USGS land/sea mask dataset

An excellent free, Global data resource... The land/sea mask has been improved and now includes ocean boundaries from the World Vector Shoreline (WVS) data set. The boundaries for U.S. Great Lakes and St. Lawrence seaway come from the Political/Ocean (ponet) layer of the Digital Chart of the World (DCW). The boundaries of inland water bodies come from the perennial water bodies and polygon features (double line rivers) of the Drainage Network (dnnet) layer of the DCW. The land/sea mask has been generalized during the process of rasterizing the vector data to 1-km grid cells. A 1-km cell is assigned to land or water based on the dominant area of a cell. World Vector Shoreline boundaries have precedence over DCW. Grab it at http://edc2.usgs.gov/1KM/land_sea_mask.php