The Structured Generalized Markup Language and the eXtensible Markup Language are used to effectively manage and publish geospatial dataset descriptions. Metadata documents in these formats are software-independent, so they can be viewed on the WWW, used with word processors, imported to and exported from databases, and they can form the basis for consistent digital library catalogues and other online search systems. Each metadata document can be expressed and validated in many different ways.
See also: IndexGeo / Geospatial metadata / SGML and XML
"The Standard Generalised Markup Language (SGML) is an international standard for the definition of device-independent, system-independent methods of representing text in electronic form. SGML is a metalanguage, that is, a means of formally describing a language." [from A Gentle Introduction to SGML]
The Australian Spatial Data Directory (ASDD) can use geospatial dataset descriptions as catalogue records in SGML and XML formats.
"SGML is necessarily sophisticated: It is providing a much-needed service by allowing the exchange of information at any level of complexity among software, hardware, storage and presentation systems (including database management and publishing applications) without regard to the manufacturer's name on the label. And it is doing all this with the authority of an International Standard." [from Introduction to the SGML PRIMER]
At its very basic form, SGML describes particular elements of text with easily recognisable nametags in opening and closing pairs. These elements can be parents and children, thus describing a hierarchical, structured relationship between the elements of a document.
<document> <title>My Life</title> <author>Fred Nerk</author> <abstract>Not much</abstract> </document>
SGML is independent of vendors, platforms, and software. There are three core components to SGML - the Document Type Definition (DTD) which defines the allowed structure and content, the many document instances (each .sgml file), and the SGML Declaration (which defines lots of parameter settings for the authoring or processing software).
"XML is designed to make it easy and straightforward to use SGML on the Web: easy to define document types, easy to author and manage SGML-defined documents, and easy to transmit and share them across the Web. It defines an extremely simple dialect of SGML ..." [from the XML specification]
"The goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. For this reason, XML has been designed for ease of implementation, and for interoperability with both SGML and HTML." [from the XML specification]
"XML itself is not a single markup language: it's a metalanguage to let you design your own markup language. A regular markup language defines a way to describe information in a certain class of documents (eg HTML). XML lets you define your own customized markup languages for many classes of document. It can do this because it's done in SGML, the international standard metalanguage for markup languages." [from the XML Frequently Asked Questions]
The Australasian spatial data community has decided to use SGML and XML as the format for geospatial dataset description documents which contribute to the Australian Spatial Data Directory (ASDD).
There are many reasons for this choice:
The following applications can be used to produce, manage, view, and process the SGML documents. Some of them will also handle XML documents. Until the WWW browsers support XML documents directly (and even then to support old browsers) content providers will need to convert XML documents to HTML.
To find out more about SGML, read A Gentle Introduction to SGML and find many resources at the The SGML/XML Web Page. XML information can also be found at the World Wide Web Consortium's W3C XML page. See also the W3C documentation for SGML, XML, and Structured Document Interchange and Metadata and Resource Description.
The Australia New Zealand Land Information Council (ANZLIC) and the ASDD home page have more information about the Australian Spatial Data Directory.
Last Modified: 2 April 2000