I can't imagine this change having any traction for the coordinate variable of time, as some of the answers show.
However, if one thinks about non-coordinate variables, standard_names of local_time and sidereal_time seem arguably useful.
Even allowing for Roy's perfectly valid arguments against the use of local time, at least if it's declared 'local_time' you can back out the appropriate information, if you have latitude and longitude, as Benno suggests. (Of course, that assumes the people in the ship are resetting the time zone at exactly the right moment ... )
Arguing against your request, since CF won't use ISO8601 in its coordinate time variable, the ISO local time interpretation does not come into play. That being the case, if you want anyone else in the CF community to be able to use your files without getting Really Irritated (see Roy's post), the nicest thing to do would be to create a separate local time variable that your scientist can use, and have the coordinate time be in UT. But I might guess your CF-reading applications won't support that view of the world (rotating the data around the local_time variable) gracefully.
In any case, it may be important going forward for us to be Really Clear about whether we're talking about the coordinate variable time, or an independent variable. Different rules can and will apply.
John
On Oct 22, 2010, at 08:46, Julia Collins wrote:
> Hello,
>
> Benno's comment provides a convenient entry point for me to describe a
> recent problem.
>
> On Fri, 22 Oct 2010, Benno Blumenthal wrote:
>> Note: if the time zone is omitted the default is UTC, and if both time
>> and time zone are omitted the default is 00:00:00 UTC.
>
> To clarify, this is the time zone interpretation used in the current CF
> standard.
>
>> If we take that to constrain ISO8601 as well, we have made progress.
>> If we can do such a thing.
>
> Actually, I would like to propose the opposite: that CF follow the ISO
> standard of interpreting an omitted time zone as *local time*. I have some
> raster data which represents values at 0400 local time across many time
> zones. The time units that I think make intuitive sense are:
>
> time:units = "days since 1601-01-01T00:00:00" ;
>
> No time zone is indicated, therefore according to the ISO standard I'm
> referring to local time. An example time unit for this data set would be
> one that decodes to 2007-07-19 04:00. From the ISO point of view, this
> means 0400 at local time. From the CF point of view, since a time zone
> omission implies UTC, my time value is interpreted as 0400+00:00 (0400
> UTC), which is incorrect. The correct UTC value varies over the x axis. If
> instead I want to indicate a local time in CF, what local time do I use?
> The only place I see to indicate time zone is in the time units string
> itself. I haven't figured out a way in CF to describe the zone for each
> data point. There's no doubt a way to do this using cells, but that comes
> at a cost of complexity. The most elegant solution I see is to follow the
> ISO assumptions, which allows me to indicate a generic local time in the
> time units string.
>
> Part of my argument for following the ISO approach to time zone
> specification is that it is the *ISO* approach. My guess is that when it
> comes to time interpretation by a wide audience, ISO is perceived (and
> known) as a more general standards body than CF. I don't think it's wise
> for CF to constrain ISO. Rather, the established ISO standards should
> be constraining CF.
>
> I know that switching to the ISO convention for interpreting the meaning of
> an existing (or not) time zone indicator is an unpleasant suggestion, since
> it's not backwards compatible in the least. However, I think this is a
> change that should be considered. For now, I've resigned myself to
> creating files that are not quite CF-compliant.
>
> Julia
> --
> Julia Collins
> National Snow and Ice Data Center
> http://nsidc.org/
> collinsj at nsidc.org
> +1 303.492.6405
>
> _______________________________________________
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John Graybeal <mailto:jgraybeal at ucsd.edu>
phone: 858-534-2162
System Development Manager
Ocean Observatories Initiative Cyberinfrastructure Project:
http://ci.oceanobservatories.org
Marine Metadata Interoperability Project:
http://marinemetadata.org
Received on Fri Oct 22 2010 - 15:52:35 BST
