ISO Citation Identifiers
What are Citation Identifiers in ISO metadata?
The ISO Citation can include any number of identifiers, each of which is defined as a value uniquely identifying an object within a namespace. The identifiers have a MD_Identifier type so they include an authority citation for the namespace and a code that uniquely identifies the citation within that namespace.
The FGDC model did not include an identifer as part of the citation, although identifiers are used to identify sources used or produced in the Lineage Section. That approach identifies the sources for a single dataset within the metadata record and links those sources to the appropriate process steps. The identifiers are only unique within that particular lineage section.
The FGDC approach is consistent with the scientific paper model of FGDC citations. The references for a scientific paper are listed in the references section of the paper. In the text of the paper they are given abbreviations relative to that reference list. A reference to Doe, 1978a is to the first paper publised by Doe during 1978 referenced in the paper. This is not necessarily the first paper published by Doe during 1978. One might say that the scope of the FGDC citation abbreviations is limited to a particular lineage section of a particular metadata record.
Adding identifiers to the citation is a significant extension of the scope of these identifiers. They are now associated directly with the citation and have a scope that is described by the namespace for the identifier. A namespace is an abstract concept that describes a container in which all names are unique. As a rule, names in a namespace cannot have more than one meaning, that is, two or more things cannot share the same name. A namespace is also called a context, as the valid meaning of a name can change depending on what namespace applies. In the scientific paper case described above, the namespace for the idntifier Doe, 1978a (and for the other references) is the paper in which they are referred to. Consider the word "bonnet" as another example. In the American namespace this is a word for a hat. In the British namespace it is a cover for the engine of a car (hood in the American namespace).
The addition of identifiers to the citation also allows us to treat those identifiers in ways that are similar to primary keys in a database table. In that case, the database table is the namespace in which the primary keys are guarenteed to be unique. For example, we can search for all datasets that include data from a particular source using the citation identifier. That was impossible using the FGDC approach.