The GFM product output layer S-1 Observed Flood Extent identifies the pixels covered by flood-water, mapped using Sentinel-1 (S-1) SAR backscatter intensity. Pixels that are normally under water (identified based on the monthly Sentinel-1 Reference Water Mask) are not part of the Sentinel-1 Observed Flood Extent. Sentinel-1 Observed Flood Extent is derived using the GFM ensemble flood mapping algorithm, as described in the dedicated section of this PUM.
To map flood extent pixels for a certain date, the algorithm uses as input the S-1 data overpass plus offline-generated S-1 SAR parameters and auxiliary thematic datasets such as Exclusion Mask and topography (e.g. digital elevation models and HAND index). The relative orbit path information, to select the corresponding offline-generated S-1 SAR parameters, is extracted from the S-1 Metadata. During the near real-time operation of the GFM product, the acquisition month of the S-1 scene is retrieved from the S-1 Metadata and the corresponding monthly Sentinel-1 Reference Water Mask is cropped to the extent of the processed S-1 scene. All the processing is done at the 20 metres spatial resolution of the S-1 pre-processed data cube.
For further information on how the product is generated please check also: https://extwiki.eodc.eu/GFM/PDD/GFMoutputLayers