Dear Balaji, Roy, Martin I agree completely with what Roy is saying - that labelling something as psu or dimensionless in the units string should have no bearing whatsoever on the storage precision of the numerical value.
However, regarding the scaling, the CF standard name table currently lists the units of practical salinity as "1". Up to and including version 28 of the standard name table we had the units of sea_water_practical_salinity and change_over_time_in_sea_water_practical_salinity listed as "1e-3" which is still a dimensionless number, but scaled. There are two things to say about this: (1) This change was only made after considerable debate on the CF mailing list in 2015, starting with http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2015/058205.html; (2) Although the decision was taken to change the scaling in the canonical units to "1", this does not in any way prevent 1e-3 being used in individual data files, if that helps with storage precision, because the two differ only by a scale factor and are in all other aspects equivalent. Therefore, I don't see the need to have "psu" as a unit in UDunits. It's a matter for the data provider to choose the scaling in his or her data files. Martin is correct that "psu" is not a unit - it refers to a dimensionless number on a calibrated scale. Best wishes, Alison ------ Alison Pamment Tel: +44 1235 778065 Centre for Environmental Data Analysis Email: [email protected]<mailto:[email protected]> STFC Rutherford Appleton Laboratory R25, 2.22 Harwell Campus, Didcot, OX11 0QX, U.K. From: CF-metadata [mailto:[email protected]] On Behalf Of Lowry, Roy K. Sent: 18 July 2017 14:37 To: V. Balaji - NOAA Affiliate Cc: [email protected]; Juckes, Martin (STFC,RAL,RALSP) Subject: Re: [CF-metadata] Is "psu" a valid cf unit? Dear Balaji, I think there are some crossed wires here. The dimensionless Practical Salinity and a Practical Salinity in PSU are exact numeric equivalents. The only difference is the name that's given to the unit of measure - that's why to make this crystal clear CF includes the scaling factor of 10^-3. So I don't think that this can affect storage precision. Cheers, Roy. Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to [email protected]<mailto:[email protected]>. Please also use this e-mail if your requirement is urgent. ________________________________ From: V. Balaji - NOAA Affiliate <[email protected]<mailto:[email protected]>> Sent: 18 July 2017 14:12 To: Lowry, Roy K. Cc: [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]> Subject: Re: [CF-metadata] Is "psu" a valid cf unit? The ocean modeling community is adamant that they will continue to use PSUs for salinity: it's this unit, rather than its SI or dimensionless equivalent, that gives the maximum digits of precision in a floating-point representation of salinity. It's a valid concern, and CF should perhaps reconsider, with a new discussion giving weight to the views of modelers as well as physical oceanographers. Thanks, Lowry, Roy K. writes: > Hello Martin, > > > This topic has been debated at length in CF. To cut a long story short, the > term 'Practical Salinity Unit' was coined when the 1978 Practical Salinity > scale was devised. However, the term fell out of favour with the physical > oceanographic community whose current recommended practice is that Practical > Salinity - a ratio - should be a dimensionless number. CF followed this > recommendation and so PSU is not a part of CF. > > > Have a dig around in the mailing list archive if you want to find out more. > > > Cheers, Roy. > > > Please note that I partially retired on 01/11/2015. I am now only working 7.5 > hours a week and can only guarantee e-mail response on Wednesdays, my day in > the office. All vocabulary queries should be sent to > [email protected]<mailto:[email protected]>. Please also use this > e-mail if your requirement is urgent. > > > ________________________________ > From: CF-metadata > <[email protected]<mailto:[email protected]>> > on behalf of [email protected]<mailto:[email protected]> > <[email protected]<mailto:[email protected]>> > Sent: 18 July 2017 13:43 > To: [email protected]<mailto:[email protected]>; > [email protected]<mailto:[email protected]>; > [email protected]<mailto:[email protected]> > Subject: [CF-metadata] Is "psu" a valid cf unit? > > Hello David, all, > > Is "psu" a valus CF unit? It is not in Udunits, but it is added in cf-python > as a unit alias and also appears to be accpeted by the cf-checker. I can't > see any mention of it in the CF Convention document: the latter only lists > level, layer, and sigma_level permitted departures from Udunits, > > regards, > Martin > > _______________________________________________ > CF-metadata mailing list > [email protected]<mailto:[email protected]> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata CF-metadata Info Page - University Corporation for ...<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> mailman.cgd.ucar.edu This is an unmoderated list for discussions about interpretation, clarification, and proposals for extensions or change to the CF conventions. > CF-metadata Info Page - University Corporation for > ...<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> CF-metadata Info Page - University Corporation for ...<http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> mailman.cgd.ucar.edu This is an unmoderated list for discussions about interpretation, clarification, and proposals for extensions or change to the CF conventions. > mailman.cgd.ucar.edu > This is an unmoderated list for discussions about interpretation, > clarification, and proposals for extensions or change to the CF conventions. > > > > ________________________________ > This message (and any attachments) is for the recipient only. NERC is subject > to the Freedom of Information Act 2000 and the contents of this email and any > reply you make may be disclosed by NERC unless it is exempt from release > under the Act. Any material supplied to NERC may be stored in an electronic > records management system. > ________________________________ > -- V. Balaji Office: +1-609-452-6516 Head, Modeling Systems Group, GFDL Mobile: +1-917-273-9824 Princeton University Email: [email protected]<mailto:[email protected]> https://www.gfdl.noaa.gov/v-balaji-homepage v-balaji-homepage - Geophysical Fluid Dynamics Laboratory<https://www.gfdl.noaa.gov/v-balaji-homepage> www.gfdl.noaa.gov<http://www.gfdl.noaa.gov> Bio Dr. V. Balaji is affiliated with Princeton University's Cooperative Institute on Climate Sciences. He has headed the Modeling Systems Group at NOAA's Geophysical ... ________________________________ This message (and any attachments) is for the recipient only. NERC is subject to the Freedom of Information Act 2000 and the contents of this email and any reply you make may be disclosed by NERC unless it is exempt from release under the Act. Any material supplied to NERC may be stored in an electronic records management system. ________________________________
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