Editing Topography (section)

==Forms of topographic data==
Terrain is commonly modelled either using vector ([[triangulated irregular network]] or TIN) or gridded ([[raster image]]) mathematical models. In the most applications in [[environmental science]]s, land surface is represented and modelled using gridded models. In civil engineering and entertainment businesses, the most representations of land surface employ some variant of TIN models. In [[geostatistics]], land surface is commonly modelled as a combination of the two signals – the smooth (spatially correlated) and the [[Soil-surface roughness|rough]] (noise) signal.

In practice, surveyors first sample heights in an area, then use these to produce a Digital Land Surface Model in the form of a [[Triangulated irregular network|TIN]]. The DLSM can then be used to visualize terrain, drape remote sensing images, quantify ecological properties of a surface or extract land surface objects. The contour data or any other sampled elevation datasets are not a DLSM. A DLSM implies that elevation is available continuously at each location in the study area, i.e. that the map represents a complete surface. Digital Land Surface Models should not be confused with Digital Surface Models, which can be surfaces of the canopy, buildings and similar objects. For example, in the case of surface models produces using the lidar technology, one can have several surfaces – starting from the top of the canopy to the actual solid earth. The difference between the two surface models can then be used to derive volumetric measures (height of trees etc.).

===Raw survey data===
Topographic survey information is historically based upon the notes of surveyors. They may derive naming and cultural information from other local sources (for example, [[Boundary (topology)|boundary]] delineation may be derived from local [[cadastral]] mapping). While of historical interest, these field notes inherently include errors and contradictions that later stages in map production resolve.

===Remote sensing data===
As with field notes, remote sensing data (aerial and satellite photography, for example), is raw and uninterpreted. It may contain holes (due to cloud cover for example) or inconsistencies (due to the timing of specific image captures). Most modern topographic mapping includes a large component of remotely sensed data in its compilation process.

===Topographic mapping===
{{Main|Topographic map}}
[[File:Europe topography map.png|thumb|A map of [[Europe]] using elevation modeling]]
In its contemporary definition, topographic mapping shows relief. In the United States, [[USGS]] topographic maps show relief using [[contour lines]]. The USGS calls maps based on topographic surveys, but without contours, "planimetric maps."

These maps show not only the contours, but also any significant streams or other bodies of water, [[forest cover]], built-up areas or individual buildings (depending on scale), and other features and points of interest.

While not officially "topographic" maps, the national surveys of other nations share many of the same features, and so they are often called "topographic maps."

Existing topographic survey maps, because of their comprehensive and encyclopedic coverage, form the basis for much derived topographic work. Digital Elevation Models, for example, have often been created not from new remote sensing data but from existing paper topographic maps. Many government and private publishers use the artwork (especially the contour lines) from existing topographic map sheets as the basis for their own specialized or updated topographic maps.<ref>See for example the publications of [http://www.ngmapstore.com/jump.jsp?itemType=CATEGORY&itemID=54 National Geographic Trails Illustrated Maps] {{Webarchive|url=https://web.archive.org/web/20061116063243/http://www.ngmapstore.com/jump.jsp?itemID=54&itemType=CATEGORY |date=2006-11-16 }} and [[DeLorme]] products.</ref>

Topographic mapping should not be confused with [[geological map]]ping. The latter is concerned with underlying structures and processes to the surface, rather than with identifiable surface features.

===Digital elevation modeling===
{{Main|Digital elevation model}}
[[File:Maps-for-free Sierra Nevada.png|thumb|Relief map: [[Sierra Nevada (Spain)|Sierra Nevada Mountains]], [[Spain]] ]]
[[File:Mtm-05277e 3d.png|thumb|3D rendering of a [[Digital elevation model|DEM]] used for the [[topography of Mars]]]]
The digital elevation model (DEM) is a [[raster graphics|raster]]-based [[Digital data|digital]] dataset of the topography ([[hypsometry]] and/or [[bathymetry]]) of all or part of the Earth (or a [[telluric planet]]). The [[pixels]] of the dataset are each assigned an elevation value, and a header portion of the dataset defines the area of coverage, the units each pixel covers, and the units of elevation (and the zero-point). DEMs may be derived from existing paper maps and survey data, or they may be generated from new satellite or other remotely sensed [[radar]] or [[sonar]] data.

===Topological modeling===
[[File:Earth_dry_elevation.stl|thumb|[[STL (file format)|STL 3D model]] of Earth without liquid water with 20× elevation exaggeration]]
A [[geographic information system]] (GIS) can recognize and analyze the spatial relationships that exist within digitally stored spatial data. These topological relationships allow complex spatial [[Model (abstract)|modelling]] and analysis to be performed. Topological relationships between geometric entities traditionally include adjacency (what adjoins what), containment (what encloses what), and proximity (how close something is to something else).
* reconstitute a sight in synthesized images of the ground,
* determine a trajectory of overflight of the ground,
* calculate surfaces or volumes,
* trace topographic profiles,