[CF-metadata] [cf-satellite] Sharing quality flags amongmultiple variables

From: Jonathan Gregory <j.m.gregory>
Date: Sat, 5 Nov 2011 09:16:18 +0000

Dear all

Referring to Mike's comment. I agree that the ancillary_variables attribute
indicates that the status_flag variable is associated with its data variable,
and that alone identifies it to some extent, but (a) it doesn't specifically
indicate its purpose, since there could be more than one ancillary variable
for a given data variables (status flag, standard error, number of
observations, ...); (b) the status flag variable can be regarded as a data
variable in its own right, and as such needs a standard_name to be
self-describing; it is quite possible, for example, that variables for
status_flag or number_of_observations might be stored in different netCDF
files from the variables they describe, and then the correspondence would
depend on them being distinguishable e.g. by
  standard_name="sea_water_salinity number_of_observations"
  standard_name="sea_water_potential_temperature number_of_observations".
However, when the standard_name modifiers were introduced, I don't think we
foresaw the possibility that several data variables might need to share the
same ancillary variable e.g. when the number of observations for salinity and
temperature are the same.

Referring to Upendra's comment. We could introduce both changes, but I think
we should do that only if really necessary for existing examples that are
likely to represent common use-cases. We should not complicate the CF standard
more than we have to! I would say the same about Mike's ingenious scheme for
include-statements.

If it would serve the purpose that began this discussion, personally I would
favour the first solution, and generalise the use of the standard_name att,
like Edward said e.g.

float thetao(lat,lon);
  thetao:standard_name="sea_water_potential_temperature";
  thetao:ancillary_variables="nobs flags";
  thetao:units="degC";
float so(lat,lon);
  thetao:standard_name="sea_water_salinity";
  thetao:ancillary_variables="nobs flags";
  thetao:units="psu"; // not allowed currently---but that's a different story!
int nobs(lat,lon);
  nobs:standard_name="sea_water_potential_temperature sea_water_salinity number_of_observations";
int flags(lat,lon);
  flags:flag_values = 0, 1, 2;
  flags:flag_meanings = "accepted_value range_outlier failed_inversion_check";

That is, we would change the text in CF sect 3.3 that reads

"A standard name is associated with a variable via the attribute standard_name
which takes a string value comprised of a standard name optionally followed by
one or more blanks and a standard name modifier (a string value from Appendix
C, Standard Name Modifiers)."

to

"A standard name is associated with a variable via the attribute standard_name
which takes a string value that can have either of two forms. The first form
is a standard name alone. The second form is a blank-separated list beginning
with one or more standard names and ending with a single standard name
modifier (a string value from Appendix C, Standard Name Modifiers)
i.e. standard_name [standard_name ...] standard_name_modifier. This second
form permits a single variable to provide ancillary data (see Section 3.4) for
several variables that have various standard names."

Naturally this would require change to software, such as the CF_checker, which
analyses the standard_name att, but it would not require any change to
software which uses the complete att simply as an identifying string, to label
plots, etc.

What does you think?

Cheers

Jonathan
Received on Sat Nov 05 2011 - 03:16:18 GMT