Editing Geographic information system (section)

===Terrain analysis===
[[File:Dem.jpg|thumb|right|300px|Hillshade model derived from a [[digital elevation model]] of the Valestra area in the northern Apennines (Italy)]]
{{main|Geomorphometry}}
{{see also|Surface gradient}}
Many geographic tasks involve the [[terrain]], the shape of the surface of the earth, such as [[hydrology]], [[Earthworks (engineering)|earthworks]], and [[biogeography]]. Thus, terrain data is often a core dataset in a GIS, usually in the form of a raster [[Digital elevation model]] (DEM) or a [[Triangulated irregular network]] (TIN). A variety of tools are available in most GIS software for analyzing terrain, often by creating derivative datasets that represent a specific aspect of the surface. Some of the most common include:
* [[Grade (slope)|Slope or grade]] is the steepness or gradient of a unit of terrain, usually measured as an angle in degrees or as a percentage.<ref name=Jones1998>{{cite journal |doi= 10.1016/S0098-3004(98)00032-6 |author = Jones, K.H.|year=1998 |title=A comparison of algorithms used to compute hill slope as a property of the DEM |journal=Computers and Geosciences |volume=24 |issue=4 |pages =315–323 |bibcode=1998CG.....24..315J}}</ref> 
* [[Aspect (geography)|Aspect]] can be defined as the direction in which a unit of terrain faces. Aspect is usually expressed in degrees from north.<ref name="Skidmore1989">{{cite journal |author1 =Chang, K. T.|year=1989|title=A comparison of techniques for calculating gradient and aspect from a gridded digital elevation model|journal=International Journal of Geographical Information Science|volume=3|issue=4|pages =323–334 |doi=10.1080/02693798908941519|url=https://research.utwente.nl/en/publications/470392f6-30d7-45df-806b-a29aa84aaa72 }}</ref> 
* Cut and fill is a computation of the difference between the surface before and after an [[Earthworks (engineering)|excavation]] project to estimate costs.
* [[Hydrological model]]ing can provide a spatial element that other hydrological models lack, with the analysis of variables such as slope, aspect and watershed or [[Catchment area (human geography)|catchment area]].<ref name="Heywood">{{cite book |vauthors=Heywood I, Cornelius S, Carver S |year=2006 |title=An Introduction to Geographical Information Systems |publisher=Prentice Hall |place=Essex, England |edition = 3rd}}</ref> Terrain analysis is fundamental to hydrology, since water always flows down a slope.<ref name="Heywood" /> As basic terrain analysis of a [[digital elevation model]] (DEM) involves calculation of slope and aspect, DEMs are very useful for hydrological analysis. Slope and aspect can then be used to determine direction of [[surface runoff]], and hence flow accumulation for the formation of streams, rivers and lakes. Areas of divergent flow can also give a clear indication of the boundaries of a catchment. Once a flow direction and accumulation matrix has been created, queries can be performed that show contributing or dispersal areas at a certain point.<ref name="Heywood" /> More detail can be added to the model, such as terrain roughness, vegetation types and soil types, which can influence infiltration and evapotranspiration rates, and hence influencing surface flow. One of the main uses of hydrological modeling is in [[GIS in environmental contamination|environmental contamination research]]. Other applications of hydrological modeling include [[GIS and hydrology|groundwater and surface water mapping]], as well as flood risk maps.
* [[Viewshed analysis]] predicts the impact that terrain has on the visibility between locations, which is especially important for wireless communications.
* [[Shaded relief]] is a depiction of the surface as if it were a three dimensional model lit from a given direction, which is very commonly used in maps.

Most of these are generated using algorithms that are discrete simplifications of [[vector calculus]]. Slope, aspect, and surface curvature in terrain analysis are all derived from neighborhood operations using elevation values of a cell's adjacent neighbours.<ref name="Chang">{{cite book |author=Chang, K. T. |year=2008 |title=Introduction to Geographical Information Systems |publisher=McGraw Hill |place=New York |page =184}}</ref> Each of these is strongly affected by the level of detail in the terrain data, such as the resolution of a DEM, which should be chosen carefully.<ref name="Longley2005">{{cite journal |author1 =Longley, P. A. |author2 =Goodchild, M. F. |author3 =McGuire, D. J. |author4 = Rhind, D. W. |year=2005 |title=Analysis of errors of derived slope and aspect related to DEM data properties |journal=Geographic Information Systems and Science |publisher=John Wiley and Sons |place=West Sussex, England |page=328}}</ref>