I am looking at level-2 OLCI products, more specifically at the T865, A865, CHL and TSM products. In the metadata of those products there is a scale factor and an offset factor given. For example the T865 has a scale factor of 0.006299213.
I want to compare the T865 values with AERONET-OC AOT measurements, but at the moment it seems that the T865 values are a lot higher than the AERONET-OC values. Does this mean I have to multiply the OLCI T865 values by the scale factor first? For this comparison I use the pixel extraction tool, maybe it has something to do with this?
Yes, probably you need to scale the values but not with the scaling value you have found. This is only used to convert from the raw int8 data type to a floating point data type.
The AOT on OLCI is in the range of 0.0 to 0.6 (OLCI AOT). But in products the range goes up to 0.8, maybe the documentation is not up to date.
I’m not sure in what range AERONET provides its data. It think it depends on the level and maybe also on the station.
If you know the range of your AERONET data, you know the conversion factor.
I just wanted to scale the OLCI data and leave the AERONET data as it is. When you say the AOT range of OLCI is between 0.0 to 0.6 how can I get the scaling factor from this? Or did I misunderstand you?
You should use 0.8 as maximum. It is not yet clear what’s correct, but I think it is 0.8. I’ve asked the creators of the data, but didn’t get an answer yet.
If you know the range of Aeronet you can compute the factor. Let’s assume it is between 0 to 4.
factor = 4 / 0.8
In this case you need to multiply all the T865 values by 5.
Maybe this is a simplistic solution, but I think it works.
I’ve got an official response. I just cite it here:
There is “maximum allowed optical thickness” defined in an ADF that prevents the selection of any aerosol model that fits the signal with an AOT (at 865) higher than this threshold. The threshold is used in the aerosol identification. The current value of the threshold is 0.8 and is in line with the LUT generation.
Additionally, there is a “threshold for flagging the aerosol optical thickness” that raises the TAU06 flag. This threshold does not invalidate the data and allows the data to be included in the product.
AOT values between 0.6 and 0.8 are therefore normal.