This article is about the study of Earth's surface shape and features. For discussion of land surfaces themselves, seeTerrain. For other uses, seeTopography (disambiguation).
Topography is the study of forms and features ofland surfaces. The topography of an area may refer to landforms and features themselves, or a description ordepiction in maps.
Topography is a field ofgeoscience andplanetary science, and is concerned with local detail in general, including not onlyrelief, but alsonatural, artificial, andcultural features such as roads, land boundaries, and buildings.[1] In the United States, topography often means specifically relief, even though theUSGStopographic maps record not justelevation contours, but also roads, populated places, structures, land boundaries, and so on.[2]
Topography in a narrow sense involves the recording of relief orterrain, the three-dimensional quality of the surface, and the identification of specificlandforms; this is also known asgeomorphometry. In modern usage, this involves generation of elevation data in digital form (DEM). It is often considered to include the graphic representation of the landform on amap by a variety ofcartographic relief depiction techniques, includingcontour lines,hypsometric tints, andrelief shading.
The termtopography originated inancient Greece and continued inancient Rome, as the detailed description of a place. The word comes from theGreekτόπος (topos, "place") and-γραφία (-graphia, "writing").[3] In classical literature this refers to writing about a place or places, what is now largely called 'local history'. In Britain and in Europe in general, the word topography is still sometimes used in its original sense.[4]
Detailed military surveys inBritain (beginning in the late eighteenth century) were calledOrdnance Surveys, and this term was used into the 20th century as generic for topographic surveys and maps.[5] The earliest scientific surveys in France were theCassini maps after the family who produced them over four generations.[6] The term "topographic surveys" appears to be American in origin. The earliest detailed surveys in the United States were made by the "Topographical Bureau of the Army", formed during theWar of 1812,[7] which became theCorps of Topographical Engineers in 1838.[8] After the work of national mapping was assumed by theUnited States Geological Survey in 1878, the term topographical remained as a general term for detailed surveys and mapping programs, and has been adopted by most other nations as standard.
In the 20th century, the term topography started to be used to describe surface description in other fields wheremapping in a broader sense is used, particularly in medical fields such asneurology.
An objective of topography is to determine the position of any feature or more generally any point in terms of both a horizontalcoordinate system such as latitude, longitude, andaltitude. Identifying (naming) features, and recognizing typical landform patterns are also part of the field.
There are a variety of approaches to studying topography. Which method(s) to use depends on the scale and size of the area under study, its accessibility, and the quality of existing surveys.
Work on one of the first topographic maps was begun in France byGiovanni Domenico Cassini, the great Italian astronomer.
Even though remote sensing has greatly sped up the process of gathering information, and has allowed greater accuracy control over long distances, the direct survey still provides the basic control points and framework for all topographic work, whether manual orGIS-based.
In areas where there has been an extensive direct survey and mapping program (most of Europe and the Continental U.S., for example), the compiled data forms the basis of basic digital elevation datasets such asUSGS DEM data. This data must often be "cleaned" to eliminate discrepancies between surveys, but it still forms a valuable set of information for large-scale analysis.
The original Americantopographic surveys (or the British "Ordnance" surveys) involved not only recording of relief, but identification of landmark features and vegetative land cover.
Topographic surveys using laser scanners, commonly known aslidar (LIghtDetectionAndRanging), is a method for capturing high-resolution spatial data of landscapes, architectural structures, and terrains with a vertical accuracy of 10 centimeters. These surveys utilise a laser scanner that emits millions of laser pulses every second. The travel time of these pulses as they are reflected or bounce back from the ground are measured, allowing for the creation of a detailed point cloud that represents the scanned environment.[9][10] Products of lidar includeDigital elevation models (DEMs), which are a representation of the bare earth topographic surface (excluding vegetation, buildings and other surface objects).[11]
Besides their role in photogrammetry, aerial and satellite imagery can be used to identify and delineate terrain features and more general land-cover features. Certainly they have become more and more a part ofgeovisualization, whethermaps orGIS systems. False-color and non-visiblespectra imaging can also help determine the lie of the land by delineating vegetation and other land-use information more clearly. Images can be in visible colours and in other spectrum.
Photogrammetry is a measurement technique for which theco-ordinates of the points in3D of an object are determined by the measurements made in twophotographic images (or more) taken starting from different positions, usually from different passes of an aerial photography flight. In this technique, the common points are identified on eachimage. A line of sight (orray) can be built from the camera location to the point on the object. It is the intersection of its rays (triangulation) which determines the relative three-dimensional position of the point. Known control points can be used to give these relative positions absolute values. More sophisticatedalgorithms can exploit other information on the scene known a priori (for example, symmetries in certain cases allowing the rebuilding of three-dimensional co-ordinates starting from one only position of the camera).
SatelliteRADAR mapping is one of the major techniques of generating Digital Elevation Models (see below). Similar techniques are applied inbathymetric surveys usingsonar to determine the terrain of the ocean floor. In recent years, lidar, a remote sensing technique that uses a laser instead of radio waves, has increasingly been employed for complex mapping needs such as charting canopies and monitoring glaciers.
Terrain is commonly modelled either using vector (triangulated irregular network or TIN) or gridded (raster image) mathematical models. In the most applications inenvironmental sciences, 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. Ingeostatistics, land surface is commonly modelled as a combination of the two signals – the smooth (spatially correlated) and therough (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 aTIN. 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.).
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 delineation may be derived from localcadastral mapping). While of historical interest, these field notes inherently include errors and contradictions that later stages in map production resolve.
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.
In its contemporary definition, topographic mapping shows relief. In the United States,USGS topographic maps show relief usingcontour 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.[12]
Topographic mapping should not be confused withgeological mapping. The latter is concerned with underlying structures and processes to the surface, rather than with identifiable surface features.
The digital elevation model (DEM) is araster-baseddigital dataset of the topography (hypsometry and/orbathymetry) of all or part of the Earth (or atelluric planet). Thepixels 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 sensedradar orsonar data.
STL 3D model of Earth without liquid water with 20× elevation exaggeration
Ageographic information system (GIS) can recognize and analyze the spatial relationships that exist within digitally stored spatial data. These topological relationships allow complex spatialmodelling 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,
In mathematics the concept of topography is used to indicate the patterns or general organization of features on a map or as a term referring to the pattern in which variables (or their values) are distributed in a space.
