Jonathan,
I didn't see your initial statement that you reference as enforcing
anything on data consumers, or I would have raised this earlier.
Let's think about this in a simpler context. If I, as a data producer,
store values in a data variable in units of kilograms and designate it
as such with the units attribute, that doesn't mean every data consumer
should feel like they must display the values as kilograms. They are
free to select the units they prefer and, as long as they do the
conversion correctly, that is completely fine.
In a more complex and somewhat analogous case, if I as a data producer
store my geographic coordinates in latitudes and longitudes, and
properly designate the units and the reference datum that I used, data
consumers can display my data using those latitudes and longitudes, or
they can display my data using a projected coordinate system where they
have converted my latitudes and longitudes into X and Y values, or
whatever else they choose (and I hope that whatever else they might do
is valid!). What I must do as a data producer is accurately identify the
geographic Coordinate Reference System that is the basis for my
geographic coordinate values (and then make sure that my coordinate
values are correct if I started in some different CRS).
A properly formed time variable needs to have contents that are metric
(by that I mean that you can do differencing math with the values), and
if it contains real-world time observations it needs to be anchored to a
recognizable point in history. The system we have in place using elapsed
time values with a reference epoch does just that.
The decision about how to represent the time values as timestamps for
display purposes should be left up to the data consumer. You may have
used a reference epoch expressed using the Proleptic Gregorian calendar,
but if I would rather express timestamps on my plots using the Julian
calendar, that's my business. Perhaps the discipline I am working in has
a convention of using Modified Julian Days, so I am going to convert
everything to days since 1858-11-17 00:00:00.
Whatever my choice of output form for times, it is crucial that I know
where the elapsed time values stored in my time variable are anchored in
history, and that is what we should be trying to make clear with the
units and calendar attributes (and any other time-related attributes
that might arise).
Grace and peace,
Jim
On 6/9/15 1:21 PM, Jonathan Gregory wrote:
> Dear Jim
>
> You wrote
>> The calendar only specifies how the reference date and time
>> are to be interpreted. It says nothing about either the time
>> variable values or the decoding that should be used to turn those
>> elapsed time values into dates and times. That choice is entirely up
>> to the data consumer. If a data producer started with a set of
>> Julian calendar dates and created a time variable, and a data user
>> prefers to use Proleptic Gregorian dates, there is no problem. One
>> is not more correct than the other.
> You are right to point to this as a point of disagreement. I thought we had
> discussed this earlier e.g. in
> http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2015/058224.html
> I wrote
>> Clarify that in the CF convention the choice of "calendar" implies the
>> particular set of rules that is used to convert between date-times (YYYY-MM-DD
>> hh:mm:ss i.e. sets of six numbers) and time coordinates in units of elapsed
>> time since a reference time.
> and I believe that this arose from an earlier discussion about this being a
> CF-specific use of the term "calendar". Maybe I have misunderstood you now.
>
> I think the data producer is the person who decides what the data means. If
> the producer has Julian calendar timestamps and encodes with Julian rules
> as a time coordinate variable, the data-user is wrong to decode them with
> any other rules into timestamps or interpret them as being in any other
> calendar. Why would that be a useful thing to do? I agree with your earlier
> posting and email that there is a range of timestamps which refer to the same
> points in time in the Gregorian and Julian calendars (long ago, before they
> drifted apart) so for that range of dates it would not matter if the data-
> user changed the calendar, since they're indistinguishable. But that is a
> special case. If you come up to present, a given time-stamp refers to a
> different instance in time in the Gregorian and Julian calendars, just like
> it does between UTC and GPS calendars. For model calendars, it would be
> nonsense for time coordinates encoded in the 360-day calendar to be decoded
> in the proleptic Gregorian calendar, for example.
>
> Perhaps we view time coordinates in different ways? I think the timestamps
> are the primary information, and the time coordinates are an encoded version.
> We do it like that for efficiency of storage, and convenience and robustness
> of processing, since string-valued timestamps are relatively awkward. It has
> also the great advantage that the encoded time coordinate is also an elapsed
> time variable, so it can be used to check monotonicity and for calculations.
> This is a common need, since time is a continuous coordinate.
>
> Best wishes
>
> Jonathan
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Received on Wed Jun 10 2015 - 07:51:21 BST
