Using the ACDD keywords approach

Keywords are important to properly identify the purpose of a specific dataset. They are used to identify the content of the dataset (e.g. variables), relevant thematic relations and other aspects of a dataset. For keywords to be useful in search and filtering operations, application of controlled vocabularies, thesauri, taxonomies or ontologies is required. An example for how to add keywords and at the same time identify the controlled resource used for these keywords is provided below. The two key ACDD elements in question are

  1. keywords
  2. keywords_vocabulary

According to the ACDD conventions proper identification of keywords are done according to:

If you are using a controlled vocabulary for the words/phrases in your "keywords" attribute, this is the unique name or identifier of the vocabulary from which keywords are taken. If more than one keyword vocabulary is used, each may be presented with a prefix and a following comma, so that keywords may optionally be prefixed with the controlled vocabulary key. Example: 'GCMD:GCMD Keywords, CF:NetCDF COARDS Climate and Forecast Standard Names'.

An example of proper usage for this purpose is:

:keywords = "GCMDSK:Earth Science > Cryosphere > Sea Ice > Ice Types, GCMDSK:Earth Science > Oceans > Sea Ice > Ice Types, Earth Science > Climate Indicators > Cryospheric Indicators > Sea Ice Types, GCMDLOC:Geographic Region > Northern Hemisphere, GCMDLOC: Vertical Location > Sea Surface, GCMDPROV: Government Agencies-non-US > Norway > NO/MET > Norwegian Meteorological Institute" ;


:keywords_vocabulary = "GCMDSK:GCMD Science Keywords:, GCMDPROV:GCMD Providers:, GCMDLOC:GCMD Locations:" ;

This example is extended with inclusion of the URI of the controlled vocabulary in use to simplify subsequent utilisation of the information in search engines. Inclusion of the URI is not covered by the ACDD standard but is rcommended for integration with projects listed below since it allows improved services to be offered. The URI is identified using a second ':' in the keywords_vocabulary attribute.

Relevant GCMD keywords are available at The primary focus for ACDD is on the Science Keywords ( and it is necessary to properly separate between the vocabularies used, also for GCMD. Other relevant vocabularies to use in this context are:

But only GCMD Science Keywords are required.

Below is a list of prefixes for various vocabularies presented with URIs. This list is built into the data ingestion services for ADC, NorDataNet and SIOS. The landing page for all GCMD vocabularies is For easy usage the CSV versions are available through

Table of controllled vocabularies actively used.
Prefix Vocabulary URI Mandatory Comment
GCMDSK GCMD Science Keywords Yes  
GCMDLOC GCMD Locations No  
GCMDPROV GCMD Providers No  
CFSTDN CF Standard Names No Area and Region List are not used although present in the same URI.
GEMET INSPIRE Themes No Mandatory for some purposes.
NORTHEMES GeoNorge Themes No Mandatory for some purposes.