Difficulty processing L3 mapped CHL data

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des.inge
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Joined: Fri Mar 04, 2022 9:37 am America/New_York

Difficulty processing L3 mapped CHL data

by des.inge » Fri Mar 04, 2022 9:58 am America/New_York

Hello,

I am a first time satellite data user.
I am trying to plot the 2018 MODIS Aqua chlor-a mapped data, but I am running into issues.

I first plotted the data without any processing and the concentrations were negative, which is not correct. So I went to look at the user guide for the data here: https://oceancolor.gsfc.nasa.gov/docs/technical/ocean_level-3_smi_products.pdf . I noticed that I have to apply a scaling equation of Base**((Slope*l3m_data) + Intercept).

So, I tried to apply the equation. Firstly, I noticed that in the file attributes that is no Slope or Intercept information. In the pdf the Base=10. Therefore, I applied the following in MATLab: base=10; chl_scaled=base.^(chl);. When I plotted the chl_scaled it gave me values at 4x10^91 which is also impossible.

I then thought I may have read it wrong and just applied a log10(chl), but that gave me imaginary numbers...

I am currently lost as to how to solve this problem and if I am missing a step or just following the wrong instructions. Any help or guidance will be greatly appreciated.

The file that I am currently attempting to use has the following name: A20181522018181.L3m_MO_CHL_chlor_a_4km.nc. I have attached the images that I produced to show my results.

Thanks,
Inge
Attachments
Raw_unprocessed.png
Raw_unprocessed.png (60.64 KiB) Not viewed yet
base_power_chl.png
base_power_chl.png (43.43 KiB) Not viewed yet

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OB SeaDAS - xuanyang02
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Re: Difficulty processing L3 mapped CHL data

by OB SeaDAS - xuanyang02 » Mon Mar 07, 2022 1:27 pm America/New_York

I am not sure if you are aware that you can use SeaDAS to display the ocean color data -- https://seadas.gsfc.nasa.gov/

Here is the image for A20181522018181.L3m_MO_CHL_chlor_a_4km.nc
Screen Shot 2022-03-07 at 1.26.03 PM.png
Screen Shot 2022-03-07 at 1.26.03 PM.png (158.55 KiB) Not viewed yet

des.inge
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Joined: Fri Mar 04, 2022 9:37 am America/New_York

Re: Difficulty processing L3 mapped CHL data

by des.inge » Tue Mar 08, 2022 4:36 am America/New_York

Hi xuanyang02,

I have given it a look and that is also why I know I am doing some thing wrong.

I would also like to get the mean and sd and compare to a biogoechemical model so that is why I would like to use the data.

Thanks

OB.DAAC - SeanBailey
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Re: Difficulty processing L3 mapped CHL data

by OB.DAAC - SeanBailey » Tue Mar 08, 2022 10:38 am America/New_York

Not all the L3 mapped products are distributed as scaled integers. The chlorophyll product is store as floating point values, so no scaling is required. The equation you tried would only be necessary if the product was stored with a logarithmic scaling (rarely used - most products are linearly scaled, so value = scale_factor * scaled value + add_offset - but again, the chlorophyll data are not scaled.)

Just read the data as is :)

Sean

des.inge
Posts: 3
Joined: Fri Mar 04, 2022 9:37 am America/New_York

Re: Difficulty processing L3 mapped CHL data

by des.inge » Tue Mar 08, 2022 10:47 am America/New_York

Thank you very much!

I realised this and I now understand that in the attributes where it says suggested_image_scaling_type=LOG is when you plot the image to use a log scale.

Baby steps to getting it right, but getting there.

Thanks for the help.

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