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July 2004 |
WFS, SVG and the future of Internet GIS
What's GML, WFS and SVG?
The Geography Markup Language (GML) is "an XML grammar written in XML Schema for the modeling, transport, and storage of geographic information including both the spatial and non-spatial properties of geographic features" (OGC 2003a). It was developed as an implementation specification by the Open GIS Consortium to foster data interoperability and exchange between different systems.
GML models the world in terms of features based on the OGC Abstract Specification (OGC 2003b). A feature is an abstraction of a real world phenomenon; a geographic feature is any real-world object that is associated with a location. GML 2.1 models the world in terms of simple features, which are "features whose geometric properties are restricted to 'simple' geometries for which coordinates are defined in two dimensions and the delineation of a curve is subject to linear interpolation" (OGC 2001). GML 3.0 can represent real-world phenomenon using more complex feature types, "including features with complex, non-linear, 3D geometry, features with 2D topology, features with temporal properties, dynamic features, coverages, and observations" (OGC 2003a p.18).
GML is based on XML. XML is a data description language with strict hierarchical data structures to facilitate data search and discovery on the Web. As XML is mainly used to describe data, GML is mainly concerned with the representation of the geographic data content. The most important characteristic of GML is that it separates the content from the representation.
GML is very flexible and extensible in encoding geographic features. It allows users to define their own tags or elements to describe features. The descriptive tags of GML store data without any proprietary vendor specific format. While maintained to be compatible with the standards, these tags may be created to suit the user's own data model. The schema file in GML defines these user-created tags. GML offers standard ways to describe these spatial features and their corresponding properties in terms of GML Schema.
The WFS is an OpenGIS implementation specification (OGC 2002) that allows a client to retrieve
Fig 4 An example of feature-level data query (Peng, forthcoming)
geospatial data encoded in GML from multiple Web Feature Services. The WFS is written in XML and uses GML to represent features, but the database (or datastore in OGC's term) could be in any format. In fact, the structure of those databases should be opaque to client applications. Any access to the database should be through the WFS interface. WFS has a tight relationship with GML. The WFS allows the client applications to access, query, create, update, and delete data elements from the GML feature database server. The WFS provides interfaces for four basic data manipulation operations on GML features: create a new feature instance, delete a feature instance, update a feature instance, and get or query features based on some spatial or non-spatial query constraints. Client applications can post requests for feature level data in XML. Such a request can include query or data transformation operations, which can be applied to one or more features in one or more database, locally or remotely. The WFS server reads and passes the request and returns the result in the form of GML.
SVG is an XML-based language used to describe an image, especially for display in a Web browser. It is a standard developed by the W3C (World Wide Web Consortium). The use of the word "scalable" in SVG has two meanings. First, vector graphic images can easily be made scalable, i.e., they are not limited to a single and fixed pixel size. This means SVG format can be displayed on any device of any size and any resolution without changing the image clarity. This contrasts with raster image files, which are difficult to modify without loss of information. Second, a particular technology can accumulate a larger number of files, a large number of users, and a wide variety of applications on the Web (W3C 2001). Other characteristics of SVG include a smaller file size and searchable text information. Text information inside SVG is still text and can be searchable, while text information inside the raster file becomes integrated into the image and is no longer recognized as text. Further, SVG is also based on XML and therefore conforms to other XML-based standards and technologies, such as XML Namespace, XLink, and XPointer. XLink and XPointer allow for linking from within SVG files to other files on the Web, like a GML data element, HTML pages, or other SVG files (Boye 1999).
GML, SVG, and WFS are standard technologies, and each has a unique role on the Web and Internet GIS. Combining them provides a greater potential in the development of Internet GIS. A framework is proposed to use GML, SVG, and WFS together to create a new kind of Internet GIS which is open, standard-based, interoperable, and with true vector graphics capabilities.
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