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[CF-metadata] Scaling ?

From: Pepe S. D. Juevara <sdj>
Date: Mon, 21 Mar 2005 11:21:16 +0100

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

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Received on Mon Mar 21 2005 - 03:21:16 GMT

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