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Scaling and Offset: S3A/B, VIIRS

Posted: Tue Jan 16, 2024 12:43 pm America/New_York
by slsmit34
Hello,
I noticed in variables for 'add_offset: 0.05' and 'scale_factor: 2.0E-6' in the Rrs Band_attributes for S3A/S3B EFR Ocean Color (OC) product (OLCIS3B_L2_EFR_OC) from OB.DAAC. Are these automatically applied to the .nc file, or do I need to scale and add the factor to the rrs measurements in the file to get the correct rrs?
For example, Rrs_560 at an area in a S3A scene is 0.00738 sr^-1. Should I multiply this by 2.0E-6 and add 0.05 to it to get the correct Rrs? That seems wrong...
I also saw these scale_factors and add_offset in VIIRS NPP and NOAA-20 Ocean Color (OC) products.
Thanks!
Best,
Sami

Re: Scaling and Offset: S3A/B, VIIRS

Posted: Fri Jan 19, 2024 2:22 pm America/New_York
by OB.DAAC - SeanBailey
The answer depends upon the netCDF reader client (for example, SeaDAS does, as does the python netCDF4 API). Most should apply them, so you wouldn't need to do so manually. If you're finding the values don't seem geophysically reasonable, it is likely the scale_factor and add_offset were not applied. The equation is very simple:

geophysical_value = stored_value * scale_factor + add_offset

Sean