HILT masking even with NIR-SWIR AC

[ana_dogliotti]

Hello,
I work in very turbid waters (R. de la Plata) and I've always processed MODIS images using the switching NIR-SWIR AC algo (until v.7.0), even there are bands that saturate like 748 nm. But now that I'm moving to SeaDAS 7.3.x I find that the turbid waters are masked with HILT flag (even though I removed this mask) probably due to 748 nm saturation. I know that this band is used to calculate the T index, but instead of masking these pixels can't they be processed directly with the SWIR bands instead of flagging/masking them?

These are the l2gen options I'm using:

aer_opt=-9
aer_swir_short=1240
aer_swir_long=2130
aer_wave_short=748
aer_wave_long=859
brdf_opt=0
cloud_thresh=0.018
cloud_wave=2130.0
maskhilt=0

Thanks for your help

Ana

[OB.DAACx - SeanBailey]

Ana,

Unfortunately, no.  However, you can run the SWIR-only option, which would not use the NIR bands (and thus avoid the saturation problem).
Since your ROI is saturated, it is likely the T-index would put it into the SWIR selection of the NIR/SWIR switching anyway.
(simply set aer_opt=-2, aer_wave_short=1240 and aer_wave_long=2130)

Sean

[ana_dogliotti]

Thanks Sean for your replay.

Indeed, I can use the SWIR bands (opt -1 since I don't want the iterative NIR correction), but it's a shame since it used to work nicely and was very useful in my area where there are extremely turbid waters right next to quite clear waters on the other side of the turbidity front. That was the beauty of switching. I guess I'll have to process twice and blend the results myself. :(
Thanks again,

Ana

[pmarroig]

Hi,

I am working with a very turbid lagoon, processing L1A MODIS Aqua files to L2. I've made several test with the atmospheric correction, however all of the options give lots of pixel masked as HILT.
As I am working with more than 10 years of images, if this was in just few it wouldn't affect the analysis, but it happens that there are months of missing data (i am studing a transect in the lagoon).
I've made tests using aer_opt= -9, -1 and -2, using cloud_thresh=0.018 and cloud_wave=2130.0 Both of the tests give the same masked pixels. When i removed the HILT mask, i could have values in just one band.

There are others options in the processing that could retrive more "valid" pixels?

Thanks for your help.

Patrícia.

[OB.DAACx - SeanBailey]

Patricia,

The HILT flag is set for MODIS when one or more bands saturate.  The 1km "ocean" bands are apt to saturate (especially in the red/NIR) over turbid waters.
No atmospheric correction option will change that.  However, if it is only the NIR bands that are saturating (these are used in the atmospheric correction)
you might have some success using the aer_wave_long=859 as the 859 band is a "land" band and does not saturate.  If that still doesn't work,
try aer_wave_short=859 with  aer_wave_long=2130.  Or choose one of the options that fixes the aerosol, so there will be no need to
have unsaturated bands to do an aerosol model selection.

Regards,
Sean

[pmarroig]

Thanks for your reply Sean,

From what i've observed in the data only the NIR bands are saturated . I will try this options. Hope those solutions could provide more information to work!

Best Regards,
Patrícia.