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62 scalar auxiliary coordinate clarifications taylor13 cf-conventions@… "This proposal is to clear up some apparent inconsistencies in the description of allowable attributes for auxiliary coordinate variables.
At the beginning of section 6.2 of the conventions, it states:
""In some situations a dimension may have alternative sets of coordinates values. Since there can only be one coordinate variable for the dimension (the variable with the same name as the dimension), any alternative sets of values have to be stored in auxiliary coordinate variables. For such alternative coordinate variables, there are no mandatory attributes, but they may have any of the attributes allowed for coordinate variables.""
But in paragraph 4 of Chapter 5 it states:
""The axis attribute is not allowed for auxiliary coordinate variables.""
in contradiction to the statement in section 6.2.
Furthermore, according to the definition of a scalar coordinate variable (section 1.2), it is supposed to be ""functionally equivalent to either a size one coordinate variable or a size one auxiliary coordinate variable."" The only way for this to be possible is for both types of scalar coordinates to include the same attributes. Currently the CF document forbids use of the axis attribute in conjunction with auxiliary coordinate variables, but allows its use for coordinate variables. To make the two consistent, the following change to paragraph 4 of Chapter 5 is proposed:
Replace ""The axis attribute is not allowed for auxiliary coordinate variables. Auxiliary coordinate variables ..."" by ""If an axis attribute is attached to an auxiliary coordinate variable, it can be used by applications in the same way the axis attribute is used in conjunction with coordinate variables. Note that if this attribute is missing, it may still be possible to determine if a particular dimension should be associated with the auxiliary coordinate variable. For example, auxiliary coordinate variables ...""
Note that in Example 5.2, the axis attribute ""X"" is associated with xc, and if the above change were adopted, the axis attribute could also appear in conjunction with the auxiliary coordinate variable ""lon"". Would this cause problems?
Also, some folks might misinterpret the paragraph at the end of section 2.4, which states that ""The advantage of using a coordinate variable is that all its attributes can be used to describe the single-valued quantity, including boundaries."" to mean that this is an ""advantage over a a size one auxiliary coordinate variable"", when in fact it means an ""advantage over *omitting* the scalar variable"". I therefore propose rewriting this sentence to read:
""The use of a scalar coordinate variable is encouraged, when appropriate, because the coordinate attributes (including axis attribute and the cell bounds) can be defined to more fully describe the quantity of interest.""
" defect closed high cf-conventions fixed