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One of the most powerful aspects of GML is the freedom it gives users to define their own custom application schemas. While this capability provides extraordinary flexibility to data modellers, it also creates significant challenges, particularly when the data is interpreted.
Since GML is a relatively new and complex technology, there may be misconceptions about what it does and does not solve. These issues will be discussed and clarified.
In addition, there have been several significant deployments of GML as a solution to data archive as well as data interchange. Several implementations will be surveyed and an analysis of the different approaches will be presented. The lessons learned from these early adopters would be of great interest to anyone planning to use GML in their own projects for data input, archive, or interchange.
GML - Experiences from the Field
Historically, the task of moving geographic data from one format to another has been difficult. As a result, users with large data stores have been locked into a single vendor's format and have been restricted to using one vendor's analysis and decision support tools. The Geography Markup Language (GML) attempts to alleviate these difficulties by increasing organizations' ability to share geographic information. GML, which is based on the eXtensible Markup Language (XML), is an open and non-proprietary specification used for the transport and storage of geographic information. This paper presents a configurable XML translation engine that enables translation between XML-based formats and other GIS formats. It then describes how the XML translation engine can be automatically configured to read datasets that are based on any arbitrary GML user application schema.
The XML specification provides a standard way for defining markup languages for textual documents; it is a meta-language that allows users to design and format the structural relationships of their documents using strict lexical and syntactical constraints. XML documents are stored in plain text, which introduces numerous beneficial consequences. Because XML is human-readable, a plain text editor can be used to view documents; it is also easily transmitted across platforms and over the Internet. In addition, plain text is vendor-neutral, so information that is stored in XML is not locked into a proprietary binary format. XML thus enables disparate systems to share information easily - and, since GML is defined with XML, it inherits all of XML's benefits.
GML uses the W3C XML Schema Definition Language to define and constrain the contents of its XML documents. The GML v2.0 Specification defines some basic conformance requirements for users to develop their own application schemas. Software applications attempting to process any arbitrary GML user application schema must understand GML and all of the technologies upon which GML depends, including the W3C XML Schema.
Many free parsers are available for XML. These parsers may be used by GIS applications as a base building block for implementing GML software modules. Most XML parsers provide the option for validating an XML document through a W3C Schema document. GIS applications can utilize these parsers to read and validate documents for arbitrary GML user application schemas. It must be noted that software applications must still interpret the output from the XML parsers into their own local meaningful context. The software application must know what each XML element in the GML dataset means, whether the element refers to a feature, a property of a feature, or a feature collection. It is not enough for the GIS application software to use the XML parser to validate the dataset according to a schema: the application must also understand how GML uses the W3C Schema to define a geographic feature and its properties. GML introduces an extraordinary flexibility by letting users define their own application schemas suitable for their own domains; however, this same flexibility also presents a substantial difficulty for writing GML software applications.
It is more or less trivial to write a software component that works on a particular GML user application schema. The GML software component can even bypass the schema processing, since all the processing logic for that particular domain can be hard-coded into the software component. The W3C XML Schema document only needs to be examined if, in addition to processing, the user wishes to perform validation on the GML datasets with the XML parser.
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