Ordnance Survey – Great Britain's national mapping agency
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6.2: Standards (5)
XML and GML
In general, mark-up languages use tags to associate a rule to interpret the content of a set of information. In HTML this means the visual formatting and association of HTTP hyperlinks with text and images. For example, the <font> tag can be used to instruct a browser application to display a piece of text in a certain style.
In a similar way, eXtensible Markup Language (XML) uses tags to give meaning and context on the content of a set of information. In an XML document, tags could encode the fact that Southampton is a city. But alternatively they could state that, in a particular document, Southampton refers to a node on a network of shipping lines, or in another, a football club. This is where the extensible bit comes in. In XML you can define your own set of tag types as long as the tag set applicable to your document is defined in a separate schema. XML provides for self-describing data. This makes it very useful as a standard format for exchanging information because computer programs can interpret the content of XML packets without any prior exposure. XML makes system development more flexible and is rapidly becoming the standard for information interchange on the Internet.
The emergence of XML has led to the creation of a wide range of mark-up languages specific to particular subject matter such as maths, chemistry and medicine. There is also a Geography Mark-up Language (GML) that allows spatial data to be stored and transferred between systems over a network. It allows points, lines and polygons to be encoded along with their attributes and the spatial reference system on which they are based, for example, the National Grid. There is already a lot of interest in GML from other communities, including the mobile phone industry and the general Internet community. GML is fast becoming the definitive method of describing geographical data and simple location information on the Internet. The specification for GML 2.0 can be viewed on the OGC public web site. Ordnance Survey has adopted GML as the format for OS MasterMap.