GCMD DIF

(Source: Major, Gene (Undated). About us, Welcome to the GCMD. Retrieved February 2, 2004. http://gcmd.gsfc.nasa.gov/Aboutus/ (and sub-pages). )

The NASA Global Change Master Directory (GCMD) is an on-line system with information about Earth science data sets, and is intended for the use of the research science community. The GCMD offers descriptions of Earth science data sets (metadata) using a defined set of fields known as the Directory Interchange Format (DIF). It has evolved over a twelve-year period and serves the user community in the discovery of Earth science data. The GCMD's philosophy for using the DIF is that the specific set of attributes, the content metadata, is the critical set needed for a user to determine if the data sets returned from a database query are those that define viable data sets for the user's needs. Using a specific set of fields also helps to "normalize" the search for data sets through the use of several alternative search engines.

The Global Change Master Directory was developed by the Catalog Interoperability (CI) working group on data management. This group consisted of representatives from universities and government agencies including NASA, NOAA, and USGS. The objective was to implement an interoperable, network-connected data information system, which became the International Directory Network (IDN) of the Committee on Earth Observation Satellites (CEOS)

The Interagency Working Group on Data Management for Global Change (IWGDMGC) adopted the directory as a centralized directory to facilitate global change research in the USA. IWGDMGC members included NASA, NOAA, USGS, NSF, EPA, DOE, USDA, DOS, and other US agencies. The directory became part of the International Directory Network (IDN) guided by the Working Group on Information Systems and Services (WGISS) of the Committee on Earth Observing Satellites (CEOS). There are five coordinating nodes of the CEOS IDN for the Asian, American, European, and African continents, and another coordinating node for those countries participating in Antarctic research. These coordinating nodes provide a path for researchers within a country or region to participate in the CEOS IDN. Cooperating nodes may support directories specializing in a specific subject, or, although not required, may maintain the complete IDN database.

The DIF is a de facto metadata standard used to describe Earth science data sets. A DIF record consists of a collection of fields that detail specific information about the data. Five fields are required in the DIF in order for the DIF to load into DIF databases, including Entry_ID, Entry_Title, Parameters, Data_Center, and Summary. Four more fields let users search and understand the data: Parameters, Temporal_Coverage, Spatial_Coverage, Location, and Data Set Citation. Three fields help users understand a dataset and decide whether it meets their needs: Entry_Title, Summary, and Data_Resolution. The Data_Center field lets users access the data when needed. Lastly, the Data Set Citation field allows proper citation and acknowledgement. Some of the DIF fields are open-ended text; others have lists of "valids" (i.e., multiple-choice).

As a search and retrieval system, GCMD uses two complementary search engines: free- text and controlled vocabulary. It also allows users to combine the text search with a geographic location and/or time period of interest. The controlled vocabulary search interface is implemented using MD-Oracle, which is the Relational Database Master Directory software. The free text search interface is implemented using Isearch and Z39.50 software.

For more detail about GCMD, please visit http://gcmd.gsfc.nasa.gov