Topography Examples

[Emmaline Sasportas]

Topographycombines top- with graph-, a root meaning "write" or "describe". The topography of the Sahara Desert features shifting sand dunes and dry, rocky mountains. A topographic (or topo) map not only shows the surface features of a region but also indicates the contours and approximate altitude of every location, by means of numerous curving lines, each indicating a single elevation. In other words, it shows a "three-dimensional" picture on a two-dimensional surface. Topo maps are commonly used by hikers, surveyors, government workers, and engineers, among other people.

Inverted topography is a classic type of landform in the American southwest with its young monogenetic volcanic fields and rapid arid-land erosion. Many volcanic deposits, whether they are lava flows or ash-flow-tuffs, flow down river valleys and topographic lows and even fill them. Because they are resistant to erosion, the volcanic rocks may erode away more slowly than the rocks that had made the sides of the former valleys. With time and this process of differential erosion, lava flows that once travelled down valleys later stand as mesas. This process reverses the elevation of volcanic features relative to other features; e.g., lava-filled valleys become mesas, and, in turn, former topographic high points become valleys.

Inverted topography can sometimes help geologists determine the relative age of volcanic deposits. In areas with numerous lava flows of varying ages, the oldest will stand the highest above the current landscape with progressively younger flows becoming closer to the current land surface.

The most common type of inverted topography is formed by basaltic lava flows. But inverted topography can also form in locations of former valleys that were filled by ash-flow tuffs or lava domes as these rock types are also frequently harder than surrounding country rocks.

Figure (right): Schematic illustrations of the formation of inverted topography. Top graphic: Lava flows travel in topographic lows and valleys below surrounding topographic highs. Middle graphic: The hard volcanic rock protects the underlying country rock from erosion, eroding more slowly than the surrounding topography. Bottom graphic: stream valleys in which lava once flowed stand as high mesas above modern valleys.

NPS image by Trista Thornberry-Ehrlich after Muehlberger et al. (2005, p. 4).

Capulin Volcano NM sits within the Raton-Clayton Volcanic Field which experienced three phases of activity during the last 9 million years. The older lava flows stand at an elevation above the current ground surface level, whereas younger lava flows, like those erupted from Capulin Volcano, are the current ground surface level.

Left image

The Johnson Mesa lava flow was erupted approximately 8 million years ago and now holds up a low mesa west of Capulin Volcano NM. Jose Butte is a cinder cone that erupted about 2.5 million years ago.

Credit: Photo by Allyson Mathis.

Examples of inverted topography are found near Grand Canyon. Some of the basaltic lava flows capping mesas and buttes in the region provide important age constraints for the age of the landscape as geologists can determine the age of the lava flows and calculate rates of erosion using the age information and how high the lava flow is above the regional ground level.

Volcanic inverted topography is recognized for having geoheritage values since these landscapes can be easy to understand, can readily be observed, and result from the passage of geologic time and geologic change.

Topography is a field of geoscience and planetary science and is concerned with local detail in general, including not only relief, but also natural, artificial, and cultural features such as roads, land boundaries, and buildings.[1] In the United States, topography often means specifically relief, even though the USGS topographic maps record not just elevation contours, but also roads, populated places, structures, land boundaries, and so on.[2]

Topography in a narrow sense involves the recording of relief or terrain, the three-dimensional quality of the surface, and the identification of specific landforms; this is also known as geomorphometry. 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 a map by a variety of cartographic relief depiction techniques, including contour lines, hypsometric tints, and relief shading.

Detailed military surveys in Britain (beginning in the late eighteenth century) were called Ordnance 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 the Cassini 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 the War of 1812,[7] which became the Corps of Topographical Engineers in 1838.[8] After the work of national mapping was assumed by the U.S. 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.

An objective of topography is to determine the position of any feature or more generally any point in terms of both a horizontal coordinate system such as latitude, longitude, and altitude. Identifying (naming) features, and recognizing typical landform patterns are also part of the field.

A topographic study may be made for a variety of reasons: military planning and geological exploration have been primary motivators to start survey programs, but detailed information about terrain and surface features is essential for the planning and construction of any major civil engineering, public works, or reclamation projects.

Surveying helps determine accurately the terrestrial or three-dimensional space position of points and the distances and angles between them using leveling instruments such as theodolites, dumpy levels and clinometers.GPS and other global navigation satellite systems (GNSS) are also used.

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 or GIS-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 as USGS 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.

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 of geovisualization, whether maps or GIS systems. False-color and non-visible spectra 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 the co-ordinates of the points in 3D of an object are determined by the measurements made in two photographic 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 each image. A line of sight (or ray) 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 sophisticated algorithms 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).

Satellite RADAR mapping is one of the major techniques of generating Digital Elevation Models (see below). Similar techniques are applied in bathymetric surveys using sonar to determine the terrain of the ocean floor. In recent years, LIDAR (LIght Detection And Ranging), 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.

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 local cadastral 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 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.

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.[9]

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