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#98: Clarify constraints on ancillary variable use
----------------------------+-----------------------------------------------
 Reporter:  markh           |       Owner:  [email protected]
     Type:  defect          |      Status:  new                          
 Priority:  medium          |   Milestone:                               
Component:  cf-conventions  |     Version:                               
 Keywords:                  |  
----------------------------+-----------------------------------------------
 == 1. Title ==

     Clarify constraints on ancillary variable use

 == 2. Moderator ==

     required

 == 3. Requirement ==

     An ancillary variable should not be able to be referenced by a data
 variable in the case where the ancillary variable is defined with respect
 to NetCDF file dimensions which the data variable with the referencing
 ancillary_variables attribute is not defined with respect to.

     For example, if
 {{{
     data(x,y,z)

     anc(x,y,t)
 }}}
     then data may not have an attribute of
 {{{
     data:ancillary_variables = "anc"
 }}}

 The dimensions '''anc''' is defined with respect to are not the set or a
 subset of the dimensions '''data''' is defined with respect to so it
 cannot be an ancillary variable of '''data'''.

 == 4. Initial Statement of Technical Proposal ==

     [http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch03s04.html
 Ancillary Variables]

     '''additional text:'''

     For a variable in a NetCDF file to be referenced as an
 ancillary_variable by a data variable, this ancillary variable must be
 defined with respect to the set, or a subset of the dimensions defining
 the referencing data variable.

 == 5. Benefits ==

     The limitations on use of Ancillary Variables will be made clear and
 explicit and may be checked for compliance by software.

 == 6. Status Quo ==

     None Available. Currently compliant files may be created but not be
 understood by data users.

-- 
Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/98>
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