To ensure that all dataset descriptions are of a consistent type, the Document Type Definition (DTD) defines the metadata elements and their order and structure. A DTD allows different instances of documents of the same type to be automatically processed in a uniform way.
See also: IndexGeo / Geospatial metadata / DTD
"The type of a document is formally defined by its constituent parts and their structure ... a special purpose program (called a parser) can then be used to process a document, claiming to be of a particular type, and check that all of the elements required for that document are indeed present and correctly ordered." [from A Gentle Introduction to SGML]
The Australian Spatial Data Directory (ASDD) can use geospatial dataset descriptions as catalogue records in the format of Standard Generalized Markup Language (SGML) and the eXtensible Markup Language (XML).
This insistence on a uniform document type means that the XML dataset description documents can be readily transferred between information systems. They can be consistently processed to ensure that they are valid and that search facilities can rely on certain elements, such as the geographic extents and keywords.
The Australia New Zealand Land Information Council (ANZLIC) has released the ANZMETA DTD to define geographic dataset descriptions in Australia and New Zealand. The ANZMETA DTD and supporting documentation can be found at http://www.auslig.gov.au/anzmeta/
These hints explain some of the notations used within the ANZMETA DTD and other DTDs. A Gentle Introduction to SGML is an excellent reference for the basics.
Element occurrence indicators ...
none ... exactly one occurrence (required and non-repeatable) ? ... zero or one occurrence (optional and non-repeatable) * zero or more occurrences (optional and repeatable) + ... one or more occurrences (required and repeatable)
Element arrangement indicators ...
, ... one element followed by the other in the specified order (sequence) | ... one element or the other (choice)
<!ELEMENT spdom (place*, bounding)>
So the ANZMETA spatial domain <spdom> element specifies zero or more optional <place> elements, followed by a single required <bounding> element to describe the Minimum Bounding Rectangle.
Parameter entities ...
Parameter entities provide a mechanism for defining reusable content within the DTD. For example, in the ANZMETA DTD the following Parameter entity is defined:
<!ENTITY % list "ul | ol">
which means that a list can be either an unordered list (ul) or an ordered list (ol).
Now that this parameter entity is defined, then it can be used wherever a list element is required by using the following notation:
which will insert the replaceable text ul | ol at that point in the DTD.
Last Modified: 31 August 1999