What is a DIF?
A Short History of the Directory Interchange Format (DIF)
- by Lola Olsen
Before metadata became a ubiquitous buzzword, a descriptive and standardized format for exchanging information about scientific data sets was conceived and implemented. The Directory Interchange Format, the DIF, was the product of an Earth Science and Applications Data Systems Workshop (ESADS) held February 24-26, 1987 on catalog interoperability (CI). The workshop recommended that a "...first step towards data system interoperability, Catalog Interoperability (CI), the ability to find information about data held at other sites...", be made.
In the summer of 1987, the Catalog Interoperability Working Group (consisting of several U.S. Federal and international agencies) defined the type of information and level of detail that would be contained by the DIF. The DIF structure was "frozen" on September 18, 1987, and the population of NASA's Master Directory (NMD) prototype commenced. By December 1987, over 100 DIF entries were available in the prototype NMD database. After several demonstrations, workshops, and feedback from the scientific community, the Directory Interchange Format (DIF) was formally approved and adopted by a CI science advisory group at a CI workshop in 1988.
In 1989, the Committee on Earth Observation Satellites (CEOS) Data Working Group (DWG) began attending the CI Workshop meetings and provided valuable feedback on the DIF structure. The CEOS International Directory Network (IDN) was soon established under the auspices of the CEOS Working Group on Data, through the Catalog Subgroup, to foster the exchange of information among international agencies. The first release of the IDN was named the Prototype International Directory (PID) in 1990. [Actual DIF exchange procedures were agreed on by February 1991.]
In 1990, the Interagency Working Group on Data Management for Global Change (IWGDMGC) adopted the directory as a prototype to facilitate global change research - in response to the challenge by the Earth System Science Committee (ESSC). Thereafter, the NMD was renamed the Global Change Master Directory (GCMD) for its Earth sciences applications.
The DIF has enjoyed over 20 years of success. The DIF structure has been flexible enough to evolve with growing metadata requirements, especially for the geospatial disciplines. In the U.S., the Office of Management and Budget's (OMB's) Circular A-16 for the improved coordination of spatial data among federal agencies led to the establishment of the Federal Geographic Data Committee (FGDC) and the FGDC Clearinghouse. The GCMD serves as NASA's FGDC Clearinghouse node for geospatial metadata. Elements of the Content Standard for Digital Geospatial Metadata (CSDGM) were incorporated in the DIF in 1994.
In the late 1990s, the geospatial community began work towards the development of an international standard for geospatial metadata. The International Organisation for Standardisation (ISO) Technical Committee's [(TC)211] Metadata Standard 19115 (previously known as 15046-15) (see: http://www.isotc211.org/) sought to provide "a consistent suite of geographic information schemata that allows geographic information to be integrated with information technology. The goal of this work item is to produce a schema for geographic information metadata."
The ISO 19115/TC211 geospatial metadata standard was adopted June 2004. Required elements and appropriate modifications were approved by the CEOS IDN Interoperability group and incorporated into the DIF to achieve full ISO compatibility.
The DIF does not compete with other metadata standards. It is simply the "container" for the metadata elements that are maintained in the IDN database, where validation for mandatory fields, keywords, personnel, etc. takes place.
The DIF is used to create directory entries which describe a group of data. A DIF consists of a collection of fields which detail specific information about the data. Eight fields are required in the DIF; the others expand upon and clarify the information. Some of the fields are text fields, others require the use of controlled keywords (sometimes known as "valids").
The DIF allows users of data to understand the contents of a data set and contains those fields which are necessary for users to decide whether a particular data set would be useful for their needs.
The DIFGuide document provides information
about each field of the DIF, including its syntax, specifications,
recommendations, and examples. Several example DIFs are also provided.
|This document should be cited as:
Directory Interchange Format (DIF) Writer's Guide, 2013.
Global Change Master Directory.
National Aeronautics and Space Administration.