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

From: Seth McGinnis <mcginnis>
Date: Mon, 10 Dec 2012 15:17:50 -0700

The datum should always be interpreted according to the calendar system
specified in the file.

That's how the time coordinate is handled in the output from every model
that I've seen (which is at least a dozen), so I'd say do it that way just for
the sake of backwards compatibility. But it's not clear to me that any
alternative is even possible. What would it mean to say that my time
coordinate uses a 360-day calendar, but that the start date is Jan 1, 1900
in the mixed gregorian calendar? Or worse, if the start date was Jan 31?
It doesn't really make any sense.

Likewise, units of months and years should also be interpreted according
to the specified calendar.

Using months and years as units is cautioned against in the spec, but if one
were to do it, I'd say the expectation of nearly every end user and data
provider would be that they would be in the same units as the calendar.
Certainly, if you had a file of monthly data with a units attribute of "months"

on the time variable, it would be obvious what was meant if the time
coordinate went 1-2-3-4-5, and very confusing if it had some kind of non-
integer spacing that corresponded to (days in month)*12/365.25...

Cheers,

--Seth


On Sun, 9 Dec 2012 19:55:39 +0000
 Jon Blower <j.d.blower at reading.ac.uk> wrote:
>
>Also, there are a couple of ambiguities in CF, which have also been brought up
>before but not resolved. Time axes are recorded as a sequence of numbers
>representing a certain number of intervals since a specified datum. The datum
>is part of the units string of the time axis and is recorded in a kind of
>pseudo-ISO format (e.g. "1970-1-1 0:0:0"). Should the datum always be
>interpreted as a date/time in the mixed Gregorian/Julian calendar (as
>specified by UDUNITS) or in the calendar system specified in the CF file
>(which would probably be more sensible)? Also, should interval lengths (e.g.
>months and years) be interpreted as the UDUNITS lengths (this is specified in
>CF but is not always helpful) or should these vary by calendar (which is again
>perhaps sensible, and some data providers do this)?
Received on Mon Dec 10 2012 - 15:17:50 GMT