To produce this map, topographical data from the GTOPO30 global digital elevation model (USGS EROS Data Center 1996) were used to generate slope and local (5km radius) elevation range on a 30 arc-second grid of the world. These parameters were combined with elevation to arrive at the empirically derived definitions of the six mountain classes displayed on this map. (A spatial filter was applied to classify isolated individual 'non-mountain' grid cells in mountain areas according to the predominant class of their immediate neighbours.) The global mountain area thus defined totals 35.8 million km2, or 24% of the Earth's land surface.