Hi Nan, Yes, I think "resolution" is an appropriate term, thanks very much for pointing this out. I think of resolution (loosely) as being the accuracy to which something is knowable, and it's a property of a measurement technique, not an actual measurement (is this right?). It seems to fit my requirement.
I wonder if we can combine a new "resolution" attribute with Steve Hankin's "point_spacing" attribute to solve the issue? I'm going to take the liberty of modifying Steve's idea to not just indicate that an axis is evenly-spaced, but also to provide the spacing. E.g. for a typical OSTIA dataset, both "resolution" and "point_spacing" might be "1 day". This effectively gives a gapless time axis: we have information for all time instants from the start to the end of the time axis. In contrast, an NWP forecast might have "resolution" of "1 ms" (or whatever... I don't know) but a "point_spacing" of 6 hours. The time axis is therefore "gappy": we essentially have no information between the snapshots and would have to interpolate to find a field at 03:00Z. Does this sound reasonable? My worry is the subtle distinction between "resolution" and "bounds/cell_methods", which I would have to think through properly at some more sociable hour. Cheers, Jon -----Original Message----- From: Nan Galbraith [mailto:[email protected]] Sent: 03 June 2010 21:23 To: Jon Blower; [email protected] Subject: Re: [CF-metadata] bounds/precision for time axis Hi Jon - I think the term 'resolution' might be more correct in this case than 'precision' - These are the definitions we use, although I can't quite find the source: * Resolution is the fineness to which an instrument can be read. * Precision is the fineness to which an instrument can be read repeatably and reliably. I'm also using the attribute 'C_format' - don't recall exactly which convention uses that, but it's a good way to indicate 'vagueness' - but I'm not sure how or if it could be used to indicate that the dates are only good to whole days. Cheers - Nan > Hi Mike, > > This is useful, thanks. However, I'm not sure that this concept of > "uncertainty" is exactly the same as my notion of "nominal precision" > (maybe I've chosen a bad name). For example, I know for sure that an > OSTIA daily analysis field is considered representative of a certain > day, but the specification of exactly how the field is representative is > complex. This, to me, is not quite the same as saying that the field > represents 12 noon on that day, plus or minus 12 hours. It's more > "imprecision" than "uncertainty". > > Would you agree with this distinction? > > Cheers, Jon > > -----Original Message----- > From: McCann, Mike [mailto:[email protected]] > Sent: 03 June 2010 19:47 > To: Jon Blower; Steve Hankin > Cc: [email protected] > Subject: Re: [CF-metadata] bounds/precision for time axis > > Hi Jon, > > The OceanSITES data management team has recognized a desire to include > an > attribute named "uncertainty" for all of our in situ measurements that > are > in our netCDF files (that also follow the CF convention). > > We chose the attribute "uncertainty" after much discussion around terms > such > as "precision" and "accuracy" because we felt that "uncertainty" > encompasses > several aspects of metrology and is more usable and simple to understand > for > the consumers of the data. > > This attribute can also be extended to the coordinate variables, e.g. If > the > precision of my clock and upstream sampling techniques was 5 seconds I'd > assign 5 to the .uncertainty attribute on my time coordinate. > > Would this approach work for your data? > > -Mike > > > > -- ******************************************************* * Nan Galbraith (508) 289-2444 * * Upper Ocean Processes Group Mail Stop 29 * * Woods Hole Oceanographic Institution * * Woods Hole, MA 02543 * ******************************************************* _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
