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Threshold Definition

Posted: Sat Nov 11, 2023 8:13 am America/New_York
by cassandra21
Hi everyone,

Currently, I am working with monthly statistics derived from L2map MODIS-Aqua satellite parameters.
When conducting a monthly statistical analysis on the maximum values in the Cape Verde region, specifically POC, I found that around 70% of the registered values align with the maximum threshold set by NASA for L3 images. If the threshold were to be increased, what would be the outcome? Are the thresholds established to prevent external contamination in the data?

Thank you :)

Re: Threshold Definition

Posted: Tue Dec 05, 2023 10:44 am America/New_York
by OB General Science - guoqingw
Hello,

The POC algorithm is mainly for open ocean or Case 1 waters, and it relies on the fact that variation in Rrs(λ) at blue wavelengths are primarily driven by phytoplankton pigments and covarying matters. For coastal or optically complex case 2 waters, the empirical relationship used in the algorithm may not always hold. For the threshold, you may want to check the corresponding Rrs(λ) ratios for your study area.

The following is the algorithm description for your reference:
https://oceancolor.gsfc.nasa.gov/resources/atbd/poc/;

And the original publication:
https://bg.copernicus.org/articles/5/171/2008/bg-5-171-2008.pdf

Hope that helps.
Guoqing

Re: Threshold Definition

Posted: Wed Apr 03, 2024 8:21 am America/New_York
by cassandra21
Hi!

Silly question: if POC algorithm is mainly for open ocean or case 1 waters, we can assume that the same applies for Kd490 and Chl a, correct?

Re: Threshold Definition

Posted: Fri Apr 05, 2024 9:28 am America/New_York
by OB General Science - guoqingw
They are global algorithms, based on similar assumptions, and use the blue-green band ratios. Chlorophyll algorithm is a little complicated with the blend of a band difference and band ratio algorithms.