Cardinality describes the association between two entities as either mandatory or optional. For example, a forest must have more than one tree, but a tree may or may not be in a forest. The cardinality aspects of the data model are shown as a circle for optional and a line for mandatory. The association between a forest and a tree with the cardinality is shown below in Figures 2-2 and 2-3 from the Cadastral Data Content Standard.
In Figure 2-2 below:
Figure 2-2 Entities, Associations, and Cardinality
The associations and their cardinality are read in a clockwise manner, which means the associations in Figure 2-2 are read as shown below in Figure 2-3.
Figure 2-3 Reading Entities, Associations, and Cardinality
The above relationship reads that a forest must contain many trees, and a tree may or may not be within a forest.
Learning the Cadastral Data Content Standard
Presented by the United States Department of the Interior Bureau of Land Management, and
the Federal Geographic Data Committee Cadastral Subcommittee