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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. 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 can also be included in version 1.0 of the United States National Information Exchange Model. 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: Profile GML profiles are logical restrictions to GML, and may be expressed by a document, an XML schema or both. 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: 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: Since the profile aims to provide a simple entry point, it does not provide support for the following: 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, a community or 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. A list of known publicly available GML Application Schemas is being assembled. Some other markup languages for geography use schema constructs, but GML builds on the existing XML schema model instead of creating a new schema language, the route taken by languages such as Google's KML. 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. 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 has multiple ways to represent coordinates. For example, the 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 To make GML coordinates accessible through the XML DOM, GML 3.0 introduced the The coordinates of a The For GML data servers (WFS) and conversion tools that only support GML 1 or GML 2 (i.e. only the 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. 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: An srsName attribute attached to a geometry object specifies the object's CRS, as shown in the following example: 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. 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. Examples Polygons, Points, and LineString objects are encoded in GML 1.0 and 2.0 as follows: Note that LineString objects, along with LinearRing objects, assume linear interpolation 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: 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 xsi:schemaLocation="http://www.myphotos.org MyGoodPhotos.xsd"> Note that when using the Point Profile, the only geometry object is the ' See also | |||||||
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