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:[email protected]] Sent: 02 June 2010 22:58 To: Jon Blower Cc: Jonathan Gregory; [email protected] 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:[email protected]] On Behalf Of > Jonathan Gregory > Sent: 02 June 2010 19:13 > To: Jon Blower > Cc: [email protected] > 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 > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > -- Steve Hankin, NOAA/PMEL -- [email protected] 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 _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
