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Fine-tuning the CHRIS atmospheric correction over water
Hello all,

I am trying to atmospherically correct a CHRIS image that was acquired over a coastal maritime system.

Most of the processed water reflectances have negative values at the 410 nm channel (see the attached screenshot). The problem looks very much like the atmospheric over-correction problem that is commonly encountered in SeaWiFS and MODIS imagery in the blue region.

I would like to look a little bit more in depth into the correction scheme to find out the problem and hopefully improve it but the toolbox is such a blackbox that I don't know where to start from... The only information I can get from the processed file is that AOT @ 550nm is 0.271. Nothing more! It does not even mention the spectral characteristics of AOT which, I believe, is crucial in the problem of negative reflectances. Moreover, the spectral shapes of the corrected reflectances do not resemble at all the typical water reflectances we are accustomed to (inlcuding the ones presented in the Chris atmospheric correction ATBD, figure 13, p24). Our waters are Mediterranean waters that are very poor in chlorophyll and CDOM. It is normal that there is no fluorescence signal in our spectra but the rest of the spectra has such an awkward shape that one wonders if they are coherent.

Is there a way to investigate all the parameters involved in the correction (including the ones other than we initially provided to the UI) and
be able to fine-tune the behaviour of the algorithm so that acceptable water-level reflectances can be obtained?


Attachments: visat_screenshot2.png (778.1k)
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RE: Fine-tuning the CHRIS atmospheric correction over water
11/30/09 8:57 AM as a reply to servet cizmeli.
The source code for the CHRIS-Box can be obtained from our Subversion repository at https://www.brockmann-consult.de/svn/os/chris-box/.

In order to check out the source code install Subversion and type
svn co https://www.brockmann-consult.de/svn/os/chris-box/trunk/ PATH_TO_YOU_WORKING_DIRECTORY.

The source code for the atmospheric correction is located in the chris-atmospheric-correction directory. The source code you are concerned about is most likely to be found in the ComputeSurfaceReflectancesOp class. The original IDL source code can be found in the chris-atmospheric-correction/src/idl directory.

We know that the algorithm has some deficiencies. According to Luis Guanther, who developed the algorithm, solving the issues would require some major changes.
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