[CF-metadata] MSG CPP standard name for time_offset_of_observation / pixel_delta_time
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-- Janine Aquino NCAR/EOL/CDS& RAF 303-497-8691 On 2/3/11 12:18 PM, tim.nightingale at stfc.ac.uk wrote: > Hello All, > > I would love to see a time coordinate offset. Another use could be > for rastered data, such as that generated by some scanning satellite > instruments, where the along-track time stamp and the across-track > time offsets can be separated. In these cases the time and time_offset > variables could have completely different indices: > > dimensions: > i = 40000; // along-track index > j = 1000; // across-reack index > variables: > double time(i) ; > :standard_name = "time" > ... > double delta_t(j) ; > :standard_name = "time_offset" ; > :ref_coordinate = "time" ; > ... > double pixel(i,j) > > > on the assumption that pixel_time(i,j) = time(i) + delta_t(j). This > would be somewhat more efficient than the conventional: > > dimensions: > i = 40000; // along-track index > j = 1000; // across-track index > variables: > double time(i,j) ; > :standard_name = "time" > ... > double pixel(i,j) > > for storage space and would be a natural use for a coordinate offset. > > I'm not completely clear how any of the offset schemes should couple > the coordinate/coordinate_offset pair to the :coordinates attribute of > subsequent data variables, as both the data variable and coordinate > offset would refer upwards to the base coordinate. Should we: > > a) trust the reading software to identify any associated offsets for > a coordinate variable? (What happens if there's more than one?) > > b) add :coordinate_offsets attributes or similar to data variables, > listing all relevant coordinate offsets, in analogy to the > :coordinates attribute? (To avoid possible ambiguity/duplication, > should we then leave out the corresponding :coordinates entry, as > the offset :ref_coordinate attribute identifies this for us?) > > c) where appropriate, include the coordinate offset, and not the base > coordinate, in the :coordinates attribute, as the reading software > can then work its way unambiguously up the chain of references? > (You could conceivably also have offsets of offsets of coordinates, > with delta_delta_t:ref_coordinate = "delta_t", and so on!) > > d) something else? > > Regards, > > Tim. > > -------------------------------------------------------------------- > Tim Nightingale > RAL Space > STFC Rutherford Appleton Laboratory > Harwell Oxford > Didcot Phone: +44/0 1235 445914 > OX11 0QX Fax: +44/0 1235 445848 > United Kingdom Email: tim.nightingale at stfc.ac.uk > -------------------------------------------------------------------- > > > > > > > > > On 03/02/2011 12:20, "Bentley, Philip"<philip.bentley at metoffice.gov.uk> > wrote: > >> Hi folks, >> >> I wonder if there isn't a more generic pattern sitting behind these >> proposals. On the assumption that people are likely to want to use >> similar offsets for other coordinate variables - e.g. x_offset, >> y_offset, z_offset, lat_offset, lon_offset, and so on - would there be >> any merit in specifying the more general-purpose standard_name of >> 'coordinate_offset', which would then be used in association with a new >> attribute called, say, 'base_coordinate' or 'ref_coordinate' (comparable >> to Jonathan's proposed 'relative_to' attribute) which would define the >> reference/datum coordinate variable? >> >> Then, in CDL, we'd have something like... >> >> variables: >> double time(time) ; >> :standard_name = "time" ; >> ... >> double delta_t(time) ; >> :standard_name = "coordinate_offset" ; # or just plain "offset" ? >> :ref_coordinate = "time" ; >> >> ... >> >> (In some cases, of course, the time coordinate might be a scalar >> coordinate, in which case the dimensions of the above two variables >> would be different). >> >> Adopting an approach along these lines would seem to confer the >> advantage of avoiding a proliferation of standard names of the form >> 'X_offset'. That said, however, there are probably some CF nuances that >> I've not thought of :-) >> >> Regards, >> Phil >> >>> -----Original Message----- >>> From: cf-metadata-bounces at cgd.ucar.edu >>> [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of plieger >>> Sent: 03 February 2011 09:14 >>> To: Jonathan Gregory >>> Cc: CF metadata >>> Subject: Re: [CF-metadata] MSG CPP standard name for >>> time_offset_of_observation / pixel_delta_time >>> >>> Dear Jonathan, >>> >>>> Do you think the standard name alone is sufficient? Proposing a new >>>> attribute is more work than proposing a standard_name, >>> since it means >>>> amending the CF standard. I can see there could be value in a >>>> relative_to attribute, but it might be an unnecessary >>> complexity. I wonder what you and others think. >>> >>> The standard name alone is sufficient for our case. I agree >>> with you that we should not add unnecessary complexity to the >>> CF standard. I think we will use the standard name 'time_offset'. >>> >>> Best regards, >>> Maarten >>> >>> >>> >>> On 02/02/2011 05:15 PM, Jonathan Gregory wrote: >>>> Dear Maarten >>>> >>>> What you write about pixeltime as an aux coord var (y,x) and your >>>> ncdump look sensible to me. I think that's all fine. >>>> >>>>> The standard_name time_offset (s) seems good to me. In the >>> long name >>>>> we can add an explanation that this variable deals with the time >>>>> offset for each pixel. >>>> OK. So that is a definite proposal for a new standard name. >>>> >>>> Do you think the standard name alone is sufficient? Proposing a new >>>> attribute is more work than proposing a standard_name, >>> since it means >>>> amending the CF standard. I can see there could be value in a >>>> relative_to attribute, but it might be an unnecessary >>> complexity. I wonder what you and others think. >>>> Best wishes >>>> >>>> Jonathan >>>> >>> >>> -- >>> Maarten Plieger >>> KNMI, R&D Information and Observation Technology, De Bilt >>> (t) +31 30 2206330 >>> _______________________________________________ >>> CF-metadata mailing list >>> CF-metadata at cgd.ucar.edu >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> >> _______________________________________________ >> CF-metadata mailing list >> CF-metadata at cgd.ucar.edu >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadataReceived on Thu Feb 03 2011 - 14:11:21 GMT
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