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Up Topic SeaDAS / Non-SeaDAS Packages (e.g. MATLAB, ENVI, GIS, etc) / Obtaining values from 256 color image (locked)
- By gverutes Date 2010-01-28 16:47
I have successfully imported the SMI/HDF files for chlorophyll and SST into ENVI and ArcGIS...

My question: is there any way to translate the pixel values (0-255) into actual measured values like degrees Celsius (SST) or mg/m^3 (chlorophyll).  I assume 255 = No data.

I know approximate ranges are provided for say SST (-2 to 45 degrees Celsius), but would appreciate direction on how to get exact SST values for each pixel value.

Thanks!
- By norman Date 2010-01-29 15:06
We have not stored pixel values in our SMI files as 8-bit
integers for some time now.  If you are getting MODIS
files from us, they contain scaled chlorophyll and SST
values stored in 16-bit integers.  The "no data" value
is 65535.  Scaling information is included within each SMI
file.  For example:

                :Parameter = "Chlorophyll a concentration" ;
                :Measure = "Mean" ;
                :Units = "mg m^-3" ;
                :Scaling = "logarithmic" ;
                :Scaling Equation = "Base**((Slope*l3m_data) + Intercept) = Parameter value" ;
                :Base = 10.f ;
                :Slope = 5.8137757e-05f ;
                :Intercept = -2.f ;

and

                :Parameter = "Sea Surface Temperature" ;
                :Measure = "Mean" ;
                :Units = "deg-C" ;
                :Scaling = "linear" ;
                :Scaling Equation = "(Slope*l3m_data) + Intercept = Parameter value" ;
                :Slope = 0.00071718497f ;
                :Intercept = -2.f ;

Regards,

Norman
- By norman Date 2010-01-29 15:08
P.S.

Note that we will switch to storing the geophysical values
as 32-bit floating point numbers once we have reprocessed
the data.  (SeaWiFS SMI chlorophyll values are already
stored in this format.)

Norman
- By maheshshiramgon Date 2011-04-21 03:46
Hi Norman,

Can we use these scaling equation for getting actual parameter values from l3m_data table?

Please advise I am struggling to use SST and Chl values from MODISA Mapped products of ocean color.

I am upto the point to extract data from l3m_data files of HDF4 for SST downloaded from MODISA Mapped products and I tried to use formula l3m_data values/1000 as actual values except 65535.
but this formula is showing numbers somewhat unrealistic and hence I want to use scaling equation if you say its a correct method to get real parameter vlaues.

Kindly suggest if any tools or scripts to use on windows XP OS to get actual parameter values from these HDF4 files

Looking for your response

Regards
mahesh
- By norman Date 2011-04-21 08:27
Hi Mahesh,

The scaling information for the SST l3m_data data set is
stored in the HDF file.  For example:

ncdump -h A2011110.L3m_DAY_SST_9
netcdf A2011110 {
dimensions:
  fakeDim0 = 2160 ;
  fakeDim1 = 4320 ;
  fakeDim2 = 2160 ;
  fakeDim3 = 4320 ;

variables:
  short l3m_data(fakeDim0, fakeDim1) ;
    l3m_data:Fill = -1s ;
    l3m_data:Scaling = "linear" ;
    l3m_data:Scaling Equation = "(Slope*l3m_data) + Intercept = Parameter value" ;
    l3m_data:Slope = 0.00071718497f ;
    l3m_data:Intercept = -2.f ;
.
.
.

The above slope = 47/65534.  Thus the 16-bit data represent temperatures
in the range [-2,45] with 65535 being reserved for absent data.

The best tool I can suggest to you is the SeaDAS software that is
distributed and supported by our group.
http://oceancolor.gsfc.nasa.gov/seadas/

Regards,
Norman
Up Topic SeaDAS / Non-SeaDAS Packages (e.g. MATLAB, ENVI, GIS, etc) / Obtaining values from 256 color image (locked)



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