Charlie,
My thought is that you should not put a cell_methods attribute on a true
coordinate variable. A true coordinate variable is like a number line.
It is a set of numbers that should be thought of as independent of the
acquired data. This gets a bit fuzzy when it comes to a time variable
where the times come from the acquired data, but I think the concept
should be adhered to. If the time values truly end up being shifted
within each time bounds based on the distribution of the times of the
acquired data values, then it seems to me that the variable holding
these time values should be made an auxiliary coordinate variable
instead of a true coordinate variable. (That is, give the variable a
name that is different than the dimension name and associate it with the
data variable via the coordinates attribute.) Such a time variable would
be the equivalent of a latitude or longitude auxiliary coordinate
variable, where fill values, cell_methods, etc can be applied, as they
represent either acquired data values or transformations of underlying
independent coordinates. (The underlying independent coordinates don't
have to be present in the file.)
Anyway, that's my view. What do others think?
Grace and peace,
Jim
On 5/31/15 8:17 PM, Charlie Zender wrote:
Aloha CFers,
Is it correct to add a cell_methods = "time: mean" attribute to the
time coordinate when the coordinate is averaged over time?
NCO's ncra does this (though the NERC CF checker doesn't like it).
It's clear from the CF docs that ncra time-averaging a variable like
wind(time) from an array to a degenerate (size 1) array should
add a cell_methods = "time: mean" attribute to wind.
Yet should it add cell_methods to the time coordinate itself?
My take is that it should. The time_bounds variable, if any,
will show the original extent of the temporal range, and the time
coordinate value contains the mean of the original range.
Yet an NCO user is making a good case that cell_methods are only
for non-coordinate variables. His point is that many variables with
different cell_methods can all contain the same coordinate, so that
the coordinate should not have cell_methods. My response is that
cell_methods refers to the bounds variable of the coordinate, not
the coordinate values themselves.
Is the question clear? If not, I can supply CDL...
Mahalo,
Charlie
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