Terrain



Terrain, or relief, is the third or vertical dimension of land surface. When relief is described underwater, the term bathymetry is used. Topography has recently become an additional synonym, though in many parts of the world it retains its original more general meaning of the description of place. Terrain is used as a general term in physical geography, referring to the lie of the land. This is usually expressed in terms of the elevation, slope, and aspect. Terrain affects surface water flow, weather and climate patterns.

Importance of terrain
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 be better farming soils than steeper, rockier uplands.
 * In terms of environmental quality, agriculture, hydrology, and other interdisciplinary sciences ; understanding the terrain of an area assists the understanding of watershed boundaries, drainage characteristics, groundwater systems, 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. Additionally, when the terrain of a larger area is taken into account, weather predictions become invaluable to those calculating crop yield and estimating natural hazards.
 * Terrain is militarily critical because it determines the ability of armed forces to take and hold areas and to move troops and material into and through areas. An understanding of terrain is basic to both defensive and offensive strategy.
 * Terrain is important in understanding weather patterns. Two areas close to each other geographically may differ radically in precipitation levels or timing because of elevation differences or a "rain shadow" effect.
 * In addition, terrain can be a useful tool to help a map reader understand the characteristics of the land portrayed, and to increase interest in the map.

Representing Terrain
There are many ways to represent terrain. While 3-D representations can be created (such as a raised relief map) there are also many ways that have been devised to show terrain on 2-D maps. Examples of such techniques include contour lines, tanaka contours, hachures, resolution bumping, illuminated relief, bump mapping, digital elevation models such as the USGS Digital elevation models, hyposemetric tinting and triangulated irregular networks. Spot height labels can also be used to give elevation values of important points. This method is generally used to supplement one of the other methods listed above.

When representing terrain on a map where terrain is not the primary message of the map, care must be used to depict terrain in a way that will not distract from the main purpose of the map.

Geomorphology
Geomorphology is in large part the study of the formation of terrain or topography. Terrain is formed by concurrent processes:
 * Geological processes - migration of tectonic plates, faulting and folding, volcanic eruptions, etc.
 * Erosional processes - glacial, water, wind erosion, and landslides.
 * Extraterrestrial - meteorite impacts

Tectonic processes, such as orogenies and uplifts, cause the land to be elevated, whereas erosional and weathering processes wear the land away, smoothing and reducing features. The relationship of erosion and tectonics rarely (if ever) reaches equilibrium. These processes are also co-dependent; however, the full range of their interactions is still a topic of debate.

Land surface parameters are quantitative measures of various morphometric properties of a surface. The most common examples are used to derive slope or aspect of a terrain or curvatures at each location. These measures can also be used to derive hydrological parameters that reflect flow or erosion processes. Climatic parameters are based on the modeling of solar radiation or air flow.

Land surface objects or landforms are definite physical objects (lines, points, areas) that differ from the surrounding objects. The most typical examples are lines of watersheds, stream patterns, ridges, break-lines, pools, borders of specific landforms, etc.