Dear John/Jonathan
Thank you for your replies. I had thought that something as ubiquitous as log scaling a variable would already have a CF-convention treatment. However, if I am interpreting the replies correctly this is not the case. There are two scenarios that have been addressed: (a) how to store a geophysical variable that is defined as a logarithm, e.g. dB. (b) how to pack a float as a logarithm with scale_factor & add_offset to cast it to a byte. Case (b) describes our scenario, e.g. to store chlorophyll concentration in sea water as a log scaled values? My understanding now is: 1. If log(chlorophyll) is stored it cannot have the standard_name "chlorophyll_concentration_in_sea_water". 2. Units mg/m3 are incorrect as the log(chlorophyll) is dimensionless. 3. Creating a base attribute is OK but is not a standard CF attribute, so not useful to 3rd party software. To remain CF-convention compliant, the relevant CDL could be: byte chl(latitude, longitude) ; chl:long_name = "Logarithm (base 10) of chlorophyll-a concentration [mg/m3]" ; chl:scale_factor = "0.0150000" ; chl:add_offset = "-2.00000" ; Would this be the recommended approach? As a future modification of the convention would it be possible to leverage the cell_methods attribute by adding 'log10' as a keyword? Or is it already possible using the 'comment' modifier e.g. chl:cell_methods = "lat: lon: point comment: log10 scaled"; NOTE: is it necessity to include a bounds attribute if the cell_method is defined as a point measurement? Kind Regards Steve -- Dr Stephen Emsley T: +44 (0)1752 764 295 ARGANS Limited M: +44 (0)7912 515 418 -----Original Message----- From: Jonathan Gregory [mailto:[email protected]] On Behalf Of Jonathan Gregory Sent: 27 September 2010 08:33 To: John Graybeal Cc: Stephen Emsley; [email protected]; [email protected]; Samantha Lavender Subject: Re: [CF-metadata] Logarithmic scaling and CF-Convention Dear John and Steve Any conversion to logarithmic units requires a reference value, because the quantity computed is log(value/reference). The base of the logarithm (10 or e) also has to be specified. If this conversion is used for packing, those two numbers could be recorded as attributes and the transformation could be reversed. If the conversion is done to produce a new geophysical quantity, the reference level and logarithm base are part of the definition of the quantity and could be recorded in the standard_name definition. For instance, "acoustic intensity level" in decibels is the logarithm of the "acoustic intensity" in W m-2, with a reference of 1e-12 W m-2 according to wikipedia. Cheers Jonathan
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