Level-1C Masks - Sentinel-2 MSI Technical Guide - Sentinel Online
The masks included in the Level-1C are:
- Level-1B technical masks (defective pixels, no data pixels, etc) reprojected to the Level-1C geometry
- Cloud Masks recomputed starting from the tile images
The mask calculation is based on the 60 m resolution and the constituent spectral bands (Band 1 at 443 nm or Band 2 at 490 nm, Band 10 at 1 375 nm and a SWIR of either Band 11 at 1 610 nm or Band 12 at 2 190 nm) are used.
To minimise the potential errors arising as a result of an outlying value (such as a geometric error, or the presence of a highly reflective object in the VIR) and to ensure that any pixel flagged as cloud-free is truly cloud-free, an additional morphology-based filtering step is undertaken. Under this filtering, the presence of spatially isolated pixel values is mitigated (erosion), followed by a dilation of local values into the gap.
The morphology operation is applied separately to the opaque (value of 1) cloud mask, and the cirrus (value of 2) cloud mask, and the precise size of the neighbouring pixel area involved in dilation and erosion is taken from the associated GIPP.