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What is ACDD?

Attribute Conventions for Data Discovery (ACDD)… As a student fellow for the Documentation Cluster this term has been discussed regularly over the last few years.  But what is ACDD? Why is it important? Why do we keep talking about it?  These are all questions I have asked myself.


What is ACDD?

The governing body for the convention is the ESIP Documentation Cluster. From the ESIP wiki: “These conventions identify and define a list of NetCDF global attributes recommended for describing a NetCDF dataset to discovery systems such as Digital Libraries. Software tools will use these attributes for extracting metadata from datasets, and exporting to Dublin Core, DIF, ADN, FGDC, ISO 19115 etc. metadata formats.” BUT what does THAT mean?

This, of course, all has to do with metadata (who guessed that already?).  The convention is a list of recommendations of how to describe attributes.  Attributes can be messages to the applications to generate metadata in code or descriptive information about the metadata.  These conventions allow for the translation of metadata between multiple formats.  This means that more computer programs can understand the information. In the case of ACDD the conventions allow the metadata to be understood by programs that are searching for data (data discovery tools).

ACDD is also referred to as Unidata Dataset Discovery Conventions (UDDC) but ACDD is preferred.  OCDD is also referred to as version 2.0 of ACDD.  The purpose of ACDD is to allow dataset discovery and facilitate mapping between dataset metadata (notably netCDF) and ISO 19115.  As each organization, discipline, country, region, or other group uses and develops their own metadata forms and standards conventions such as ACDD become essential for locating data on the internet. Via THREDDS Data Server you can download scientific dataset (especially georeferenced data such as net CDF, DF, and GRIB) using your web browser or even subsets of the data without downloading the whole datasets with OPeNDAP.

The convention defines highly recommended, recommended, and suggested attributes to be included into metadata.  Each includes the title for the attribute and a definition.  The current version (1.1) can be found here (link will always direct to the current convention).  The standard has recently been migrated from the NOAA page (wiki) to the ESIP wiki.  In addition to the current version there is one or more working version (1.2 and 2.0) that are not considered stable and are subject to change.


Versions 1.2 vs 2.0?

This is a topic that has been brought up regularly during Documentation Telecoms.  There is some confusion about the naming as some of the committee for version 1.2 would like to name it version 2.0, but there is a page with version 2.0.  Version 1.2 is the working version.  The final changes to 1.2 should be approved at the September 2014 telecom.  Version 2.0 (or the next version) is under development.  It will include grouping, a means to flatten 2.0 to function like 1.2, and will be similar to parameters in ISO. These convention will be used in NetCDF4 extended or HDF5.  The final shape of version 2.0 is still being determined – follow (or join) the conversation in the Documentation Cluster Telecoms and email list.


What is next?

The August and September Documentation Telecons are a forum where ACDD 1.2 is being approved.  Join us on September 18, 2014 at 2pm EDT to join the discussion.  The meeting will address any remaining topics needed to approve the final release of this version of ACDD.