Global Change Master Directory (GCMD) Keywords

GCMD Keywords are a hierarchical set of controlled Earth science vocabularies that ensure Earth science data and services are described in a consistent manner.

Global Change Master Directory (GCMD) Keywords are a hierarchical set of controlled Earth Science vocabularies that help ensure Earth science data, services, and variables are described in a consistent and comprehensive manner and allow for the precise searching of metadata and subsequent retrieval of data, services, and variables. Initiated over twenty years ago, GCMD Keywords are periodically analyzed for relevancy and will continue to be refined and expanded in response to user needs. The categories of GCMD Keywords are as follows:

  • Earth Science
  • Earth Science Services
  • Data Centers/Service Providers
  • Projects
  • Instruments/Sensors
  • Platforms/Sources
  • Locations
  • Horizontal Data Resolution
  • Vertical Data Resolution
  • Temporal Data Resolution
  • URL Content Types
  • Granule Data Formats
  • Measurement Names
  • Chronostratigraphic Units

The information provided on this page defines how GCMD Keywords are structured, used and accessed. It also provides information on how users can participate in the further development of the keywords.


GCMD Keyword Forum

The GCMD Keyword Forum, now part of the Earthdata Forum, provides keyword users and metadata providers with an area for discussion of topics related to GCMD Keywords. Participants are invited to use this forum to ask questions, submit keyword requests, discuss trade-offs, and track the status of keyword requests.

GCMD Earthdata Forum

More Information About the Keywords

GCMD Keyword Structure

GCMD Keywords are organized using a multi-level hierarchical structure. This hierarchical structure provides a framework by which concepts can be classified and related. The Earth Science and Science Services keyword sets use the following hierarchical structure:

Category > Topic > Term > Variable > Detailed Variable

Categories and Topics represent disciplines and high level concepts. Terms and Variables define subject areas and parameters. Detailed Variables are uncontrolled values that can be added by users to more specifically describe data.

All other keyword sets use hierarchies designed for a specific concept. For example, "Instruments" uses Short Name > Long Name. Short name and long name define the name of the instrument used to acquire the data.

Earth Science Keywords Structure

The Earth Science Keywords have a six-level keyword structure with the option for a seventh uncontrolled field. Category and Topic levels define how the keywords are organized and the associated Earth science discipline within the hierarchy. The Term and Variables levels define the subject area, measured variables/parameters, and the hierarchical-type relationship for the subject area.

Keyword Level Example
Category Earth Science
Topic Atmosphere
Term Weather Events
Variable Level 1 Subtropical Cyclones
Variable Level 2 Subtropical Depression
Variable Level 3 Subtropical Depression Track
Detailed Variable (Uncontrolled Keyword)

Earth Science Services Keywords Structure

The Earth Science Services Keywords have a four-level keyword structure with the option for a fifth uncontrolled field. Category and Topic levels define how the controlled keywords are organized and the associated Earth Science Services discipline associated with the keyword in the hierarchy. The Term and Variable levels define the subject area, the type of software/service being described, and the hierarchical-type relationship for the subject area.

Keyword Level Example
Service Category Earth Science Services
Service Topic Data Analysis and Visualization
Service Term Geographic Information Systems
Service Variable Web-Based Geographic Information Systems
Detailed Variable (Uncontrolled Keyword)

Data Center/Service Provider Keywords Structure

The Data Center/Service Provider Keywords have a four-level keyword structure and a short name/long name that define the data center, service provider, organization, or institution responsible for distributing the data or developing the service.

Keyword Level Example
Level 0 Government Agencies-U.S. Federal Agencies
Level 1 NASA
Level 2 -
Level 3 -
Short Name LP DAAC
Long Name Land Processes Distributed Active Archive Center

Project Keyword Structure

The Project Keywords have a short name/long name that define the name of the scientific program, field campaign, or project from which the data were collected.

Keyword Level Example
Short Name ESIP
Long Name Earth Science Information Partners

Instrument/Sensor Keyword Structure

The Instrument/Sensor Keywords have a four-level keyword structure and a short name/long name that define the name of the instrument used to acquire the data.

Keyword Level Example
Category Earth Remote Sensing Instruments
Class Passive Remote Sensing
Type Spectrometers/Radiometers
Sub Type Imaging Spectrometers/Radiometers
Short Name MODIS
Long Name Moderate-Resolution Imaging Spectroradiometer

Platform/Source Keyword Structure

The Platform/Source Keywords have a three-level keyword structure a short name/long name that define the name of the platform used to acquire the data.

Keyword Level Example
Basis Space-based Platforms
Category Earth Observation Satellites
Sub Category -
Short Name Aqua
Long Name Earth Observing System, Aqua

Location Keyword Structure

The Location Keywords have a five-level keyword structure with the option for a sixth uncontrolled field and define the name of a place on Earth, a location within Earth, a vertical location, or a location outside of the Earth.

Keyword Level Example
Location Category Continent
Location Type North America
Location Subregion 1 United States of America
Location Subregion 2 Maryland
Location Subregion 3 Baltimore
Detailed Location (Uncontrolled Keyword)

Horizontal Data Resolution Keyword Structure

The Horizontal Data Resolution Keywords have a range structure and define the horizontal resolution of the data.

Keyword Level Example
Resolution Range 1 km - < 10 km

Vertical Data Resolution Keyword Structure

The Vertical Data Resolution Keywords have a range structure and define the vertical resolution of the data.

Keyword Level Example
Resolution Range 1 km - < 10 km

Temporal Data Resolution Keyword Structure

The Temporal Data Resolution Keywords have a range structure and define the vertical resolution of the data.

Keyword Level Example
Resolution Range 1 minute - < 1 hour

URL Content Type Keyword Structure

The URL Content Type Keywords have a content type and type/subtype structure and define the type of URL being referenced.

Keyword Level Example
URL Content Type Distribution URL
Type Get Data
Sub-Type Earthdata Search

Granule Data Formats Keyword Structure

The Granule Data Formats Keywords have a one-level keyword structure and represent the format of the data that is distributed by the data center.

Keyword Level Example
Granule_Data_Format GRIB

Measurement Names Keyword Structure

The Measurement Names Keywords have a three-level keyword structure and define an observable property, usually geophysical, geo-biophysical, physical, or chemical.

Keyword Level Example
Context_Medium atmosphere
Object air
Quantity pressure_anomaly

Chronostratigraphic Units Types Keyword Structure

The Chronostratigraphic Keywords have a five-level keyword structure and define the geologic time of the data.

Keyword Level Example
Eon Phanerozoic
Era Cenozoic
Period Neogene
Epoch Miocene
Stage Tortonian
Community Use of GCMD Keywords

NASA’s Earth Observing System Data and Information System (EOSDIS) as well as numerous other U.S. and international agencies, research universities, and scientific institutions use GCMD Keywords as an authoritative vocabulary, taxonomy, or thesaurus to describe Earth science data, services, and visualizations. Many of these organizations also use GCMD Keywords to create ontologies for the Semantic Web, and to search and access Earth science data within online metadata catalogues. A representative list of organizations using GCMD Keywords is shown below.

GCMD Keyword Governance and Community Guide Document

The GCMD Keyword Governance and Community Guide Document, Version 1.0, is intended to provide the community with a comprehensive resource that describes the governance structures and process for reviewing proposed changes to GCMD Keywords. The document includes an overview of the structure of the keywords, requirements and recommendations to consider when requesting keyword changes, and instructions for submitting change requests. The process described in this document will be the basis of all future GCMD Keyword reviews. The community is invited to provide feedback.

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The Change Process Overview diagram conveys the end to end process of a keyword including initial request, triage, approval, notification, and implementation.

Additional Information

Access the GCMD Keywords

Submit a Keyword Request

Keyword Release Announcements

GCMD Keyword Forum

GCMD Keyword Viewer

The GCMD Keywords should be cited as:

Global Change Master Directory (GCMD). 2023. GCMD Keywords, Version 18.0, Greenbelt, MD: Earth Science Data and Information System, Earth Science Projects Division, Goddard Space Flight Center, NASA. URL (GCMD Keyword Forum Page): https://forum.earthdata.nasa.gov/app.php/tag/GCMD+Keywords.

Contact GCMD User Support for questions, comments, or technical issues regarding GCMD Keywords.

Subscribe to the GCMD/IDN Interoperability Email List to receive keyword release announcements.

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