Karl,
On 6/29/15 2:00 PM, Karl Taylor wrote:
Hi all,
I haven't followed all this as closely as I would have liked, but will
hazard some comments anyway:
1. I think we should require that elapsed time (recorded by the
time-coordinate in CF files) be correct no matter what the calendar.
So samples taken at a specific interval have identical
time-differences (calculated from the difference between successive
time coordinate values) whether or not the a leap second was
introduced (or dropped?) or not.
I agree, in principle. The way leap seconds worm their way into elapsed
times is by using the "wrong" software when calculating elapsed times
from timestamps. POSIX (that is, Unix/linux) time conversion functions
do not normally account for leap seconds, so if you use them with UTC
times, you put yourself at risk a couple of different ways.
2. I think we should allow for basetime (the reference time stamp) to
be recorded either as UTC or GPS. Absence of either of these would
imply "GPS", which will therefore apply to all data written until now
under CF.
This is great for real-world acquired times, but model times are in a
non-real time system - neither GPS nor UTC. There is also the
possibility of reference times recorded in the TAI time system, but we
can handle that by adding yet another calendar (and so on for sidereal
time, etc, etc).
3. Folks *generating* CF files should use algorithms that correctly
convert their timestamps to elapsed time (which is recorded in the
files). Then users can regenerate the timestamps correctly by looking
at whether UTC or GPS (or neither) appears.
They should, but many haven't in the past, and the added complexity of
getting UTC time conversion functions that understand leap seconds (one
is going to be added at midnight July 1!) is overkill. When your time
resolution is on the order of 1 hour, none of this matters.
4. Couldn't all of the above be simply accommodated with a single
"gregorian" calendar, but with the basetime (reference time stamp in
the units attribute) including either "UTC" or "GPS" at the end (or
neither for compatibility with previous written CF data)? Examples:
"days since 1990-1-1 0:0:0UTC" or "days since 1950-1-1 0:0:0 GPS"
(which would be equivalent to "days since 1950-1-1 0:0:0"
This would be another way to tackle it. In the past, CF also allowed for
time zone offsets, so you would need to add that as well.
5. Data would not be considered CF-compliant if the elapsed time
(recorded by the time coordinate) were incorrect because it had been
incorrectly converted by the data provider. "UTC" would indicate that
the basetime and conversion of elapsed time to timestamps should
follow the rules of UTC and include leap seconds. "GPS" (or absence
of "UTC" for backward compatibility) would indicate that thebasetime
and conversion of elapsed time to timestamps should follow the rules
of GPS and *not* include leap seconds.
Note that UTC and GPS in the reference times (or in the calendar) don't
say anything about how to create timestamps. It tells you how to read
the reference time. How you produce timestamps from the elapsed times is
up to the data consumer. It is entirely correct to take a time variable
with a reference time in GPS, correctly convert the reference time to
UTC, and then produce UTC timestamps. The information in the time
variable tells me what I have in hand (what the data producer did), not
what I'm supposed to do with it.
I'm sure I have missed some important use case where the above simple
scheme would be inadequate. (Or perhaps I'm just completely out to
lunch, in which case please forgive me).
From my perspective there are two gaps. One is model time, which is
entirely non-physical (has no reference point that ties it into the real
world), and the other is the very real and likely continuing case where
the (in-)precision of the time measurements makes the whole thing moot.
I'm thinking that Nan's suggestion of specifying an uncertainty for the
elapsed time values (which when left off could imply that "anything
goes" as far as leap seconds is concerned) may be an elegant way through
this.
We could (whether in the calendar or the units) have:
* gregorian, which would not take an uncertainty attribute and would
indicate that the time base and precision are unknown (good for
models and for backward compatibility)
* gregorian + utc, which would take an uncertainty attribute and would
indicate that the reference time stamp is expressed in UTC, and that
elapsed times have no artifacts to the level of the uncertainty
(true UTC with "wrong" conversions but only accurate to 1 minute
would validly fit in this category)
* gregorian + gps, which would be like gregorian + utc, except that
the reference time stamp is expressed in GPS.
Grace and peace,
Jim
best regards,
Karl
On 6/29/15 9:21 AM, Jonathan Gregory wrote:
Dear Tim and Nan
If I have understood correctly, I think your two emails suggest that we do need
a distinction of the precise and imprecise cases. As usual, I believe that CF
should not prescribe to users what they should do; its aim is to allow them to
describe what they have done. Different levels of precision are needed for
different datasets.
Following the emails that Jim and I exchanged, we could distinguish:
gregorian: Real world-times, but without specifying whether UTC or GPS
timestamps are intended, nor whether the encoding was done with or without leap
seconds. The decoded times could differ by several seconds from UTC. I think
this is Nan's use-case.
gregorian_nls: UTC timestamps were encoded without leap seconds, with a
reference UTC timestamp. I think this is Tim's use-case. This is not accurate
according to UTC but it can be decoded precisely as intended. Jim points out
that it's not a real-world calendar, but it's not far off.
Have I correctly described these as your cases?
In addition, we propose two other calendars:
gregorian_utc: The encoded and reference timestamps are UTC, and the encoding
is done with leap seconds allowed for. Hence the time coord is an accurate
elapsed time.
gregorian_gps: The encoded and reference timestamps are GPS, and the encoding
is done without leap seconds. Again, the time coord is accurately elapsed time.
I think this is the use-case which originally started this thread!
Best wishes
Jonathan
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