Hi Tim -
I don't think there's any difference between GPS and UTC times
before 1981-07-01, when the first leap second was accepted (if that's
the right term), so, if your reference time is before that date, you
can use the GPS calendar attribute, and not worry about it being used
to muck with your reference date.
I've argued that 'instrument and measurement datasets' generally have
time values that can't be accurate enough to care about leap seconds,
and you've expressed the opposite view. To be honest, my instrument
clocks are mysterious, at best, and I often have to override them. Then,
to top things off, I use Matlab to generate elapsed time for CF, and have
no clue whether that software knows anything about leap seconds.
Maybe an uncertainty attribute should be a best practice for time variables;
OceanSITES recommends doing that, but I'm not sure often it's included - or
noticed by data users.
Cheers - Nan
On 6/29/15 6:16 AM, Timothy Patterson wrote:
> I understand that for climate or forecast data that 30+ seconds of inaccuracy may not be significant, but even though the C and F in "CF Conventions" stand for "Climate and Forecast", the conventions are also being increasingly adopted for instrument and measurement datasets. In these cases, time accuracy at small time scales becomes more important, which is why seeing a proposed convention that allows the time to be written ambiguously (so that there may or may not be discontinuities or offsets) is rather disconcerting, like telling a climate scientist that the netCDF encoding of his temperature data may or may not have introduced an inaccuracy of a few Kelvin in some readings.
>
> Under the CF1.6 conventions, I believe the base time or epoch time was always expressed in UTC and the calendar attribute was applied to the encoded time count. Using this convention, I could specify a UTC start time and then have a simple GPS-like count of elapsed seconds (with no discontinuities introduced by leap seconds). Under the proposals for the 1.7 convention, this doesn't seem possible. The epoch and time count must both be expressed either as UTC or as GPS time and the only "mixed calendar" options introduce the above mentioned ambiguity.
>
> Regards,
>
> Tim Patterson
--
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* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
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Received on Mon Jun 29 2015 - 09:56:03 BST
