188.8.131.52.3.4 Land Identification (step 2.6.26) and Smile Effect Correction (step 2.1.6)
The purpose of this classification is to identify using geo-physical data, land from water pixels in cases where the Level1b a priori classification leads to ambiguities which may occur from:
· geo-location error;
· land /ocean atlas error: uncharted land or water, etc.;
· transient emerged land: tidal flats, etc.
This cases are identified using the Surface Confidence Map, an atlas identifying zones of low-confidence in the a priori land/water classification map used in the level1b. When the Surface Confidence Map indicates high confidence classification, the Land Identification radiometric tests are by-passed and the a priori classification is kept.
First, a test on the reflectance corrected for gaseous absorption at 665 nm is performed to identify the darkest pixels. The TOA reflectance at 665 nm is compared to a threshold interpolated from a LUT.
For the pixels having a reflectance smaller than this threshold, a second test is made to compare the TOA reflectance at 665 nm with the TOA reflectance at 865 nm ; if the TOA reflectance at 665 nm is greater than the reflectance at 865 nm, the pixel is classified as water.
Land in water
The purpose of this test is to identify pixels of emerged land, flagged as "water" in the L1B product. It is the opposite of the Inland water test.
The purpose of the Smile Effect Correction is to correct TOA reflectance (already corrected for stratospheric aerosols and gaseous absorption) for small scale variations due to non-constant central wavelength of a given band across the field of view. Correction is made only for a subset of bands for which those variations can induce severe distortions after corrections based on fixed wavelength scheme (e.g. Rayleigh diffusion correction). This subset of bands, which is specific to each land and water surface type, should ensure smoothness of reflectance local variations with wavelength and allow a good estimation of the reflectance derivative using neighbour bands.