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 r?o -
> *************************************************
>
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Received on Mon Mar 21 2005 - 08:16:32 GMT
