Metadata and Vocabularies

Metadata is used to describe the aspects of something. In the MMI community, the item being described could be almost anything related to the marine community, such as a data set or a marine service.

A metadata standard, also known as a content standard, is used to define the containers for the metadata. It is a list of possibly important descriptors for the item. Since the list contains many possible containers (descriptors), any specific container may or may not be of use for a particular item 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 determine all the possible values that may be used in the container. If someone fills the container with text that is not included in the accepted values, it will not be permitted to stay in the container.

As an example, the diagram below shows the relationship between the metadata descriptors in the metadata standard, the terms in the controlled vocabulary, and the data.

Suppose we are going to purchase a vehicle. In this example, the vehicle represents the item we are going to describe. A hypothetical metadata standard to assist us in vehicle selection might consist of an extensive list of important containers that describe any possible vehicle. These containers could include model name, color, number of doors, number of passengers, and type of spare tire.

Note that the descriptors—the containers—in the list do not apply to every possible vehicle. For example, a motorcycle has no spare tire, and thus the descriptor for spare does not apply.

Metadata for a Vehicle

Consider the Number of Doors container. 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. No other value for the number of doors is possible in passenger vehicles.

When using a controlled vocabulary, the values chosen for the data must conform to the terms in the controlled vocabulary. In this example, the color of the Vespa might be called cobalt by the manufacturer, but because we are using a controlled vocabulary, we must choose an accepted term. Cobalt is not an accepted value, so we need to choose one from the accepted list, in this case, blue.

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.

Suggested Citation

Isenor, A. 2011. "Metadata and Vocabularies." In The MMI Guides: Navigating the World of Marine Metadata. Accessed July 16, 2020.