Thanks Steve, I agree that a "point_spacing:even" attribute would be
helpful to clients.
However, I think that the question of point spacing is separate to that
of nominal precision. A time axis may be evenly spaced with one point a
day, but the time axis values could be considered instantaneous
(00:00:00.000Z), or each timestep could be considered "representative"
of the whole day.
I guess the combination of "cell_methods=cell" and "point_spacing:even",
plus the separation between the first two points along the axis might
together give the nominal precision.
Perhaps instead of "point_spacing=even" we could have
"regular_point_spacing=1.0" to make things slightly easier on clients?
(Irregular axes would simply omit the regular_point_spacing attribute.)
(I worry a little that this mechanism wouldn't allow a nominal precision
of a month or year, which might be common for palaeo data, since months
and years are not of exactly equal length.)
Cheers, Jon
-----Original Message-----
From: Steve Hankin [mailto:Steven.C.Hankin at noaa.gov]
Sent: 02 June 2010 22:58
To: Jon Blower
Cc: Jonathan Gregory; cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] bounds/precision for time axis
Jon Blower wrote:
> Dear Jonathan,
>
> Yes, I take your point. Another option would be to allow a simple
> attribute on a time axis "nominal_precision" or something similar,
which
> could take values like "1 day", "1 month" etc. This could be in
> addition to the bounds and cell_methods attributes.
> The problem with
> using the bounds/cell_methods approach in isolation is that it
requires
> a client to look at the bounds for every point along the time axis to
> verify that the bounds for each point is the same length.
>
Hi Jon,
This comment applies equally to spatial axes; as-is a CF client
application must examine the full coordinate axis and its bounds in
order to determine if an axis is regularly spaced whether time or
space. And yes, this is a particularly acute problem for time axes,
where it is not uncommon to have many thousands of points in a CF
dataset.
There have been discussions in the past of the merits of a
"point_spacing" attribute:
my_axis:point_spacing = "even";
I'd be totally supportive of adding this attribute to CF, myself. It
offers significant efficiencies to clients and it addresses your
question reasonably well (to quickly infer a nominal spacing from the
axis coordinates or its bounds).
- Steve
> Best wishes,
> Jon
>
> -----Original Message-----
> From: Jonathan Gregory [mailto:jonathan at met.reading.ac.uk] On Behalf
Of
> Jonathan Gregory
> Sent: 02 June 2010 19:13
> To: Jon Blower
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] bounds/precision for time axis
>
> Dear Jon
>
> If we decide to let cell_methods indicate "vagueness" I don't think
it's
> a
> problem if the bounds themselves are precise. The precision is
somewhat
> irrelevant, as you say, but it shouldn't be misleading in view of the
> cell_methods. Changing cell_methods is a small alteration to the CF
> standard,
> whereas introducing alternative ways of representing time is a large
and
> complex change, which would require substantial alteration to software
> as well.
>
> Best wishes
>
> Jonathan
> _______________________________________________
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> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
--
Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
7600 Sand Point Way NE, Seattle, WA 98115-0070
ph. (206) 526-6080, FAX (206) 526-6744
"The only thing necessary for the triumph of evil is for good men
to do nothing." -- Edmund Burke
Received on Thu Jun 03 2010 - 02:53:04 BST
