Hi all,
I would argue that time coordinates should always go somewhere in the
middle of the interval, not at either end.
Certainly, putting the time coordinate at one end of the interval is the
easy and unambiguous choice when generating data files. But in my
experience, it tends to cause confusion and problems when *using* the
files, particularly with software that automatically generates labels
and when aggregating the data to longer timescales.
The basic problem is that a time coordinate at the end of the interval
can be ambiguous to the end user as to whether it's at the beginning or
the end, and if you're not the person who created the data, it can take
a fair amount of digging to figure out which it is. Worse, it's easy
for the user to make an incorrect assumption and not realize it.
Putting the time coordinate in the middle of the interval makes this
kind of confusion much less likely, because it's more in line with human
intuition about what interval the coordinate belongs to.
Using the midpoint can require more logic on the data creator's part to
get right (e.g., what do you do when the interval is an even number of
cycles long), but I think it's a much better approach in terms of usability.
Cheers,
--Seth
On 4/30/2015 2:13 AM, David Charles Hassell wrote:
> Hello Chris, Charlie, Karl, ....
>
> Personally, I prefer to use the first (or last) year, as this is
> unambiguous. Ambiguities about how to define the mid-year could arise
> depending on whether number of years in the climatology is odd or
> even. The mid-year could, of course, be carefully defined, but the
> definition would be arbitrary, I think.
>
> (I used "year" here, but the same argument applies to "day")
>
> All the best,
>
> David ________________________________________ From: CF-metadata
> [cf-metadata-bounces at cgd.ucar.edu] on behalf of Little, Chris
> [chris.little at metoffice.gov.uk] Sent: 30 April 2015 08:58 To:
> cf-metadata at cgd.ucar.edu Subject: [CF-metadata] Is there ambiguity in
> labelling climatological time. Was: CF-metadata Digest, Vol 144,
> Issue 25
>
> Dear Charlie, Karl, and other CF'ers,
>
> If you can all agree on a preferred convention for whether the first,
> last, mid, or some other date is used to label a climatology, or a
> way of unambiguously labelling which has been used, it would make
> lots of people happier.
>
> Does anyone know of any WMO Climate Commission, or Commission for
> Basic Systems, guidance?
>
> Best wishes, Chris
>
> -----Original Message----- From: CF-metadata
> [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of
> cf-metadata-request at cgd.ucar.edu Sent: Thursday, April 30, 2015 7:44
> AM To: cf-metadata at cgd.ucar.edu Subject: CF-metadata Digest, Vol 144,
> Issue 25
>
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> Today's Topics:
>
> 1. Is there ambiguity in labeling climatological time coordinates?
> (Charlie Zender) 2. Re: Is there ambiguity in labeling climatological
> time coordinates? (Karl Taylor) 3. Ancillary variables in coordinate
> variables (latitude, longitude, ...) (Kristian Sebasti?n)
>
>
> ----------------------------------------------------------------------
>
> Message: 1 Date: Wed, 29 Apr 2015 17:11:07 -0700 From: Charlie
> Zender <zender at uci.edu> To: CF Metadata Mail List
> <cf-metadata at cgd.ucar.edu> Subject: [CF-metadata] Is there ambiguity
> in labeling climatological time coordinates? Message-ID:
> <5541731B.50800 at uci.edu> Content-Type: text/plain; charset=utf-8;
> format=flowed
>
> Dear CF'ers,
>
> The draft 1.7 conventions example Example 7.8. Climatological seasons
> has the following for the time coordinate:
>
> time="1960-4-16", "1960-7-16", "1960-10-16", "1961-1-16" ;
>
> All else being equal, are the values
>
> time="1975-4-16", "1975-7-16", "1975-10-16", "1976-1-16" ;
>
> also be acceptable for this same example?
>
> The underlying question is whether there is permissible ambiguity in
> the time coordinate values, or if for some reason the beginning year
> (1960) must be used as in this example. An alternative choice that
> seems reasonable to me is the use of the midpoint year (1975). I'm
> unsure whether 1960 was chosen arbitrarily or because one is expected
> to apply the minimum operation discussed in this example (seasonal
> minimum temperature) to the values of the time coordinate as well.
>
> Thanks, Charlie -- Charlie Zender, Earth System Sci. & Computer Sci.
> University of California, Irvine 949-891-2429 )'(
>
>
> ------------------------------
>
> Message: 2 Date: Wed, 29 Apr 2015 18:22:45 -0700 From: Karl Taylor
> <taylor13 at llnl.gov> To: cf-metadata at cgd.ucar.edu Subject: Re:
> [CF-metadata] Is there ambiguity in labeling climatological time
> coordinates? Message-ID: <554183E5.1080104 at llnl.gov> Content-Type:
> text/plain; charset="windows-1252"; Format="flowed"
>
> Hi Charlie,
>
> I think the only guidance CF provides is:
>
> "The time coordinates should be values that are representative of
> the climatological time intervals, such that an application which
> does not recognise climatological time will nonetheless be able to
> make a reasonable interpretation"
>
> I think for your case any consecutive dates within the
> climatological period would do, but like you I'd probably choose the
> middle year (or perhaps the first year, as in the example).
>
> Hope others will correct me if I'm wrong.
>
> Karl
>
> On 4/29/15 5:11 PM, Charlie Zender wrote:
>> Dear CF'ers,
>>
>> The draft 1.7 conventions example Example 7.8. Climatological
>> seasons has the following for the time coordinate:
>>
>> time="1960-4-16", "1960-7-16", "1960-10-16", "1961-1-16" ;
>>
>> All else being equal, are the values
>>
>> time="1975-4-16", "1975-7-16", "1975-10-16", "1976-1-16" ;
>>
>> also be acceptable for this same example?
>>
>> The underlying question is whether there is permissible ambiguity
>> in the time coordinate values, or if for some reason the beginning
>> year (1960) must be used as in this example. An alternative choice
>> that seems reasonable to me is the use of the midpoint year (1975).
>> I'm unsure whether 1960 was chosen arbitrarily or because one is
>> expected to apply the minimum operation discussed in this example
>> (seasonal minimum temperature) to the values of the time
>> coordinate as well.
>>
>> Thanks, Charlie
>
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>
> Message: 3 Date: Thu, 30 Apr 2015 08:43:54 +0200 From: Kristian
> Sebasti?n <ksebastian at socib.es> To: cf-metadata at cgd.ucar.edu Subject:
> [CF-metadata] Ancillary variables in coordinate variables (latitude,
> longitude, ...) Message-ID:
> <CAGfa=MDW7dpnpZj=EOJgGrK2eQ9o1Nvtz_dcUx-EeMH4MyBWXw at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Dear CF community,
>
> We have some dataset with quality controls applied to the coordinate
> variables, such as latitude and longitude coordinate. The result are
> quality control variables that we associate as ancillary variables of
> the coordinate variables with the ancillary_variables attribute. For
> example, the LAT coordinate variable has the ancillary variable
> QC_LAT. The dataset
> http://thredds.socib.es/thredds/dodsC/drifter/surface_drifter/drifter_svp052-ime_svp017/L1/2014/dep0001_drifter-svp052_ime-svp017_L1_2014-05-25.nc
>
> The cf-conventions clarifies the use of the ancillary_variables
> attribute for data variables but not for coordinate variables. My
> question is, Is the ancillary_variables attribute in coordinates
> variables compliant with the cf-conventions?
>
> Best regards,
>
> Kristian
>
> --
>
> Kristian Sebastian Blalid SOS Division: Data Center Technical Tel:
> 971439860 - Fax: 971439979 E-mail: kristian.sebastian at socib.es
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