When one data variable provides metadata about the individual values of another data variable it may be desirable to express this association by providing a link between the variables. For example, instrument data may have associated measures of uncertainty. The attribute ancillary_variables is used to express these types of relationships. It is a string attribute whose value is a blank separated list of variable names. The nature of the relationship between variables associated via ancillary_variables must be determined by other attributes. The variables listed by the ancillary_variables attribute will often have the standard name of the variable which points to them including a modifier (Appendix C, Standard Name Modifiers) to indicate the relationship.

  float q(time) ;
    q:standard_name = "specific_humidity" ;
    q:units = "g/g" ;
    q:ancillary_variables = "q_error_limit q_detection_limit" ;
  float q_error_limit(time)
    q_error_limit:standard_name = "specific_humidity standard_error" ;
    q_error_limit:units = "g/g" ;
  float q_detection_limit(time)
    q_detection_limit:standard_name = "specific_humidity detection_minimum" ;
    q_detection_limit:units = "g/g" ;