Glint correction for WorldView imagery over turbid water

[cio]

Hi Quinten,

I was wondering what I can change in the setting file to minimize/eliminate masking of pixels in WV imagery (in shallow water areas) due to turbidity. I understand from a previous explanation of yours that this (overcorrection) is expected when turbidity is present. When I switch glint correction off, no masking occurs but I'd like glint to be reduced/removed in ACOLITE - instead of having to do a separate (manual) glint correction after atmospheric correction in ACOLITE.

Thanks.

[quinten]

Hi Chris

I am not sure if I understand correctly. By default WV processing masks water products when the reflectance is > 0.1 in the NIR band. This is quite high to be caused by turbidity, but not impossible. If your image is glinted then this will have a significant contribution in the NIR.

The glint correction assumes all reflectance in the NIR is caused by glint, which is not true for turbid and productive waters. This is the reason why at the moment there is no turbid water glint correction for imagers without SWIR bands. In the SWIR there is no signal from below the water surface so the assumption of SWIR reflectance = glint reflectance works. If you are looking at "case 1" waters over clearly visible bottoms, you can probably also use the NIR for glint correction. (See Lyzenga and Hedley papers.) From the moment you add turbidity and a NIR signal from particulate backscattering in the system you will have issues.

If you have a WV3 image with SWIR acquired at the same time (not sure if that is a thing) you could perhaps attempt using its SWIR observations for turbid water glint correction.

I hope this helps!

Quinten

[cio]

Thanks a lot Quinten!

I also noticed some masking when glint correction is switched off with DSF correction over turbid water (this is expected/not unreasonable, I think, based on y our discussion): (1) do you think using the old SWIR 'aerosol correction' option will make a difference (since this is designed for turbid waters), and (2) can ACOLITE's glint correction be used with AOLITE's old SWIR correction method?

Also, when you said 'By default WV processing masks water products when the reflectance is > 0.1 in the NIR band,' where can I increase this or change the band (as you suggested in your last sentence).

Thanks once again!

[quinten]

Hi Chris

To increase the masking threshold, add a higher l2w_mask_threshold to your settings file, e.g. l2w_mask_threshold=0.15

The SWIR based EXP correction will not work very well since it extrapolates all SWIR signal to the VNIR using an exponential. Glint has a different shape, and will likely cause negatives at shorter wavelengths. (See also https://doi.org/10.1016/j.rse.2019.03.010) I have not even tested EXP for WV and would not recommend it!

Quinten

[cio]

Great. Thanks Quinten. I'll try that!