[CF-metadata] Logarithmic scaling and CF-Convention

From: Stephen Emsley <SEmsley>
Date: Mon, 27 Sep 2010 16:45:47 +0100

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:jonathan at met.reading.ac.uk] On Behalf Of Jonathan Gregory
Sent: 27 September 2010 08:33
To: John Graybeal
Cc: Stephen Emsley; cf-metadata at cgd.ucar.edu; ludovic.bourg at acri-st.fr; 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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Received on Mon Sep 27 2010 - 09:45:47 BST