[CF-metadata] 2. Re: [cf-satellite] Sharing quality flags among multiple variables (Jonathan Gregory)

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 18 Nov 2011 15:42:37 +0000

Dear Martin

> what is the difference between a mean value and an observation count? You may add the 25th percentile to this list as well. As far as I can tell, the cell_methods attribute should be best suited for all of these and I don't see a need to work with standard_name modifiers

Though this has not been thoroughly debated,
I think the reasons why there are these two different mechanisms are that the
two functions are distinguished like this:

* cell_methods represents subgrid variation. They always imply that the
data variable formerly had a higher dimensionality or a higher resolution,
and they refer to one or more dimensions of the data on which the reduction
or collapse was done. The relationships indicated by standard_name modifiers
do not refer to particular dimensions of the data.

* The operations cell_methods records are done on the data in the variable
itself. Ancillary variables, described by standard_name modifiers, are extra
information about the data in the variable. This cannot be inferred from
the data; they are metadata, really, not a statistical reduction of data.

However, I agree there's a similarity. In particular, both of them were
motivated by a desire to avoid proliferation of standard_names because of the
need to describe very common operations that could be applied to anything,
and both of them could modify the units.

Best wishes

Jonathan
Received on Fri Nov 18 2011 - 08:42:37 GMT