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The Geography Markup Language (GML) is the XML grammar defined by the Open Geospatial Consortium (OGC) to express geographical features. GML serves as a modeling language for geographic systems as well as an open interchange format for geographic transactions on the Internet. The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language for creating special-purpose markup languages, capable of describing many different kinds of data. ...
The Open Geospatial Consortium, or OGC, is an international voluntary consensus standards organization. ...
Standards The OGC is an international voluntary consensus standards organization whose members maintain the Geography Markup Language standard. The OGC coordinates with the ISO TC 211 standards organization to maintain consistency between OGC and ISO standards work.
GML is the XML data standard for the GeoWeb infrastructure, enabling Internet-enabled devices to access geographical information, including, for example, merchant locations and traffic conditions.
GML model The original GML model was based on the World Wide Web Consortium's Resource Description Framework (RDF). Subsequently, the OGC introduced XML schemas into GML's structure to help connect the various existing geographic databases, whose relational structure XML schemas more easily define. The resulting XML-schema-based GML retains many features of RDF, including most of the primitive object types in GML 3.0: The World Wide Web Consortium (W3C) is an international consortium where member organizations, a full-time staff, and the public, work together to develop standards for the World Wide Web. ...
Resource Description Framework (RDF) is a family of specifications for a metadata model that is often implemented as an application of XML. The RDF family of specifications is maintained by the World Wide Web Consortium (W3C). ...
XML Schema, published as a W3C Recommendation in May 2001, is one of several XML schema languages. ...
Profile GML profiles are XML schemas that extend Geography Markup Language (GML) in a modular fashion. A commonly used GML profile, geometryBasic0D1D.xsd is the only one required by many applications. These profiles are intended to simplify adoption of GML, to facilitate rapid adoption of the standard. The following profiles, as defined by the GML specification, have been published or proposed for public use: XML Schema, published as a W3C Recommendation in May 2001, is one of several XML schema languages. ...
- A Point Profile for applications with point geometric data but without the need for the full GML grammar
- A GML Simple Features profile supporting vector feature requests and transactions, e.g. with a WFS
- A GML profile for GMJP2 (GML in JPEG 2000)
- A GML profile for RSS
Note that Profiles are distinct from application schemas. Profiles are part of GML namespaces (Open GIS GML) and define restricted subsets of GML. Application schemas are XML vocabularies defined using GML and which live in an application-defined target namespace. Application schemas can be built on specific GML profiles or use the full GML schema set.
GML Simple Features Profile The GML Simple Features Profile is a more complete profile of GML than the above Point Profile and supports a wide range of vector feature objects, including the following: - A reduced geometry model allowing 0d, 1d and 2d linear geometric objects (all based on linear interpolation) and the corresponding aggregate geometries (gml:MultiPoint, gml:MultiCurve, etc).
- A simplified feature model which can only be one level deep (in the general GML model, arbitrary nesting of features and feature properties is not permitted).
- All non-geometric properties must be XML Schema simple types – i.e. cannot contain nested elements.
- Remote property value references (xlink:href) just like in the main GML specification.
Since the profile aims to provide a simple entry point, it does not provide support for the following: - coverages
- topology
- observations
- value objects (for real time sensor data)
- nor support for dynamic features.
Nonetheless it supports a good variety of real world problems.
Subset tool In addition, the GML specification provides a subset tool to generate GML profiles containing a user-specified list of components. The tool consists of a pair of XSLT scripts written by Paul Daisey of the US Census Bureau. The scripts generate a profile that a developer may extend manually or otherwise enhance through schema restriction. Note that as restrictions of the full GML specification, application schemas that a profile can generate must themselves be valid GML application schemas.
The subset tool can generate profiles for many other reasons as well. Listing the elements and attributes to include in the resultant profile schema and running the tool results in a single profile schema file containing only the user-specified items and all of the element, attribute and type declarations on which the specified items depend. Some Profile schemas created in this manner support other specifications including IHO S-57 and GML in JPEG 2000.
Application schema In order to expose an application's geographic data with GML, an organization creates an XML schema specific to the application (the application schema). That schema describes the object types whose data the application must expose. For example, an application for tourism may define object types including monuments, places of interest, museums, road exits, and viewpoints in its application schema. Those object types in turn reference the primitive object types defined in the GML standard.
Note that other markup languages for geography use schema constructs, but GML builds on the existing XML schema model instead of creating a new schema language, as does Google's KML. KML (Keyhole Markup Language) is an XML-based language for managing three-dimensional geospatial data in the program Google Earth (previously called Keyhole before it was bought up by Google). ...
GML Geometries GML encodes the GML geometries, or geometric characteristics, of geographic objects as elements within GML documents. The geometries of those objects may describe, for example, roads, rivers, and bridges.
The key GML geometry object types in GML 1.0 and GML 2.0, are the following:
Note that this geometry model is identical to the geometry model in KML.
Features GML defines both features distinct from geometry objects. A feature is an application object that represents a physical entity, e.g. a building, a river, or a person. A feature may or may not have geometric aspects. A geometry object defines a location or region instead of a physical entity, and hence is different from a feature. The distinction between features and geometry objects in GML contrasts with models used in other geographic information systems (GIS) that make no such distinction. That is, although some other GIS define features and geometry objects interchangeably as items on a map, GML maintains them as separate entity types. A geographic information system (GIS) is a system for creating and managing spatial data and associated attributes. ...
In GML, a feature can have various geometric properties that describe aspects or characteristics of the feature (e.g. the feature's Point or Extent properties). GML also provides the ability for features to share a geometry property with one another by using a remote property reference on the shared geometry property. Remote properties are a general feature of GML borrowed from RDF. An xlink:href attribute on a GML geometry property means that the value of the property is the resource referenced in the link.
For example, a Building feature in a particular GML application schema might have a position given by the primitive GML geometry object type Point. However, the Building is a separate entity from the Point that defines its position. In addition, a feature may have several properties, including both an extent and a position. Any such property may share its geometry object with a properties of other features.
Coordinates Coordinates in GML represent the coordinates of geometry objects. Coordinates can be specified by any of the following GML elements: - <gml:coordinates>
- <gml:pos>
- <gml:posList>
GML has multiple ways to represent coordinates. For example, the <gml:coordinates> element can be used, as follows: <gml:Point gml:id="p21" srsName="urn:ogc:def:crs:EPSG:6.6:4326"> <gml:coordinates>45.67, 88.56</gml:coordinates> </gml:Point> Note that, when expressed as above, the individual coordinates (e.g. 88.56) are not separately accessible through the XML Document Object Model since the content of the <gml:coordinates> element is just a single string. The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language for creating special-purpose markup languages, capable of describing many different kinds of data. ...
Document Object Model (DOM) is a description of how a HTML or XML document is represented in an object-oriented fashion. ...
To make GML coordinates accessible through the XML DOM, GML 3.0 introduced the <gml:pos> and <gml:posList> elements. (Note that although GML versions 1 and 2 had the <gml:coord> element, it is treated as a defect and is not used.) Using the <gml:pos> element instead of the <gml:coordinates> element, the same point can be represented as follows:
<gml:Point gml:id="p21" srsName="urn:ogc:def:crs:EPSG:6.6:4326"> <gml:pos dimension="2">45.67 88.56</gml:pos> </gml:Point> The coordinates of a <gml:LineString> geometry object can be represented with the <gml:coordinates> element: <gml:LineString gml:id="p21" srsName="urn:ogc:def:crs:EPSG:6.6:4326"> <gml:coordinates>45.67, 88.56 55.56,89.44</gml:coordinates> </gml:LineString > The <gml:posList> element is used to represent a list of coordinate tuples, as required for linear geometries: <gml:LineString gml:id="p21" srsName="urn:ogc:def:crs:EPSG:6.6:4326"> <gml:posList dimension="2">45.67 88.56 55.56 89.44</gml:posList> </gml:LineString > For GML data servers (WFS) and conversion tools that only support GML 1 or GML 2 (i.e. only the <gml:coordinates> element), there is no alternative to <gml:coordinates>. For GML 3 documents and later, however, <gml:pos> and <gml:posList> are preferable to <gml:coordinates>.
For more information on the srsName attribute, see Coordinate Reference System below.
Coordinate Reference System A Coordinate Reference System (CRS) determines the geometry of each geometry element in a GML document. This article needs to be cleaned up to conform to a higher standard of quality. ...
Unlike KML or geoRSS, GML does not assume a single, fixed coordinate system. Instead, each coordinate system must be specified with a Coordinate Reference System. The elements whose coordinates are interpreted with respect to such a CRS include the following: KML (Keyhole Markup Language) is an XML-based language for managing three-dimensional geospatial data in the program Google Earth (previously called Keyhole before it was bought up by Google). ...
- <gml:coordinates>
- <gml:pos>
- <gml:posList>
An srsName attribute attached to a geometry object specifies the object's CRS, as shown in the following example:
<gml:Point gml:id="p1" srsName="#srs36"> <gml:coordinates>100,200</gml:coordinates> </gml:Point> The value of the srsName attribute is a Uniform Resource Identifier (URI). It refers to a definition of the Coordinate Reference System that is used to interpret the coordinates in the geometry. The CRS definition may be in a document (i.e. a flat file) or in an online web service. A Uniform Resource Identifier (URI), is an Internet protocol element consisting of a short string of characters that conform to a certain syntax. ...
The srsName URI may also be a Uniform Resource Name (URN) for referencing a common CRS definition. The OGC has developed a URN structure and a set specific URNs to encode some common Coordinate Reference Systems. A URN resolver resolves those URNs to GML CRS definitions. A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the urn scheme, and does not connote availability of the identified resource. ...
Examples Polygons, Points, and LineString objects are encoded in GML 1.0 and 2.0 as follows: Look up Polygon in Wiktionary, the free dictionary For other use please see Polygon (disambiguation) A polygon (literally many angle, see Wiktionary for the etymology) is a closed planar path composed of a finite number of sequential line segments. ...
Point, the Great Shogun of the Red Empire, is the leader of the infamous Munsterfans. ...
<gml:Polygon> <gml:outerBoundaryIs> <gml:LinearRing> <gml:coordinates>0,0 100,0 100,100 0,100 0,0</gml:coordinates> </gml:LinearRing> </gml:outerBoundaryIs> </gml:Polygon> <gml:Point> <gml:coordinates>100,200</gml:coordinates> </gml:Point> <gml:LineString> <gml:coordinates>100,200 150,300</gml:coordinates> </gml:LineString> Note that LineString objects, along with LinearRing objects, assume linear interoplation between the specified points.
Features using geometries The following GML example illustrates the distinction between features and geometry objects. The Building feature has several geometry objects, sharing one of them (the Point with identifier p21) with the SurveyMonument feature: <abc:Building gml:id="SearsTower"> <gml:name>Sears Tower</gml:name> <abc:height>52</abc:height> <abc:position> <gml:Point> <gml:coordinates>100,200</gml:coordinates> </gml:Point> </abc:position> <app:extent> <gml:Polygon> <gml:exterior> <gml:LinearRing> <gml:coordinates>100,200</gml:coordinates> </gml:LinearRing> </gml:exterior> </gml:Polygon> </app:extent> </abc:Building> <abc:Building gml:id="SearsTower"> <abc:position xlink:type="Simple" xlink:href="#p21"/> </abc:Building> <abc:SurveyMonument gml:id="g234"> <abc:position> <gml:Point gml:id="p21"> <gml:coordinates>100,200</gml:coordinates> </gml:Point> </abc:position> </abc:SurveyMonument> Note that the reference is to the shared Point and not to the SurveyMonument, since any feature object can have more than one geometry object property.
Point Profile The GML Point Profile contains a single GML geometry, namely a <gml:Point> object type. Any XML Schema can use the Point Profile by importing it and referencing the subject <gml:Point> instance: <PhotoCollection xmlns="http://www.myphotos.org" xmlns:gml="http://www.opengis.net/gml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.myphotos.org MyGoodPhotos.xsd"> <items> <Item> <name>Lynn Valley</name> <description>A shot of the falls from the suspension bridge</description> <where>North Vancouver</where> <position> <gml:Point srsDimension="2" srsName="urn:ogc:def:crs:EPSG:6.6:4326"> <gml:pos>49.40 -123.26</gml:pos> </gml:Point> </position> </Item> </items> </PhotoCollection> Note that when using the Point Profile, the only geometry object is the '<gml:Point>' object. The rest of the geography is defined by the photo-collection schema.
References - GML 3.1 specification
- Digital Earth: GeoWeb
- GeoRSS - Geographically Encoded Objects for RSS Feeds
- Recommended XMLGML encoding of common CRS definitions, Open Geospatial Consortium
- Demonstration of a Coordinate Reference System Registry, Open Geospatial Consortium
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