I am facing a “negative reflectance” problem in the Sentinel-2 images that I correct with ACOLITE.
I am finding that water pixels spectra show negative reflectance values in the SWIR bands B11 and B12, but I wonder also if all the spectra band points are shifted down. I am analysing a coastal area in the North Adriatic Sea over the Friuli Venezia Giulia region in Italy using the Sentinel-2 raster S2B_MSIL1C_20210912T100029_N0301_R122_T33TUL_20210912T120438.SAFE .
Here is the google maps view pointed to the region of interest: https://goo.gl/maps/THc5M7fH71F8QdUZ7.
The geojson polygons I use to clip the Sentinel raster are listed in the txt file "Geojson_Polygons". The first polygon is that I use to apply Acolite, the second one is to clip the corrected raster into a smaller one covering only water pixels. The picture below is the mean water spectra calculated with pixels of this smaller raster.
I am applying ACOLITE with simple basic settings (see the attached file with the user settings), so I am not sure what can cause this overcorrection. I have increased the size of the region where the algorithm can evaluate the atmospheric correction, adding more inland areas, but the problem remains. Moving the polygon to another place along the coast gives the same results.
Could you give me some suggestions?
Thanks in advance for you help,
Stefano
P.s. I don’t know if this is relevant to solve the issue, but the terminal output a warning after the “Detector 1” step:
Code: Select all
Detector 1
…/ACOLITE/acolite/acolite/sentinel2/l1_convert.py:413: RuntimeWarning: Mean of empty slice
ave_vza = np.nanmean(ave_vza, axis=2)
…/ACOLITE/acolite/acolite/sentinel2/l1_convert.py:414: RuntimeWarning: Mean of empty slice
ave_vaa = np.nanmean(ave_vaa, axis=2)
Detector 2