Extending SAIF involves the definition of new types. Type definitions provide a means of modelling phenomena and ultimately of providing data in a meaningful way. A number of users can describe their data holdings in a highly consistent fashion through SAIF, easing the semantic problems associated with information sharing. New types may relate directly to spatial or temporal phenomena, or they may deal with other ideas altogether.
Type definitions may be formally expressed in SAIF using graphical techniques such as SAIF/OMT, the text based Class Syntax Notation (CSN), or both. Additionally, the interrelationships between classes and between objects are handled directly through the definitions. Later in this chapter a series of examples will be reviewed covering both graphical and CSN approaches.
The subsections below describes first the constituent parts of a class definition, then those of an enumeration definition, and finally those of a special domain construction. The discussion applies both to the formulation of the SAIF standard schema and user defined schemas.