Metadata vs. Vocabularies

Metadata is used to describe the aspects of ‘something’. In the MMI community, the ‘something’ could be most anything related to the marine community, such as a data set, a marine oriented service, etc.

A metadata standard, also known as a content standard, is used to define the ‘containers’ for the metadata. The metadata standard is like a list of possibly important items for describing ‘something’. The metadata standard describes this all-encompassing list of containers. Since the list is all-encompassing, any specific container may or may not be of use for the particular thing being described.

A controlled vocabulary is often used as the content for specific metadata containers. The controlled vocabulary, which is a managed list of acceptable terms, will predetermine all the possible ‘stuff’ that may be used in the ‘container’. If someone fills the container with text that is not included in the ‘stuff’, it will not be permitted to stay in the ‘container’. In this case, the controlled vocabulary could be defined internal or external to the metadata standard.

An example may help to distinguish the metadata containers from the terms in the controlled vocabulary. Suppose we are going to purchase a vehicle. In this example, the vehicle represents the ‘something’ we are going to describe. A hypothetical metadata standard to assist us in vehicle selection might consist of an all-encompassing list of important containers that describe any possible vehicle. These containers could include: model name, color, number of doors, number of passengers and whether or not there is a real spare tire (i.e., as opposed to the donut spare). Note that the descriptors in the all-encompassing list do not apply to every possible vehicle. For example, a motorcycle has no spare tire at all and thus the descriptor for ‘real spare’ doesn’t really apply.

Now consider the number of doors. In this case, the controlled vocabulary may be represented by the numbers 0, 2, 3, and 4. This controlled vocabulary allows for all passenger vehicles produced by all manufacturers. The zero case accounts for motorcycles; the two door case for two-door cars; and the three and four door case for the older and newer style vans (note, we are ignoring the gate or hatchback). No other value for the number of doors is possible in passenger vehicles.

In this example, the metadata standard represents the structure and metadata descriptors, while the controlled vocabulary is used to define the allowable content (e.g., the 0, 2, 3, 4 list) for the descriptors.