Earthdata Forum

Sir

While processing atmospheric correction for HICO data in SEADAS 7.1 using OCSSW,Rayleigh radiance (Lr) is given for inland water pixels (LAKE). But in next step [Lt-Lr] the whole land portion is set to 32.717.

Can you suggest a solution to process inland water.


Thanks
S.Nagendra Jaiganesh
Project Associate
IIT-Madras

As an alternative, you may wish to try the online hyperspectral atmospheric correction tool available specifically for HICO on the HICO website (://hico.coas.oregonstate.edu). This tool is based on Tafkaa 6s.  Access to the tool is currently restricted (it is still beta) - to request access, simply email me at jasmine@coas.oregonstate.edu. This tool has been used extensively to correct inland water scenes, and may help with your particular scene.The SeaDAS atmospheric correction tool (OCSSW) is currently not hyperspectral (rather, it is limited to a subset of wavebands). Development of a hyperspectral atmospheric correction tool for SeaDAS is in progress.  Jasmine NahorniakHICO team member

As Jasmine pointed out, l2gen currently only supports a "multi-spectral" HICO (16 bands matching MERIS wavelengths).   We've also not vicariously calibrated HICO.
It is possible that the data have been flagged as cloud or another mask condition that would prevent atmospheric correction by l2gen.  You could use the l2_flags to
identify which conditions exist, and potentially modify the atmospheric correction processing parameters to retrieve useful data. 

One thing to consider when working with inland water bodies is that sometimes inland water is masked as land - another condition that would prevent atmospheric correction
(our primary focus being oceans, we halt processing on pixels flagged as land).  

Regards,
Sean

Hi Sean,

Regarding what you have said above, I was wondering if it is possible not to mask the land when using l2gen to process HICO? What should I do if I want to keep all the pixels (waters and land) and apply atmospheric correction to the whole scene using l2gen?

I have similar problem with him as I am interested in a small inland lake, but l2gen masked my whole scene as land... I was thinking if we can change the flag, keep all the pixels, and then atmospherically correct the whole scene but only use the information from the water part.

Thanks in advance.

HY

You can set maskland=0 or to be really sure land=$OCDATAROOT/common/landmask_null.dat.
Keep in mind we have NOT vicariously calibrated HICO, so while you may get a retrieval, the results may not be valid.

Sean

Thanks Sean.

Another question is, can BEAM read the l2gen generated HICO data? I tried to use BEAM but failed. It says can not decode the file.

HY

Hi Sean,

Sorry for the multiple questions. I have followed your suggestion to set maskland=0 or land=$OCDATAROOT/common/landmask_null.dat, but both still didn't show me the land (land area was gray with NaN value).. I tried to set maskcloud=0 to test if my settings were wrong, but it turns out the could was successfully unmasked. I have no idea why the landmask can not be set off.

Any idea on that? Thanks.

If your BEAM installation has the latest version of the seadas_reader module, then yes. Otherwise, no.

Processing with the "null" landmask, or with the maskland=0 option set will still result in atmospheric correction failures over land. 
The l2gen program is designed to retrieve over water.  All that using the null landmask does is force l2gen to process all pixels regardless of
whether or not the landmask would have masked the pixel.  This can be useful in cases where the landmask is inaccurate (or as is often the case for HICO, the geolocation is inaccurate).
You might try alternative aerosol options.  The simplest option is aer_opt=-99, which skips aerosol removal altogether
(it is often the aerosol selection which fails in the processing of challenging scenes).

Sean

I see and now I understand the l2gen more. Thank you very much Sean!