Tennessee is a state located in the southeastern region of the United States. Tennessee is the 16th most populous of the 50 United States. Tennessee is bordered by Kentucky to the north, Virginia to the northeast, North Carolina to the east, Georgia and Mississippi to the south, Arkansas to the west, Missouri to the northwest; the Appalachian Mountains dominate the eastern part of the state, the Mississippi River forms the state's western border. Nashville is the state's capital and largest city, with a 2017 population of 667,560. Tennessee's second largest city is Memphis, which had a population of 652,236 in 2017; the state of Tennessee is rooted in the Watauga Association, a 1772 frontier pact regarded as the first constitutional government west of the Appalachians. What is now Tennessee was part of North Carolina, part of the Southwest Territory. Tennessee was admitted to the Union as the 16th state on June 1, 1796. Tennessee was the last state to leave the Union and join the Confederacy at the outbreak of the American Civil War in 1861.
Occupied by Union forces from 1862, it was the first state to be readmitted to the Union at the end of the war. Tennessee furnished more soldiers for the Confederate Army than any other state besides Virginia, more soldiers for the Union Army than the rest of the Confederacy combined. Beginning during Reconstruction, it had competitive party politics, but a Democratic takeover in the late 1880s resulted in passage of disenfranchisement laws that excluded most blacks and many poor whites from voting; this reduced competition in politics in the state until after passage of civil rights legislation in the mid-20th century. In the 20th century, Tennessee transitioned from an agrarian economy to a more diversified economy, aided by massive federal investment in the Tennessee Valley Authority and, in the early 1940s, the city of Oak Ridge; this city was established to house the Manhattan Project's uranium enrichment facilities, helping to build the world's first atomic bombs, two of which were dropped on Imperial Japan near the end of World War II.
Tennessee's major industries include agriculture and tourism. Poultry and cattle are the state's primary agricultural products, major manufacturing exports include chemicals, transportation equipment, electrical equipment; the Great Smoky Mountains National Park, the nation's most visited national park, is headquartered in the eastern part of the state, a section of the Appalachian Trail follows the Tennessee-North Carolina border. Other major tourist attractions include the Tennessee Aquarium in Chattanooga; the earliest variant of the name that became Tennessee was recorded by Captain Juan Pardo, the Spanish explorer, when he and his men passed through an American Indian village named "Tanasqui" in 1567 while traveling inland from South Carolina. In the early 18th century, British traders encountered a Cherokee town named Tanasi in present-day Monroe County, Tennessee; the town was located on a river of the same name, appears on maps as early as 1725. It is not known whether this was the same town as the one encountered by Juan Pardo, although recent research suggests that Pardo's "Tanasqui" was located at the confluence of the Pigeon River and the French Broad River, near modern Newport.
The meaning and origin of the word are uncertain. Some accounts suggest, it has been said to mean "meeting place", "winding river", or "river of the great bend". According to ethnographer James Mooney, the name "can not be analyzed" and its meaning is lost; the modern spelling, Tennessee, is attributed to James Glen, the governor of South Carolina, who used this spelling in his official correspondence during the 1750s. The spelling was popularized by the publication of Henry Timberlake's "Draught of the Cherokee Country" in 1765. In 1788, North Carolina created "Tennessee County", the third county to be established in what is now Middle Tennessee; when a constitutional convention met in 1796 to organize a new state out of the Southwest Territory, it adopted "Tennessee" as the name of the state. Tennessee is known as The Volunteer State, a nickname some claimed was earned during the War of 1812 because of the prominent role played by volunteer soldiers from Tennessee during the Battle of New Orleans.
Other sources differ on the origin of the state nickname. This explanation is more because President Polk's call for 2,600 nationwide volunteers at the beginning of the Mexican–American War resulted in 30,000 volunteers from Tennessee alone in response to the death of Davy Crockett and appeals by former Tennessee Governor and Texas politician, Sam Houston. Tennessee borders eight other states: Virginia to the north. Tennessee is tied with Missouri as the state bordering the most other states; the state is trisected by the Tennessee River. The highest point in the state is Clingmans Dome at 6,643 feet (
The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, named after Cambria, the Latinised name for Wales, where rocks from this age were first studied; the Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons of the geologic time scale, it spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period, about 541 million years ago, when hard-shelled creatures first appeared in abundance. Little is known about the Precambrian, despite it making up seven-eighths of the Earth's history, what is known has been discovered from the 1960s onwards; the Precambrian fossil record is poorer than that of the succeeding Phanerozoic, fossils from the Precambrian are of limited biostratigraphic use. This is because many Precambrian rocks have been metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain buried beneath Phanerozoic strata.
It is thought that the Earth coalesced from material in orbit around the Sun at 4,543 Ma, may have been struck by a large planetesimal shortly after it formed, splitting off material that formed the Moon. A stable crust was in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma; the term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time. "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. As of 2010, the United States Geological Survey considers the term informal, lacking a stratigraphic rank. A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area.
However, there is evidence. There is a solid record of bacterial life throughout the remainder of the Precambrian. Excluding a few contested reports of much older forms from North America and India, the first complex multicellular life forms seem to have appeared at 1500 Ma, in the Mesoproterozoic era of the Proterozoic eon. Fossil evidence from the Ediacaran period of such complex life comes from the Lantian formation, at least 580 million years ago. A diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma; these are referred to as Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic eon. By the middle of the following Cambrian period, a diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa; the increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals and other microbes formed prokaryotic mats that covered terrestrial areas.
Tracks from an animal with leg like appendages have been found in what was mud 551 million years ago. Evidence of the details of plate motions and other tectonic activity in the Precambrian has been poorly preserved, it is believed that small proto-continents existed prior to 4280 Ma, that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth"; the atmosphere of the early Earth is not well understood. Most geologists believe it was composed of nitrogen, carbon dioxide, other inert gases, was lacking in free oxygen. There is, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism.
This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have combined with other elements in Earth's crust iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed real dates to be assigned to specific formations and features; the Precambrian is divided into
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, many earth scientists use a different definition: "a clustering of nearly all continents", which leaves room for interpretation and is easier to apply to Precambrian times. Supercontinents dispersed multiple times in the geologic past. According to the modern definitions, a supercontinent does not exist today; the supercontinent Pangaea is the collective name describing all of the continental landmasses when they were most near to one another. The positions of continents have been determined back to the early Jurassic, shortly before the breakup of Pangaea; the earlier continent Gondwana is not considered a supercontinent under the first definition, since the landmasses of Baltica and Siberia were separate at the time. The following table displays historical supercontinents. There are two contrasting models for supercontinent evolution through geological time.
The first model theorizes that at least two separate supercontinents existed comprising Vaalbara and Kenorland. The Neoarchean supercontinent consisted of Sclavia; these parts of Neoarchean age broke off at ~2480 and 2312 Ma and portions of them collided to form Nuna. Nuna continued to develop during the Mesoproterozoic by lateral accretion of juvenile arcs, in ~1000 Ma Nuna collided with other land masses, forming Rodinia. Between ~825 and 750 Ma Rodinia broke apart. However, before breaking up, some fragments of Rodinia had come together to form Gondwana by ~608 Ma. Pangaea formed by ~336 Ma through the collision of Gondwana and Siberia; the second model is based on both palaeomagnetic and geological evidence and proposes that the continental crust comprised a single supercontinent from ~2.72 Ga until break-up during the Ediacaran Period after ~0.573 Ga. The reconstruction is derived from the observation that palaeomagnetic poles converge to quasi-static positions for long intervals between ~2.72–2.115, 1.35–1.13, 0.75–0.573 Ga with only small peripheral modifications to the reconstruction.
During the intervening periods, the poles conform to a unified apparent polar wander path. Because this model shows that exceptional demands on the paleomagnetic data are satisfied by prolonged quasi-integrity, it must be regarded as superseding the first model proposing multiple diverse continents, although the first phase incorporates Vaalbara and Kenorland of the first model; the explanation for the prolonged duration of the Protopangea-Paleopangea supercontinent appears to be that lid tectonics prevailed during Precambrian times. Plate tectonics as seen on the contemporary Earth became dominant only during the latter part of geological times; the Phanerozoic supercontinent Pangaea is still doing so today. Because Pangaea is the most recent of Earth's supercontinents, it is the most well known and understood. Contributing to Pangaea's popularity in the classroom is the fact that its reconstruction is as simple as fitting the present continents bordering the Atlantic-type oceans like puzzle pieces.
A supercontinent cycle is the break-up of one supercontinent and the development of another, which takes place on a global scale. Supercontinent cycles are not the same as the Wilson cycle, the opening and closing of an individual oceanic basin; the Wilson cycle synchronizes with the timing of a supercontinent cycle. However, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia. Secular trends such as carbonatites, granulites and greenstone belt deformation events are all possible indicators of Precambrian supercontinent cyclicity, although the Protopangea-Paleopangea solution implies that Phanerozoic style of supercontinent cycles did not operate during these times. There are instances where these secular trends have a weak, uneven or lack of imprint on the supercontinent cycle; the causes of supercontinent assembly and dispersal are thought to be driven by convection processes in the Earth's mantle. 660 km into the mantle, a discontinuity occurs, affecting the surface crust through processes like plumes and "superplumes".
When a slab of subducted crust is denser than the surrounding mantle, it sinks to the discontinuity. Once the slabs build up, they will sink through to the lower mantle in what is known as a "slab avalanche"; this displacement at the discontinuity will cause the lower mantle to rise elsewhere. The rising mantle can form a superplume. Besides having compositional effects on the upper mantle by replenishing the large-ion lithophile elements, volcanism affects plate movement; the plates will be moved towards a geoidal low where the slab avalanche occurred and pushed away from the geoidal high that can be caused by the plumes or superplumes. This causes the continents to push together to form supercontinents and was evidently the process that operated to cause the early continental crust to aggregate into Protopangea. Dispersal of supercontinents is caused by the accumulation of heat underneath the crust due to the rising of large convection cells or plumes, a massive heat release resulted in the final break-up of Paleopangea.
Accretion occurs over geo
A river delta is a landform that forms from deposition of sediment, carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, estuary, reservoir, or another river that cannot carry away the supplied sediment; the size and shape of a delta is controlled by the balance between watershed processes that supply sediment, receiving basin processes that redistribute and export that sediment. The size and location of the receiving basin plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers, they can impact drinking water supply. They are ecologically important, with different species' assemblages depending on their landscape position. River deltas form when a river carrying sediment reaches either a body of water, such as a lake, ocean, or reservoir, another river that cannot remove the sediment enough to stop delta formation, or an inland region where the water spreads out and deposits sediments.
The tidal currents cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. The river must carry enough sediment to layer into deltas over time; the river's velocity decreases causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta; when the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment; as a result, sediment drops out of deposits. Over time, this single channel builds a deltaic lobe; as the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation. As the slope of the river channel decreases, it becomes unstable for two reasons. First, gravity makes the water flow in the most direct course down slope. If the river breaches its natural levees, it spills out into a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.
Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope; when the channel does this, some of its flow remains in the abandoned channel. When these channel-switching events occur, a mature delta develops a distributary network. Another way these distributary networks form is from deposition of mouth bars; when this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth-bar, which splits the river into two distributary channels. A good example of the result of this process is the Wax Lake Delta. In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition.
This results in the smoothing of the planform shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan; the more the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position results in more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's-feet, are examples of rivers that do not avulse enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name and more approximate an ideal fan shape. Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi, Amazon, Ganges and Yangtze discharging along passive continental margins; this phenomenon is due to three big factors: topography, basin area, basin elevation. Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate overtime to form large river deltas.
Topography along active margins tend to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf. There are many other smaller factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, volcanic activity resulting in delta formation to exist closer to the sediment source; when sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion, delta lobe switching, auto cyclicity. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.
However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in de
Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century; the geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, the rigid outermost shell of a planet, is broken into tectonic plates; the Earth's lithosphere is composed of many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, oceanic trench formation occur along these plate boundaries; the relative movement of the plates ranges from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle. In this way, the total surface of the lithosphere remains the same; this prediction of plate tectonics is referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography and density changes in the crust. At subduction zones the cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Moon; the relative importance of each of these factors and their relationship to each other is unclear, still the subject of much debate.
The outer layers of the Earth are divided into the asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat; the lithosphere is more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere transfers heat by convection and has a nearly adiabatic temperature gradient; this division should not be confused with the chemical subdivision of these same layers into the mantle and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year; the driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust and continental crust.
Average oceanic lithosphere is 100 km thick. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones. Continental lithosphere is about 200 km thick, though this varies between basins, mountain ranges, stable cratonic interiors of continents; the location where two plates meet is called a plate boundary. Plate boundaries are associated with geological events such as earthquakes and the creation of topographic features such as mountains, mid-ocean ridges, oceanic trenches; the majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and known today. These boundaries are discussed in further detail below; some volcanoes occur in the interiors of plates, these have been variously attributed to internal plate deformation and to mantle plumes.
As explained above, tectonic plates may include continental crust or oceanic crust, most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans; the distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is fo
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
Topography is the study of the shape and features of land surfaces. The topography of an area could refer to the surface shapes and features themselves, or a description. Topography is a field of geoscience and planetary science and is concerned with local detail in general, including not only relief but natural and artificial features, local history and culture; this meaning is less common in the United States, where topographic maps with elevation contours have made "topography" synonymous with relief. Topography in a narrow sense involves the recording of relief or terrain, the three-dimensional quality of the surface, the identification of specific landforms; this is known as geomorphometry. In modern usage, this involves generation of elevation data in digital form, it is considered to include the graphic representation of the landform on a map by a variety of techniques, including contour lines, hypsometric tints, relief shading. The term topography originated in ancient Greece and continued in ancient Rome, as the detailed description of a place.
The word comes from the Greek τόπος and -γραφία. In classical literature this refers to writing about a place or places, what is now called'local history'. In Britain and in Europe in general, the word topography is still sometimes used in its original sense. Detailed military surveys in Britain were called Ordnance Surveys, this term was used into the 20th century as generic for topographic surveys and maps; the earliest scientific surveys in France were called the Cassini maps after the family who produced them over four generations. 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, which became the Corps of Topographical Engineers in 1838. 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, 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 where mapping in a broader sense is used in medical fields such as neurology. An objective of topography is to determine the position of any feature or more any point in terms of both a horizontal coordinate system such as latitude and altitude. Identifying features, recognizing typical landform patterns are 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. There are a variety of approaches to studying topography. Which method to use depend on the scale and size of the area under study, its accessibility, the quality of existing surveys. Surveying helps determine 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.
Work on one of the first topographic maps was begun in France by Giovanni Domenico Cassini, the great Italian astronomer. Though remote sensing has sped up the process of gathering information, 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, the compiled data forms the basis of basic digital elevation datasets such as USGS DEM data; this data must be "cleaned" to eliminate discrepancies between surveys, but it still forms a valuable set of information for large-scale analysis. The original American topographic surveys involved not only recording of relief, but identification of landmark features and vegetative land cover. Remote sensing is a general term for geodata collection at a distance from the subject area. Besides their role in photogrammetry and satellite imagery can be used to identify and delineate terrain features and more general land-cover features.
They have become more and more a part of geovisualization, whether maps or GIS systems. False-color and non-visible spectra imaging can help determine the lie of the land by delineating vegetation and other land-use information more clearly. Images can be 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 taken starting from different positions from different passes of an aerial photography flight. In this technique, the common points are identified on each image. A line of sight can be built from the camera location to the point on the object, it is the intersection of its rays 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. Satellite RADAR mapping is one of the major techniques of generating Digital E