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Ticket Resolution Summary Owner Reporter
#76 fixed More than one name in Conventions attribute davidhassell Dave.Allured


It is proposed to support the listing of more than one convention name in the Conventions global attribute. As of CF-1.6, only the CF identification by itself is allowed. The option of a list has been in Unidata's recommendations for a long time; see references. There was some community support for this in the recent mailing list discussion.


I would like to present this in two parts. This trac ticket is part 1, which proposes to adopt verbatim, the Unidata language for listing more than one convention name. The original Unidata recommendation allows the list of names to be separated by either spaces or commas.

Part 2, which I plan to file as a separate ticket, would propose more detailed parsing rules and would probably replace the syntax description of part 1. The main objectives of part 2 would be to accommodate both unambiguous machine parsing, and flexibility in allowable characters in convention names.

Why two tickets? I think that part 1 has a better chance of approval. It accomplishes the most important objective of multiple conventions in a workable way. If part 2 succeeds, it can replace part 1, sooner or later.

The mailing list discussion beginning December 2011 should be considered part of the record for this proposal.

Text of proposed changes

Two new paragraphs are to be added at the end of the current section 2.6.1, Identification of Conventions:

It is possible for a netCDF file to adhere to more than one set of conventions, even when there is no inheritance relationship among the conventions. In this case, the value of the Conventions attribute may be a single text string containing a list of the convention names separated by blank space (recommended) or commas (if a convention name contains blanks). This is the Unidata recommended syntax from NetCDF Users Guide, Appendix B.

When CF is listed with other conventions, this asserts the same full compliance with CF requirements and interpretations as if CF was the sole convention. It is the responsibility of the data-writer to ensure that all common metadata is used with consistent meaning between conventions.

In the conformance document, add a new second line under 2.6.1 Identification of Conventions:

  • The Conventions attribute may be a single text string containing a list of convention names separated by blank space or commas, one of which shall be the full CF string as described above.


Current language of section 2.6.1, Identification of Conventions (CF-1.6)

NetCDF Users Guide, Appendix B, Attribute Conventions

List of registered conventions, NetCDF website

Ticket #27, Use namespace tags to include other conventions, 2008
(Previous proposal and discussion of Conventions attribute, not yet adopted)

#75 fixed fix documentation and definitions of 3 grid mapping definitions davidhassell etourigny

The definitions and/or documentation for 3 grid mappings cause confusion and inconsistency when translating to/from WKT.

This ticket is the result of a discussion in the mailing list: .

Here is a summary of suggested changes:

1) lambert_conformal_conic

The 1SP (1 standard_parallel) case is not properly documented. There should be a link to and mention that latitude_of_projection_origin==standard_parallel (and the WKT "Scale factor at natural origin" = 1).

In the case that scale_factor is not 1, then the 2SP version should be used (as scale_factor is not part of this projection's parameters in CF).

Also, the EPSG codes should included in "Notes" (9801 for 1SP and 9802 for 2SP).

2) lambert_cylindrical_equal_area

The scale_factor_at_projection_origin variant has no WKT nor PROJ.4 equivalent (they both require standard_parallel) and there are no known examples using this variant. It should be dropped from the CF spec, or at least deprecated.

3) polar_stereographic

The 2 variants (standard_parallel or scale_factor_at_projection_origin) are not well documented.

The standard_parallel variant corresponds to EPSG Polar Stereographic (Variant B) (EPSG dataset coordinate operation method code 9829), while the scale_factor_at_projection_origin variant corresponds to EPSG Polar Stereographic (Variant A) (EPSG dataset coordinate operation method code 9810).

This information should be included in the "Map parameters" and "Notes" sections, to be consistent with other definitions such as mercator.

#74 fixed Allow sharing of ancillary variables among multiple data variables davidhassell rhorne@…

Currently it is not possible for multiple data variables to share an ancillary variable.

Augment the conventions to allow two or more data variables to share an ancillary variable.

Here is an example of a convention, which dovetails with the existing conventions, to provide this capability:

float swpt(lat,lon);

. .

swpt:standard_name="sea_water_potential_temperature"; swpt:ancillary_variables="nobs flags"; . .

float sws(lat,lon);

. . sws:standard_name="sea_water_salinity"; sws:ancillary_variables="nobs flags"; . .

int nobs(lat,lon);

. . nobs:standard_name="sea_water_potential_temperature sea_water_salinity number_of_observations"; . .

int flags(lat,lon);

. . flags:standard_name="sea_water_potential_temperature sea_water_salinity status_flag"; flags:flag_values = 0, 1, 2; flags:flag_meanings = "valid invalid unknown"; . .

Change the text in the CF metadata conventions section 3.3 that reads

"A standard name is associated with a variable via the attribute standard_name which takes a string value comprised of a standard name optionally followed by one or more blanks and a standard name modifier (a string value from Appendix C, Standard Name Modifiers)."


"A standard name is associated with a variable via the attribute standard_name which takes a string value that can have either of two forms. The first form is a standard name alone. The second form is a blank-separated list beginning with one or more standard names and ending with a single standard name modifier (i.e. standard_name [standard_name ...] standard_name_modifier). The standard name modifiers are defined in Appendix C, Standard Name Modifiers. This second form permits a single variable to provide ancillary data (see section 3.4) for two or more variables where each has a unique standard name."

Changes to CF software tools are required. For example, the CF_checker, which validates the standard_name attribute needs to change. However, it does not require any change to software that uses the complete attribute simply as an identifying string (e.g. to label plots, etc.)

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