[CF-metadata] Scaling ?

Brian Eaton eaton at ucar.edu
Mon Mar 21 08:16:32 MST 2005


Logrithmic scaling isn't supported by COARDS or CF.  The "base" attribute
of the variable "13m_data" is non standard.

Brian

On Mon, Mar 21, 2005 at 11:21:16AM +0100, Pepe S. D. Juevara wrote:
> Dear All,
> 
> I am trying to produce some Chlorophyll-a netCDF files which follow the COARDS-CF
> conventions.
> 
> In doing this I have stumbled upon the following problem:
> 
> My 'raw' data is stored as bytes, and in order to transform it to 'real'
> Chlorophyll I need to perform the following operation:
> real_data = 10 ^ (scale_factor * raw_data + add_offset)
> 
> Unfortunately I have noticed that the netCDF functions which access the file
> (i.e. ncview & ferret) perform the transformation in the following way:
> real_data = (scale_factor * raw_data + add_offset)
> 
> Therefore the problem is that the logarithmic scaling is not being recognized
> and the transformation is being done erroneously.
> 
> Is there any way I can force the netCDF file to interpret my attributes with
> the correct scaling equation ? Is logarithmic scaling supported by netCDF
> files ?
> 
> I am grateful for any information and/or suggestions you can give me. 
> 
> Best Regards,
> Pepe
> 
> P.S. I have attached below a copy of an ncdump -h output:
> 
> netcdf S20032132003243_L3m_CHL {
> dimensions:
>         longitude = 4096 ;
>         latitude = 2048 ;
> 
> variables:
>         float longitude(longitude) ;
>                 longitude:standard_name = "longitude" ;
>                 longitude:long_name = "Longitude" ;
>                 longitude:units = "degrees_east" ;
>                 longitude:step = "0.0878906" ;
>                 longitude:valid_min = "-179.9561" ;
>                 longitude:valid_max = "179.9561" ;
>                 longitude:axis = "X" ;
>         float latitude(latitude) ;
>                 latitude:standard_name = "latitude" ;
>                 latitude:long_name = "Latitude" ;
>                 latitude:units = "degrees_north" ;
>                 latitude:step = "0.0878906" ;
>                 latitude:valid_min = "-89.9561" ;
>                 latitude:valid_max = "89.9561" ;
>                 latitude:axis = "Y" ;
>         byte l3m_data(latitude, longitude) ;
>                 l3m_data:standard_name = "chlorophyll_concentration_in_sea_water"
> ;
>                 l3m_data:long_name = "Chlorophyll-a concentration" ;
>                 l3m_data:units = "mg/m3" ;
>                 l3m_data:base = "10.0" ;
>                 l3m_data:scale_factor = "0.0150000" ;
>                 l3m_data:add_offset = "-2.00000" ;
>                 l3m_data:valid_min = "0" ;
>                 l3m_data:valid_max = "254" ;
>                 l3m_data:_FillValue = '\377' ;
>                 l3m_data:missing_value = "255" ;
> 
> // global attributes:
>                 :Product = "Chlorophyll-a Monthly Mean" ;
>                 :Measure = "Mean" ;
>                 :Year = "2003" ;
>                 :Month = "August" ;
>                 :Start_Day = "213" ;
>                 :End_Day = "243" ;
>                 :Map_Projection = "Equidistant Cylindrical" ;
>                 :Scaling = "Logarithmic" ;
>                 :Scaling_Equation = "Chl = base**((scale_factor*l3m_data)
> + add_offset)" ;
>                 :Conventions = "COARDS-CF" ;
> }
> *************************************************
> Pepe S. D. Juevara
> 
> - No debe cambiarse de caballo al cruzar el río -
> *************************************************
> 
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> 
> 
> 
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