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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 MODEL The original GML model was based on the World Wide Web Consortium 's Resource Description Framework (RDF). Subsequently, the OGC introduced XML Schema s 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 XML Schema s 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:
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:
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. 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 System s (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 Polygon s, Point s, and ''LineString'' objects are encoded in GML 1.0 and 2.0 as follows: 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: 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 ' REFERENCES
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