[CF-metadata] CF calendars (was: problem with times in PSD dataset)

From: Jonathan Gregory <j.m.gregory>
Date: Sun, 9 Dec 2012 15:45:50 +0000

Dear Cecilia

Thanks for your posting. Are we in agreement? I think that we are.

> However, the current default is also problematic - reasons I see are:
> 1) it does send a message to modelers who find the default unusable,
> 2) because different calendar defaults are used on the modeling and
> data side, current tools are limited to particular calendars,
> affecting users,
> and 3) the mixed Julian-Gregorian calendar is an ugly beast to peg as the
> standard forevermore.

I agree with these drawbacks. If CF only dealt with the real world, obviously
the real-world calendar (mixed Julian-Gregorian) would be the default and only
calendar it had to deal with. However, at its outset CF was for models, many of
which do not use this calendar, which was nonetheless chosen as the default
because of udunits. I agree with you and others that providing such an
inconvenient default is unsatisfactory in retrospect. But here we are!

> it is a good idea to add another, strict Gregorian (error before 1582).

Thanks for seeing it like that. Yes, perhaps we should define what I suggested
as a new calendar: strict_gregorian, which is not allowed to have a ref date
before 1582, or to use negative time coordinates, or to describe dates before
1582 (which would be impossible given the first two conditions). (Note, I am
not personally the eponymous strict Gregory.)

In that case, we could change the default in the next release from gregorian
or standard to strict_gregorian. For software which is careful about versions,
this would mean that dates in the default calendar before 1582 would give an
error. This would be a safe failure, rather than a wrong answer.

> Some data sets would be CF compliant only under particular versions of CF.
> Is this the first time that a change to CF would have such an impact?

In the sense of old data becoming invalid in a *later* version, yes I think
it is. Of course new features always allow datasets which are invalid in an
*earlier* version.

In the failing cases, I think we agree that tools might offer the facility
> to provide the calendar if none was found.

> However, I imagine most tools would continue to assume the current default.
Yes, I expect so. They would still be unsafe, if datasets have been wrongly
coded with the default, but it would be no worse than now.

> This does not seem so bad to me. The removal of the default solution should
> not produce the nastier kinds of errors that you would get if you
> changed the default.

That's what I think, except that formally the above suggestion is a change to
to the default - one which installs a fail-safe mechanism.

Best wishes

Jonathan
Received on Sun Dec 09 2012 - 08:45:50 GMT