Figure 8.27 The influence of terrain on agriculture. The spatial relationship of commercial grain farming and flat terrain appears in the American Midwest, about 1960. “Flat” terrain is defined as any land with a slope of 3 degrees or less. Commercial grain farming is completely mechanized, and flat land permits more efficient machine operation. The result is this striking correlation between a type of agriculture and a type of terrain. What other factors might attract mechanized grain farming to level land? (Source: After Hidore, 1963, pp. 86, 87.)