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In physical geography, aspect is the compass direction that a topographic slope faces, usually measured in degrees from north. Aspect can be generated from continuous elevation surfaces.
Aspect in Physical Geography
Generally, aspect refers to the compass direction that a hillside or slope faces. A southern aspect is the same as a south-facing slope. Many geographic information systems (GIS) can analyze digital elevation data (elevation points, contour lines, digital elevation models, etc.) and derive both slope and aspect data sets. The degree to which sunlight strikes a hillside depends on its aspect; north-facing hillsides receive little or no sunlight; east- and west-facing slopes receive sunlight for a portion of each day (unless they are shaded by another hill or mountain); and south-facing slopes receive the greatest amount of sunlight. Because aspect affects the amount of sunlight striking the land's surface, aspect can be an important metric that aids in the siting of buildings to maximize or minimize solar gain. Aspect also can be one of a number of determinants of vegetation communities, habitat, soil moisture, evapotranspiration, and other biological and physical landscape characteristics.
In a GIS, aspect is sometimes quantified in compass degrees.
A slope on the eastern edge of the Rockies toward the Great Plains is described as having an easterly aspect. A slope which falls down to a deep valley on its western side and a shallower one on its eastern side has a westerly aspect or is a west-facing slope. The term can also be used to describe the shape or alignment of a coastline. Here, the aspect is the direction which the coastline is facing towards the sea. For example, a coastline with sea to the northeast (as in most of Queensland) has a northeasterly aspect.
In ArcGIS, aspect is derived from a raster surface. Aspect identifies the downslope direction of the maximum rate of change in value from each cell to its neighbors. Aspect can be thought of as the slope direction. The values of the output raster will be the compass direction of the aspect.
Importance of Aspect
The aspect of a slope can make very significant influences on its local climate (microclimate). For example, because the sun's rays are in the west at the hottest time of day in the afternoon, in most cases a west-facing slope will be warmer than a sheltered east-facing slope (unless large-scale rainfall influences dictate otherwise). This can have major effects on altitudinal and polar limits of tree growth and also on the distribution of vegetation that requires large quantities of moisture. In Australia, for example, remnants of rainforest are almost always found on east-facing slopes which are protected from dry westerly wind.
Similarly, in the northern hemisphere a south-facing slope will accure before it becomes more open to sunlight and warm winds and snow will therefore generally be warmer and dryer due to higher levels of evapotranspiration than a north-facing slope. This can be seen in the Swiss Alps, where farming is much more extensive on south-facing than on north-facing slopes. In the Himalayas, this effect can be seen to an extreme degree, with south-facing slopes being warm, wet and forested, and north-facing slopes cold, dry but much more heavily glaciated.
For a farmer, thus, choosing an aspect that will shelter from hot, dry winds or from cold can be critical to successful growth of crops. Most farming agencies will state in manuals whether a crop prefers a poleward or equatorward aspect (if this is important). Glaciology is also influenced by aspect: unless precipitation patterns dictate otherwise (as in Iceland where glaciers accumulate on the much wetter southwestern side) glaciers always accumulate downwards on the poleward side of mountains.
Aspect is also important in many other fields such as the broader study of terrain analysis. Under terrain analysis, aspect as well as slope are required in fields such as hydrology, conservation, site planning, and infrastructure development. Aspect is also useful in studies involving watershed boundaries, flowpaths and direction, and erosion modeling. With these last three areas GIS analysts can find the dominant vegetation types dependent on aspect. With this information, studies can be made to show how different slopes will erode over time. Aspect combined with precipitation, temp, growing periods, and slope can enhance erosion modeling. This is important for future building sites as well as assessing the building and maintaining of roads in high slope areas. Areas with a northern aspect have less access to sunlight causing more vegetation to grow on the southern slopes. This difference in vegetation causes differences in erosion depending on the specific area of the world under study. 
In some locals there are patterns of soil differences related to differences in aspect. Strong slopes with equatorward aspects tend to have soil organic matter levels and seasonal influences similar to level slopes at lower elevation whereas poleward aspects have soil development similarities to level soils at higher elevations. Soils with a prevailing windward aspect will typically be shallower, and often with more developed subsoil characteristics, than adjacent soils on the leeward where decelerating winds tend to deposit more air-borne particulate material. Outside of the tropics, soils with an aspect directed toward an early afternoon solar position will typically have the lowest soil moisture content and lowest soil organic matter content relative to other available aspects in a locale. Aspect similarly influence seasonal soil biological processes that are temperature dependent. Particulate laden winds often blow from a prevailing direction near solar early afternoon, the effects combine in pattern common to both hemispheres.
These are usually of importance only in the tropics, but there they produce many unexpected climatic effects:
- The dryness of the Dahomey Gap, due to the rain-bearing winds moving parallel to the coast.
- The summer dryness of the Coromandel Coast due to the southerly monsoon flowing parallel to the coast. Its wetness during the northeast monsoon is similarly explained.
- The anomalous late autumn rainy seasons of central Vietnam and the coastal zone of northeastern Brazil for the same reason as above.
- The unusual dryness of Port Moresby compared to the rest of New Guinea is because the National Capital District lies parallel to the trade winds which have a drying effect. In Gulf Province and Lae, which receives their full force, rainfall during southern winter is exceedingly heavy.
- The relative dryness of the Queensland coast has the same cause as with Port Moresby.
GIS&T Body of Knowledge
Aspect is covered by sections AM6-1 and CV4-4 in the GIS&T Body of Knowledge.
- Bennie, Jonathan; Mark O. Hill, Robert Baxter, and Brian Huntley (2006). "Influence of slope and aspect on long-term vegetation change in British chalk grasslands". Journal of Ecology 94 (2): 355–368. doi:10.1111/j.1365-2745.2006.01104.x.
- Bolstad, Paul. GIS Fundamentals: A First Text on Geographic Information Systems. 3rd Edition. 2008. Pg 417-421