Hi Steve,
I agree with all of these, as it seems does the community in general.
For the record, I’d like to add another objection, that might also
argue against using ISO8601 for time **metadata**. I’m not sure it’s
been explicitly called out yet that UDUNITS time specifications are
inconsistent with ISO8601 in more than one way. For example, “1900-1-1
0:0:0” is legal in UDUNITS, but not ISO8601 (fields must be
zero-padded and there must be a “T” in between the date and time).
This is in addition to the different defaults to local time or UTC
(actually I guess this is a property of CF, not UDUNITS?)
Anyway, it seems that it might be confusing to introduce two string
representations of date/times into CF, even for metadata. Are there
any important cases in which ISO8601 is more expressive than UDUNITS
syntax, or could we stick to UDUNITS for everything?
Cheers, Jon
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Steve Hankin
*Sent:* 22 October 2010 02:06
*To:* CF metadata
*Subject:* Re: [CF-metadata] time as ISO strings
Hi Jon, Benno, John and other pals,
Since this email thread already contains an element of informal voting
I'll cast my ballot: CF is a better standard *WITHOUT *admitting ISO
date strings as an encoding for time coordinates. My opinion is is
based upon this outlook:
* '/To create quality software, the ability to say “no” is usually
far more important than the ability to say “yes.”/'
(http://queue.acm.org/detail.cfm?id=1142044)
Bloat and run away complexity are a continual threat to the quality of
a standard as it evolves. I'd argue that the measure of whether a new
feature deserves inclusion should be whether it adds useful new
functionality and does so in a manner that is "clean" -- preserving
the consistency and simplicity of the standard to the degree feasible.
* Introducing ISO strings as coordinates adds no encoding power to
CF. It raises issues of precision but then fails to address them
adequately. (In fact it imports a host of ambiguities about
interpretation of precision as Jon pointed out to us in the
"metadata" versus "positioning" debate.)
* It fails the test of consistency, since it is not applicable to
virtually identical precision issues that exist for latitude,
longitude and vertical coordinates.
* the need to support two separate encodings (ISO and "days since
xxx") for the same date/time coordinate information would force
additional complexity into the standards documentation and into
clients that would hope to be generic CF applications. It wound
degrade interoperability for clients that did not support both
encodings
* it fails to address the multiple-calendar needs of CF. (creating
another inconsistency) (ISO 8601 does not standardize the
interpretation of dates prior to the 1582 Julian-Gregorian
discontinuity ... which likely means that library codes cannot
be trusted to give consistent results.)
* it does not even add a useful measure of convenience. The "-t"
option to ncdump already provides the needed convenience for
file readers. And for file creators, a new utility that would
translate ISO strings into CF time step values could probably be
written in less time than this email dialog has occupied. (If
this statement appears exaggerated consider that the effort to
develop this utility would be paid over and over in the clients
that would have to interpret this encoding.)
None of this is a comment on the utility of ISO date/time strings as
metadata. There are appropriate uses of ISO date/time strings in CF as
non-coordinate variables and attributes. The NO vote is in regard to
their use as CF coordinates.
- Steve
===================================
On 10/21/2010 11:57 AM, Jon Blower wrote:
Hi Benno,
2010-09 is not necessarily a precise specification of a month - time zones make it a little fuzzy for one thing. Separate to this, there are parallel conversations going on in the ISO/OGC community about what time strings actually mean. A metadata person might say that "2010-09" is simply a shorthand for the fuzzy concept of "September 2010" and does not represent a precise interval (i.e. a square-wave function that is 1 during September and 0 outside). Apart from the time zone issue which blurs the boundaries, this square-wave is simply not what humans mean when, for example, they tag a report as having been written in September 2010. It just distinguishes it from version 2 of the report, which was written in November. In this context, it's just a label with some temporal meaning.
These "metadata guys" are in discussion with the "positioning guys" who view date/times as precisely-defined positions within a temporal CRS. You may (or may not!) like to look at the GeoAPI mailing list, in which we are trying to figure out whether we can actually use the same Java types for both of these subtly-different views of date/times (we hope we can but haven't agreed). One might think that they are obviously the same thing, but I don't think so.
You *could* modify CF so that to represent data that are "representative of September 2010", you specify a nominal date half-way through September and set the bounds to the first and last instants of September. And perhaps use a new cell_methods of "representative". But the half-way point and the bounds would be quite (very) tedious to compute in the general case (months and years are of variable length for example and depend on the calendar system).
Of course, how the data is actually related to that interval is where the
notion of precision might come in
Actually, you've probably gathered that I also consider the notion of precision to apply to the interval itself, not just how the data relates to it.
This discussion repeats a bit of the previous discussion on this list entitled "bounds/precision for time axis". I like Jonathan's distinction between the concepts of temporal resolution and representivity:http://www.mail-archive.com/[email protected]/msg01341.html.
And just for completeness we should not that ISO8601 strings are not fixed-length, nor do they have a maximum length (in contrast to what I said before, sorry). So I can see some implementation challenges in NetCDF.
Cheers, Jon
-----Original Message-----
From:[email protected] <mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Benno Blumenthal
Sent: 21 October 2010 15:43
To: Steve Hankin
Cc: Jon Blower;[email protected] <mailto:[email protected]>
Subject: Re: [CF-metadata] New standard names for satellite obs data (time as
ISO strings)
While expressing precision in CF is an interesting issue, in this case
the Wikipedia quote is using the term in a different sense than I
(hopefully we) usually mean -- ISO8601 lets one express time intervals
succinctly in a single string, e.g. 2010-09 to mean all of september
2010, which is not an accuracy issue, it is a precise specification of
a larger interval. It lets you write 2010-09-01/10-05 as well, i.e.
it is not limited to intervals that involve special notational
boundaries. As Steve points out CF expresses this using a bounds
coordinate, i.e. giving the precise edges of each interval. Of
course, how the data is actually related to that interval is where the
notion of precision might come in, which cell methods/measures
addresses, perhaps inadequately for the purpose at hand.
ISO8601 is quite neat in the sense that it forces one to always
specify an interval, and CF software reading time bounds data and
rendering ISO8601 strings would do us all a lot of good.
Benno
On Wed, Oct 20, 2010 at 6:34 PM, Steve Hankin<[email protected]> <mailto:[email protected]> wrote:
Hi Jon,
Why do you see this as an issue of date-times as ISO strings in particular?
The same issues of precision are found in longitudes expressed as a
degrees-minutes-seconds string compared to a floating point. Or indeed to a
depth expressed as a decimal string of known numbers of digits. ("100.00"
communicates different precision than "100" though both a represented by the
same binary value.)
CF provides the bounds attribute and the cell methods/measures to clarify
(somewhat) these points. What is your proposal for improved representation
of precisions? And wouldn't a general improvement in how to specify
coordinate precision be preferable to a solution that applies to time, only?
- Steve
=============================
On 10/20/2010 9:41 AM, Jon Blower wrote:
Hi all,
I haven't followed this debate closely, but I've had cause to do a fair
amount of thinking (outside the CF context) on the pros and cons of
identifying date/times as strings or numbers. I could probably write a
very boring essay on this but in summary, they are not exactly
equivalent ways of representing the same information.
One way in which they are different is precision. A value of "x seconds
since y" has no implied precision - typically in programs we take the
precision to be milliseconds, but there's nothing to suggest this in the
actual metadata (anyone who tries to populate a GUI from CF metadata
struggles with this). Semantically it's a time instant; i.e. an
infinitesimal position in a temporal coordinate reference system.
However, an ISO8601 string can have various precisions. (The string
"2009-10" is not considered equivalent to "2009-10-01T00:00:00.000Z".)
> From Wikipedia (http://en.wikipedia.org/wiki/ISO_8601):
"For reduced accuracy, any number of values may be dropped from any of
the date and time representations, but in the order from the least to
the most significant. For example, "2004-05" is a valid ISO 8601 date,
which indicates May (the fifth month) 2004. This format will never
represent the 5th day of an unspecified month in 2004, nor will it
represent a time-span extending from 2004 into 2005."
I've argued before in a previous thread on this list that it would be
good to be able to specify the precision of time coordinates in terms of
calendar date/time fields (which isn't the same thing as providing a
tolerance value on the numeric coordinate value of a time axis).
I'm not saying that we should definitely allow time strings in CF, just
pointing out that they have some use cases we currently can't fulfil.
I'm not sure they are definitively "bad practice" in all cases.
(Regarding a technical point raised below, yes, it's a pain to represent
variable length strings in NetCDF, but there is a maximum length for
ISO8601 strings.)
Hope this helps,
Jon
-----Original Message-----
From:[email protected]
<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Lowry, Roy K
Sent: 20 October 2010 10:00
To: Ben Hetland;[email protected] <mailto:[email protected]>
Subject: Re: [CF-metadata] New standard names for satellite obs data
Dear All,
As others have said, I think this debate is irrelevant as there should
be no need for string timestamps in NetCDF. Providing a Standard Name
only encourages what I consider to be bad practice.
Cheers, Roy.
-----Original Message-----
From:[email protected]
<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Ben Hetland
Sent: 20 October 2010 09:14
To:[email protected] <mailto:[email protected]>
Subject: Re: [CF-metadata] New standard names for satellite obs data
On 19.10.2010 16:27, Seth McGinnis wrote:
What about using 'date' for string-valued times? That was my homebrew
solution when I was considering a similar problem.
If I may butt in and contribute here, I usually prefer names like
'datetime' or 'timestamp' in cases like this, because 'date' is
potentially confusing. It may not be immediately obvious to a future
reader (or programmer) that a variable called 'date' supports points in
time down to for example seconds of accuracy.
(Note that string data is a big pain to deal with in NetCDF-3, because
you're limited to fixed-length character arrays. You need to use
NetCDF-4 / HDF5 to get Strings as a data type.)
(It may not be such a practical issue with ISO 8601 strings, as a
reasonable max. length can be determined, I presume.)
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