Attribute Conventions for Data Discovery 2.0

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Version and Status

This version is designated as Version 2.0 This page always has the current version of the Attribute Convention for Data Discovery (ACDD). As it is updated, the version number at the top of the page will be updated.

See the [category page] for information on the history of this convention.

Development

Any development version of the ACDD definitions is maintained at Attribute_Convention_for_Data_Discovery_(ACDD)_Working.

Overview

The NetCDF Group at Unidata has recommended attributes for data discovery . The Attribute Convention for Data Discovery (ACDD) addresses that need, providing definitions for NetCDF global attributes that will help data to be located efficiently.

Alignment with NetCDF and CF Conventions

The NetCDF User Guide (NUG) provides basic recommendations for creating NetCDF files; the NetCDF Climate and Forecast Metadata Conventions (CF) provides more specific guidance. The ACDD builds upon and is compatible with these conventions; it may refine the definition of some terms in those conventions, but does not preclude the use of any attributes defined by the NUG or CF.

The NUG does not require any global attributes, though it recommends and defines two, title and history; CF specifies many more. ACDD 1.2 adopts all CF 1.6 global attributes with the exception of 'institution'; we specify 'creator_institution' and 'publisher_institution', to provide more provenance information. We also modify the syntax of the 'Conventions' attribute; we adopt the NUG recommendation to supply all conventions in a single attribute. This change has been approved by the CF Conventions Committee and will be part of CF 1.7, which is not yet published.

Attribute Crosswalks

Many of these attributes correspond to general discovery metadata content, so they are available in many metadata standards. This Unidata crosswalk to THREDDS page includes also includes a crosswalk to ISO 19115-2. Note that the attribute names link to the Unidata definitions. Many of these elements are included in the ISO 19115 Core specification. They are indicated in this Table by an M, O, or C in parentheses. An “M” indicates that the element is mandatory. An “O” indicates that the element is optional. A “C” indicates that the element is mandatory under certain conditions.

Additional Metadata: metadata_link attribute

Other metadata dialects (i.e. ISO 19115) can provide information about collections and more details about the dataset. If additional metadata exists, you can make users aware of it by adding a global attribute named "metadata_link" to the netCDF file. The value of this attribute is a URL that gives the location of the more complete metadata.


Maintenance of Metadata

ACDD attributes, like all NetCDF attributes, characterize their containing (parent) granules. As NetCDF data are processed (e.g., through subsetting or other algorithms), these characteristics can be altered. The software or user processor is responsible to update these attributes as part of the processing, but some software processes and user practices leave them unchanged. This affects both consumers and producers of these files, which comprises three roles:

  • developers of software tools that process NetCDF files;
  • users that create new NetCDF files from existing ones; and
  • end users of NetCDF files.

NetCDF file creators (the first two roles) should ensure that the attributes of output files accurately represent those files, and specifically should not "pass through" any source attribute in unaltered form, unless it is known to remain accurate. NetCDF file users (all three roles) should verify critical attribute values, and understand how the source data and metadata were generated, to be confident the source metadata is current.

The ACDD geospatiotemporal attributes present a special case, as this information is already fully defined by the CF coordinate variables (the redundant attributes are recommended to simplify access). Errors in these attributes will create an inconsistency between the metadata and data of the granule or file. The risk of these 'inconsistency errors' is highest for files that are aggregated into longer or larger products, or subset into shorter or smaller products, such as files from numerical forecast models and gridded satellite observations. For this reason, some providers of those data types may choose to omit the ACDD geospatiotemporal attributes from their files. If the ACDD geospatiotemporal attributes are present, checking them against the CF coordinate variables can serve as a partial test of the metadata's validity.


Metadata Link

The netCDF metadata model is focused on providing "use metadata" for the data included in the file (or granule). Other metadata dialects (i.e. ISO 19115) can provide information about collections and more details about the dataset. In order to make users aware of that additional metadata we recommend adding a global attribute named "Metadata_Link" to the netCDF file. The value of this attribute is a URL that gives the location of the more complete metadata. This element is not included in the current version of the NetCDF Attribute Convention for Dataset Discovery.

Highly Recommended

Attribute Description THREDDS ISO 19115-2 OGC CSW Rubric Category
title
A short description of the dataset.
dataset@name
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:title/gco:CharacterString (M)
Title Text Search
summary
A paragraph describing the dataset.
metadata/documentation[@type="summary"]
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:abstract/gco:CharacterString (M)
Abstract Text Search
keywords
A comma separated list of key words and phrases.
metadata/keyword
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:keyword/gco:CharacterString
Subject Text Search

Recommended

Attribute Description THREDDS ISO 19115-2 OGC CSW Rubric Category
id
The

combination of the "naming authority" and the "id" should be a globally unique identifier for the dataset.

dataset@id
/gmi:MI_Metadata/gmd:fileIdentifier/gco:CharacterString (O)
Identifier Identifier
naming_authority
keywords_vocabulary
If you are following a guideline for the words/phrases in your "keywords" attribute, put the name of that guideline here.
metadata/keyword@vocabulary /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:thesaurusName/gmd:CI_Citation/gmd:title/gco:CharacterString
Text Search
cdm_data_type
The THREDDS data type appropriate for this dataset. metadata/dataType /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:spatialRepresentationType/gmd:MD_SpatialRepresentationTypeCode
May need some extensions to this codelist. Current values: vector, grid, textTable, tin, stereoModel, video.
Other
history
Provides an audit trail for modifications to the original data. metadata/documentation[@type="history"] /gmi:MI_Metadata/gmd:dataQualityInfo/gmd:DQ_DataQuality/gmd:lineage/gmd:LI_Lineage/gmd:statement/gco:CharacterString (O) Text Search
comment
Miscellaneous information about the data. metadata/documentation
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:supplementalInformation
Text Search
date_created The date on which the data was created.
metadata/date[@type="created"] /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:date/gmd:CI_Date/gmd:date/gco:Date (M)
/gmd:dateType/gmd:CI_DateTypeCode="creation"
Responsible Party
creator_name
The data creator's name, URL, and email. The "institution" attribute will be used if the "creator_name" attribute does not exist.
metadata/creator/name
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:individualName/gco:CharacterString
CI_RoleCode="originator" (O)
Responsible Party
creator_url
metadata/creator/contact@url
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:onlineResource/gmd:CI_OnlineResource/gmd:linkage/gmd:URL
Responsible Party
creator_email
metadata/creator/contact@email /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:address/gmd:CI_Address/gmd:electronicMailAddress/gco:CharacterString Responsible Party
institution
metadata/creator/name /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:organisationName/gco:CharacterString Responsible Party
project
The scientific project that produced the data.
metadata/project
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:keyword/gco:CharacterString
and/or
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:aggregationInfo/gmd:MD_AggregateInformation/gmd:aggregateDataSetName/gmd:CI_Citation/gmd:title/gco:CharacterString
DS_AssociationTypeCode="largerWorkCitation" and DS_InitiativeTypeCode="project"
and/or

/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:keyword/gco:CharacterString with gmd:MD_KeywordTypeCode="project"

Responsible Party
processing_level A textual description of the processing (or quality control) level of the data.
metadata/documentation[@type="processing_level"]
acknowledgementA place to acknowledge various type of support for the project that produced this data.
metadata/documentation[@type="funding"] /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:credit/gco:CharacterString Responsible Party
geospatial_bounds Describes geospatial extent using any of the geometric objects (2D or 3D) supported by the Well-Known Text (WKT) format. BoundingPolygon Extent
geospatial_lat_min
Describes a simple latitude/longitude bounding box. geospatial_lat_min specifies the southernmost latitude; geospatial_lat_max specifies the northernmost latitude; geospatial_lon_min specifies the westernmost longitude; geospatial_lon_max specifies the easternmost longitude of the bounding box.
The values of geospatial_lon_min and geospatial_lon_max reflect the actual longitude data values. Cases where geospatial_lon_min is greater than geospatial_lon_max indicate the bounding box extends from geospatial_lon_max, through the longitude range discontinuity meridian (either the antimeridian or Prime Meridian), to geospatial_lon_min.
For a more detailed geospatial coverage, see the suggested geospatial attributes.
metadata/geospatialCoverage/northsouth/start
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:geographicElement/gmd:EX_GeographicBoundingBox/gmd:southBoundLatitude/gco:Decimal
BoundingBox Extent
geospatial_lat_max metadata/geospatialCoverage/northsouth/size /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:geographicElement/gmd:EX_GeographicBoundingBox/gmd:northBoundLatitude/gco:Decimal
BoundingBox Extent
geospatial_lon_min metadata/geospatialCoverage/eastwest/start /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:geographicElement/gmd:EX_GeographicBoundingBox/gmd:westBoundLongitude/gco:Decimal
BoundingBox Extent
geospatial_lon_max metadata/geospatialCoverage/eastwest/size /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:geographicElement/gmd:EX_GeographicBoundingBox/gmd:eastBoundLongitude/gco:Decimal
BoundingBox Extent
geospatial_vertical_min
Describes a simple vertical bounding box. For a more detailed geospatial coverage, see the suggested geospatial attributes. metadata/geospatialCoverage/updown/start /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:verticalElement/gmd:EX_VerticalExtent/gmd:minimumValue/gco:Real Extent
geospatial_vertical_max metadata/geospatialCoverage/updown/size /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:verticalElement/gmd:EX_VerticalExtent/gmd:maximumValue/gco:Real Extent
time_coverage_start Describes the temporal coverage of the data as a time range. metadata/timeCoverage/start /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:temporalElement/gmd:EX_TemporalExtent/gmd:extent/gml:TimePeriod/gml:beginPosition Extent
time_coverage_end metadata/timeCoverage/end /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:temporalElement/gmd:EX_TemporalExtent/gmd:extent/gml:TimePeriod/gml:endPosition Extent
time_coverage_duration metadata/timeCoverage/duration /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:temporalElement/gmd:EX_TemporalExtent/gmd:extent/gml:TimePeriod/gml:beginPosition provides an ISO8601 compliant description of the time period covered by the dataset. This standard supports descriptions of durations. Extent
time_coverage_resolution metadata/timeCoverage/resolution Extent
standard_name_vocabulary
The name of the controlled vocabulary from which variable standard names are taken.
metadata/variables@vocabulary /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:thesaurusName/gmd:CI_Citation/gmd:title/gco:CharacterString
Text Search
license Describe the restrictions to data access and distribution. metadata/documentation[@type="rights"] /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:resourceConstraints/gmd:MD_LegalConstraints/gmd:useLimitation/gco:CharacterString

Suggested

Attribute Description THREDDS ISO 19115-2 OGC CSW Rubric Category
contributor_name
The name and role of any individuals or institutions that contributed to the creation of this data.
metadata/contributor
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:individualName/gco:CharacterString
Responsible Party
contributor_role
metadata/contributor@role /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:role/gmd:CI_RoleCode
="principalInvestigator" | "author"
Responsible Party
publisher_name
The data publisher's name, URL, and email. The publisher may be an individual or an institution. metadata/publisher/name
/gmi:MI_Metadata/gmd:distributionInfo/gmd:MD_Distribution/gmd:distributor/gmd:MD_Distributor/gmd:distributorContact/gmd:CI_ResponsibleParty/gmd:organisationName/gco:CharacterString
and/or
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:individualName/gco:CharacterString
CI_RoleCode="publisher"
and/or
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:descriptiveKeywords/gmd:MD_Keywords/gmd:keyword/gco:CharacterString with gmd:MD_KeywordTypeCode="dataCenter"
Responsible Party
publisher_url
metadata/publisher/contact@url
/gmi:MI_Metadata/gmd:distributionInfo/gmd:MD_Distribution/gmd:distributor/gmd:MD_Distributor/gmd:distributorContact/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:onlineResource/gmd:CI_OnlineResource/gmd:linkage/gmd:URL
and/or
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:onlineResource/gmd:CI_OnlineResource/gmd:linkage/gmd:URL
CI_RoleCode="publisher"
Responsible Party
publisher_email
metadata/publisher/contact@email /gmi:MI_Metadata/gmd:distributionInfo/gmd:MD_Distribution/gmd:distributor/gmd:MD_Distributor/gmd:distributorContact/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:address/gmd:CI_Address/gmd:electronicMailAddress/gco:CharacterString
and/or
/gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:citedResponsibleParty/gmd:CI_ResponsibleParty/gmd:contactInfo/gmd:CI_Contact/gmd:address/gmd:CI_Address/gmd:electronicMailAddress/gco:CharacterString
CI_RoleCode="publisher"
Responsible Party
date_modified
The date on which this data was last modified.
metadata/date[@type="modified"] /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:date/gmd:CI_Date/gmd:date/gco:Date
/gmd:dateType/gmd:CI_DateTypeCode="revision"
ModifiedResponsible Party
date_issued
The date on which this data was formally issued.
metadata/date[@type="issued"] /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:date/gmd:CI_Date/gmd:date/gco:Date
/gmd:dateType/gmd:CI_DateTypeCode="publication"
Responsible Party
geospatial_lat_units
Further refinement of the geospatial bounding box can be provided by using these units and resolution attributes.
metadata/geospatialCoverage/northsouth/units /gmi:MI_Metadata/gmd:spatialRepresentationInfo/gmd:MD_Georectified/gmd:axisDimensionProperties/gmd:MD_Dimension/gmd:resolution/gco:Measure/@uom Extent
geospatial_lat_resolution metadata/geospatialCoverage/northsouth/resolution /gmi:MI_Metadata/gmd:spatialRepresentationInfo/gmd:MD_Georectified/gmd:axisDimensionProperties/gmd:MD_Dimension/gmd:resolution/gco:Measure Extent
geospatial_lon_units
metadata/geospatialCoverage/eastwest/units /gmi:MI_Metadata/gmd:spatialRepresentationInfo/gmd:MD_Georectified/gmd:axisDimensionProperties/gmd:MD_Dimension/gmd:resolution/gco:Measure/@uom Extent
geospatial_lon_resolution metadata/geospatialCoverage/eastwest/resolution /gmi:MI_Metadata/gmd:spatialRepresentationInfo/gmd:MD_Georectified/gmd:axisDimensionProperties/gmd:MD_Dimension/gmd:resolution/gco:Measure Extent
geospatial_vertical_units
metadata/geospatialCoverage/updown/units /gmi:MI_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:extent/gmd:EX_Extent/gmd:verticalElement/gmd:EX_VerticalExtent/gmd:verticalCRS Extent
geospatial_vertical_resolution
metadata/geospatialCoverage/updown/resolution
Extent
geospatial_vertical_positive
metadata/geospatialCoverage@zpositive
Extent

Highly Recommended Variable Attributes

Attribute Description THREDDS ISO 19115-2
long_name A long descriptive name for the variable (not necessarily from a controlled vocabulary). metadata/variables/variable@vocabulary_name At present the ISO 19115-2 Standard supports only one name for a variable. Standard names can be provided as keywords with the appropriate thesaurus.
standard_name
A long descriptive name for the variable taken from a controlled vocabulary of variable names. metadata/variables/variable@vocabulary_name
units The units of the variables data values. This attributes value should be a valid udunits string. metadata/variables/variable@units /gmi:MI_Metadata/gmd:contentInfo/gmi:MI_CoverageDescription/gmd:dimension/gmd:MD_Band/gmd:units
coverage_content_type An ISO 19115-1 code to indicate the source of the data. The valid values in the MD_CoverageContentTypeCode list are image, thematicClassification, physicalMeasurement, auxiliaryInformation, qualityInformation, referenceInformation, modelResult, coordinate


Translation Revisions

Determining an Order of Precedence