I've got a question and a thought to stir the pot with.
Is there any problem with having negative values for the dates, or are
we talking about having only positive and increasing values?
It seems that providing a way to specify the base unit of time would be
helpful from a conceptual standpoint, even if it is a pain (and
understandably avoided when possible) to handle in code. So if you want
your unit of time to be 1 kyear or Myear or Gyear, you could work in
that basis (with a corresponding loss of precision).
On 8/24/2011 9:16 AM, John Caron wrote:
On 8/24/2011 6:23 AM, Jonathan Gregory wrote:
Dear John
It seems to me it would be better to somehow denote the "epoch"
seperately, because its kind of silly keeping track of # millisecs
between two dates separated by 50 million years. plus its hard.
what about:
"01-01-01 12:00 epoch 50m BCE"
where the "epoch 50m BCE" is probably just carried along in the
string representation of the date.
I agree that that's a sensible way to do it for dates which are
separated
by relatively short periods in an epoch which is a long time ago.
However
it could be useful if the epoch wasn't just a string. If it was a
longinteger
number of calendar years (relative to the normal year 1) you would be
able
to interconvert dates expressed relative to different epochs. That
is, you
could tell that 10 calendar Myears since 01-01-01 epoch 20M BCE means
the
same as 0 calendar years since 01-01-01 epoch 10M BCE. This
conversion would
be accurate provided the time unit in calendar years is stored as an
integer.
Accuracy is lost if it is turned into floating-point milliseconds, I
suppose.
I agree we should be able to calculate intervals between epochs.
calendar years would also be my first choice as the unit of this.
since im going to propose some grammar that we will be stuck with for
the next 50m years, id like your (or anyone's) thoughts on the wording.
currently i have thought of
"01-01-01 12:00 epoch 50m BCE"
"01-01-01 12:00 reference 50m BCE"
but im open to anything
The other kind of example of palaeoclimate is the
one where large periods of time are spanned by the time axis e.g. if you
had a timeseries whose time axis spanned all the geological periods
since
600 Myear ago. (That would probably not be from a GCM! - but it could
be from
geological data or other Earth system models.) That sort of axis
would like
to use units of calendar Myear and they need to be stored accurately.
some possibilities that occur to me (not yet sure whats feasible in
parser):
"50 calendar Myears since 1980-01-01"
"50 Myears since 1980-01-01" (Myears == million calendar years)
"50 Myears" (assume reference is 0 CE)
About the multiples of calendar units: You have said they should be
integers.
However, perhaps fractions could be rather useful. For example, we
have the
recurrent question of where the time coordinate should be positioned
in the
interval if the data actually applies to the interval rather than the
instant.
It's a bit arbitrary and the usual advice is to put it in the middle.
For this
it would be convenient to have time-bounds for a cell of e.g. "1
calendar month
since reference" and "2 calendar months since reference", with the
time-coord
of "1.5 calendar months since reference".
it seems like a more informative coordinate value would be, eg "Feb
2001" when the bounds are [1,2] calendar months since 2001-01-01 ?
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Jim Biard
Government Contractor, STG Inc.
Remote Sensing and Applications Division (RSAD)
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