatial Consortium (OGC) White Paper." July 5, 2005.
  • "OpenGIS Consortium Adopts Revised Royalty-Free Intellectual Property Rights Policy."
• Earlier OGC news:
  • "OGC Launches Initiative to Support GML Metadata Encoding in JPEG 2000 Image Files." News story 2005-03-11.
  • "OGC Interoperability Experiment for LandXML and Geography Markup Language (GML)." News story 2004-05-12.
  • "Geography Markup Language (GML) Version 3.1 Public Release from Open GIS Consortium." News story 2004-03-26.
  • "Open GIS Consortium Issues RFQ for OGC Web Services Phase 2 Interoperability Initiative."
  • "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.