    thumb|200px|relief map of sierra nevada  terrain, or land relief, is the vertical and horizontal dimension of land surface. when relief is described underwater, the term bathymetry is used. terrain is used as a general term in physical geography, referring to the lay of the land. this is usually expressed in terms of the elevation, slope, and orientation of terrain features. terrain affects surface water flow and distribution. over a large area, it can affect weather and climate patterns. ==importance== the understanding of terrain is critical for many reasons: * the terrain of a region largely determines its suitability for human settlement: flatter, alluvial plains tend to have better farming soils than steeper, rockier uplands. * in terms of environmental quality, agriculture, and hydrology, understanding the terrain of an area enables the understanding of watershed boundaries, drainage characteristics, water movement, and impacts on water quality. complex arrays of relief data are used as input parameters for hydrology transport models (such as the swmm or dssam models) to allow prediction of river water quality. * understanding terrain also supports soil conservation, especially in agriculture. contour plowing is an established practice enabling sustainable agriculture on sloping land; it is the practice of plowing along lines of equal elevation instead of up and down a slope. * terrain is militarily critical because it determines the ability of armed forces to take and hold areas, and move troops and material into and through areas. an understanding of terrain is basic to both defensive and offensive strategy. * terrain is important in determining weather patterns. two areas geographically close to each other may differ radically in precipitation levels or timing because of elevation differences or a "rain shadow" effect. * precise knowledge of terrain is vital in 
Physical_geography Topography