Chlorophyll concentration OCI VS Chlorophyll Concentration OCX Algorithm

captain_universe · by **captain_universe** » Tue Jan 26, 2021 7:52 am America/New_York

Hello
I am currently doing a research in Marine protected areas (MPA), calculating Chlorophyll concentration through remote sensing.

I started using Terra Modis data but I dont know which of the 2 types I should use. OCX Algorithm OR OCI?

Also I read some papers referring that (OCI) algorithm is a hybrid between a bandâ€difference color index algorithm for lowâ€Chl waters
and the traditional bandâ€ratio algorithms (OCx) for higherâ€Chl waters. Does that mean that OCI contains OCX alogorithm with some more information? I am a little bit confused there.

When I used the OCX Algorithm the size of the file is 5 to 6 times bigger than OCI. Does that mean OCX Algorithm contains more information about variation?

Which of the 2 types should I use in my research and what type is the newest containing more information?

Thank you.

OB.DAACx - SeanBailey · by **OB.DAACx - SeanBailey** » Tue Jan 26, 2021 9:48 am America/New_York

Use OCI. As implemented in l2gen OCI is a blend of the Hu et al., (2012) color index algorithm (which uses a band difference approach) and the OCx (in the case of MODIS the x is 3, for 3-band ratio algorithm). See https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/.

I do not understand your statement that the OCX file size is 5 to6 times bigger than OCI, as OCI includes OCX, but switches to the band difference algorithm below concentrations 0f 0.15 mg m^-3, using a weighted average between 0.15 and 0.2 mg m^-3 and is OCX-only above 0.2 mg m^-3 .

Regards,
Sean

captain_universe · by **captain_universe** » Wed Jan 27, 2021 6:27 am America/New_York

Thank you for your immediate response.

When I said about the file I meant that the size of OCX (netCDF) is 300 MB (megabytes) while OCI is 40-50MB (megabytes) and I thought the bigger the file the more information will be included.

I will use OCI.

by **OB SeaDAS - dshea** » Wed Jan 27, 2021 8:54 am America/New_York

The size of the OCI and OCX are both float, so they will be the same size in an L2 file. The only way the L2 files could be a different size is if more products were put on one of the files. You can use "ncdump -h" on the command line or look at the list of bands using SeaDAS.

don