"This data set offers users 25 km Northern Hemisphere snow cover extent represented by four different variables. Three of the snow cover variables are derived from the Interactive Multisensor Snow and Ice Mapping System, MODIS Cloud Gap Filled Snow Cover, and passive microwave brightness temperatures, respectively. The fourth variable merges the three source products into a single representation of snow cover."

From the file: "This NASA MEaSUREs Earth System Data Record (ESDR) merges daily Northern Hemisphere snow cover extents over land derived from three independently produced sources.  Variables include snow cover extent from the Interactive Multisensor Snow and Ice Mapping System (IMS), a gap-filled snow extent product derived from the Moderate Resolution Imaging Spectroradiometer (MODIS), and a gap-filled snow extent product derived from the Special Sensor Microwave/Imager (SSMI) and Special Sensor Microwave Imager/Sounder (SSMIS).  The NSIDC Land-Ocean-Coast-Ice (LOCI) mask derived from BU-MODIS land cover data is consistently applied to each variable."