1. The CDM library uses the bounds if they are present. If only the
coordinate values are present, the CDM generates bounds. These grids
bounds are used by ncWMS and other visualization software to draw color
filled images. The IDV (I think) uses a contouring algorithm with just
the coordinate values.
2. Spatial coordinates probably want to use midpoint values.
3. I think theres a good argument that time coordinates want to use the
end-point. Seth makes the argument for numerical models. In this case,
all the output variables should have the same time coordinate. Im trying
to think of a case where thats not true (point observations, radar data
etc), and im not thinking of any.
4. Perhaps "interval of accumulation" is different enough that one
should just encode it in a separate attribute or auxiliary coordinate on
the data variable. Numerical models can have different variables with
different intervals, possibly overlapping. This is perhaps not really
the same as the bounds on the coordinate, they just share the same
codomain (time). An advantage of this approach is that you dont have to
create new coordinate variables for each data variable, which seems like
more trouble than its worth.
Seth McGinnis wrote:
In the case of 'raw' output from numerical models, it probably makes sense to
use the end-point of the time interval rather than the mid-point. That's the
moment for which the model stores the data, whether they're instantaneous
values (intensive variables) or time-averages over the previous timestep
(extensive variables).
If you used the mid-point of the interval for extensive variables, they
wouldn't have the same time coordinates as the intensive variables, which would
be very confusing. Using the end-point keeps everything aligned.
--Seth
On Thu, 12 Nov 2009 14:41:26 +0000 (UTC)
Thomas Lavergne <[email protected]> wrote:
Dear Jonathan,
----- "Jonathan Gregory" <[email protected]> wrote:
Dear Thomas
I'm not saying the coordinate *must* be the mid-point. If there's a
good reason
for it being something else, then you could choose it to be so. I was
suggesting that we could recommend it should be the mid-point if there
is
no strong basis for making another choice. We could also say that it
must not
be outside the bounds.
I agree with your recommendation.
But I was also trying to gain support on "which axis value should I choose for
my variable" and your answer does not help :-).
I have rather little basis for making the choice of the end time for
representing an accumulated quantity but, at least, CF does not forbid it. I
guess I have to seek agreement inside my scientific community and that it is
not CF's role to decide upon that.
Are there people interested in taking the discussion further? We seek the
answer to the question: "In which cases would another choice (other than
mid-point) be relevant?".
Thomas
You are right, it cannot be missing data. That would break some
applications,
anyway.
Cheers
Jonathan
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