The surface flagging algorithm is based on GEOTIFF maps that provide fixed masks for land/water classifications, land/ocean classification, tidal regions and high resolution coastline at 0.00277° resolution. Each map divides the surface of the earth into cells, for which a flag is defined identifying the type of surface. Based on these maps, an a priori classification is made to derive land, saline water, coastline, tidal regions and fresh water rivers and lakes flags.

Given the latitude and longitude of the pixel, the algorithm identifies for each map, the cell in which the pixel lies using a nearest neighbour method, and sets the flag accordingly, i.e. to '1' if it belongs to the surface type identified by the flag.