Digital elevation model
A digital elevation model is a 3D CG representation of a terrain's surface – of a planet, moon, or asteroid – created from a terrain's elevation data. A "global DEM" refers to a Discrete Global Grid. DEMs are used in geographic information systems, are the most common basis for digitally produced relief maps. While a DSM may be useful for landscape modeling, city modeling and visualization applications, a DTM is required for flood or drainage modeling, land-use studies, geological applications, other applications, in planetary science. There is no universal usage of the terms digital elevation model, digital terrain model and digital surface model in scientific literature. In most cases the term digital surface model represents the earth's surface and includes all objects on it. In contrast to a DSM, the digital terrain model represents the bare ground surface without any objects like plants and buildings. DEM is used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface.
Other definitions equalise the terms DEM and DTM, equalise the terms DEM and DSM, define the DEM as a subset of the DTM, which represents other morphological elements, or define a DEM as a rectangular grid and a DTM as a three-dimensional model. Most of the data providers use the term DEM as a generic term for DTMs. All datasets which are captured with satellites, airplanes or other flying platforms are DSMs, it is possible to compute a DTM from high resolution DSM datasets with complex algorithms. In the following, the term DEM is used as a generic term for DTMs. A DEM can be represented as a vector-based triangular irregular network; the TIN DEM dataset is referred to as a primary DEM, whereas the Raster DEM is referred to as a secondary DEM. The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc.. DEMs are built using data collected using remote sensing techniques, but they may be built from land surveying; the digital elevation model itself consists of a matrix of numbers, but the data from a DEM is rendered in visual form to make it understandable to humans.
This visualization may be in the form of a contoured topographic map, or could use shading and false color assignment to render elevations as colors. Visualizations are sometime done as oblique views, reconstructing a synthetic visual image of the terrain as it would appear looking down at an angle. In these oblique visualizations, elevations are sometimes scaled using "vertical exaggeration" in order to make subtle elevation differences more noticeable; some scientists, object to vertical exaggeration as misleading the viewer about the true landscape. Mappers may prepare digital elevation models in a number of ways, but they use remote sensing rather than direct survey data. Older methods of generating DEMs involve interpolating digital contour maps that may have been produced by direct survey of the land surface; this method is still used in mountain areas. Note that contour line data or any other sampled elevation datasets are not DEMs, but may be considered digital terrain models. A DEM implies.
One powerful technique for generating digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite, or a single pass if the satellite is equipped with two antennas, collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. Other kinds of stereoscopic pairs can be employed using the digital image correlation method, where two optical images are acquired with different angles taken from the same pass of an airplane or an Earth Observation Satellite; the SPOT 1 satellite provided the first usable elevation data for a sizeable portion of the planet's landmass, using two-pass stereoscopic correlation. Further data were provided by the European Remote-Sensing Satellite using the same method, the Shuttle Radar Topography Mission using single-pass SAR and the Advanced Spaceborne Thermal Emission and Reflection Radiometer instrumentation on the Terra satellite using double-pass stereo pairs.
The HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs. A tool of increasing value in planetary science has been use of orbital altimetry used to make digital elevation map of planets. A primary tool for this is Laser altimetry. Planetary digital elevation maps made using laser altimetry include the Mars Orbiter Laser Altimeter mapping of Mars, the Lunar Orbital Laser Altimeter and Lunar Altimeter mapping of the Moon, the Mercury Laser Altimeter mapping of Mercury. Lidar Radar Stereo photogrammetry from aerial surveys Structure from motion / Multi-view stereo applied to aerial photography Block adjustment from optical satellite imagery Interferometry from radar data Real Time Kinematic GPS Topographic maps Theodolite or total station Doppler radar Focus variation Inertial surveys Surveying and
In the field of road transport, an interchange is a road junction that uses grade separation, one or more ramps, to permit traffic on at least one highway to pass through the junction without interruption from other crossing traffic streams. It differs from a standard intersection. Interchanges are always used when at least one road is a controlled-access highway or a limited-access divided highway, though they are sometimes used at junctions between surface streets. Note: The descriptions of interchanges apply to countries where vehicles drive on the right side of the road. For left-side driving, layout of the junctions is the only left/right is reversed. A freeway junction or highway interchange or motorway junction is a type of road junction linking one controlled-access highway to another, to other roads, or to a rest area or motorway service area. In the UK, most junctions are numbered sequentially. In the US, interchanges are either numbered by interchange number. A highway ramp or slip road is a short section of road that allows vehicles to enter or exit a controlled-access highway.
A directional ramp tends toward the desired direction of travel: A ramp that makes a left turn exits from the left side of the roadway. Left directional ramps are uncommon, as the left lane is reserved for high-speed through traffic. Ramps for a right turn are always right directional ramps. A non-directional ramp goes opposite to the desired direction of travel. Many loop ramps are non-directional. A semi-directional ramp exits in a direction opposite from the desired direction of travel turns toward the desired direction. Many flyover ramps are semi-directional. A U-turn ramp leaves the road in one direction, turns over or under it, rejoins in the opposite direction. Weaving is an undesirable situation where traffic veering right and left must cross paths within a limited distance, to merge with traffic on the through lane; the German Autobahn system has Autobahn-to-Autobahn interchanges of two types: a four-way interchange, the Autobahnkreuz, where two motorways cross. Some on-ramps have a ramp meter, a dedicated mid-ramp traffic light that controls the flow of entering vehicles.
A complete interchange has enough ramps to provide access from any direction of any road in the junction to any direction of any other road in the junction. A complete interchange between a freeway and another road requires at least four ramps. Complete interchanges between two freeways have at least eight ramps, as having fewer would reduce capacity and increase weaving. Using U-turns, the number for two freeways can be reduced to six, by making cars that want to turn left either pass by the other road first make a U-turn and turn right, or turn right first and make a U-turn. Depending on the interchange type and the connectivity offered other numbers of ramps may be used. For example, if a highway interchanges with a highway containing a collector/express system, additional ramps can be used to link the interchanging highway with the collector and express lanes respectively. For highways with high-occupancy vehicle lanes, ramps can be used to service these carriageways directly, thereby increasing the number of ramps used.
An incomplete interchange has at least one or more missing ramps that prevent access to at least one direction of another road in the junction from any other road in the junction. A cloverleaf interchange is a two-level, four-way interchange where all turns across opposing traffic are handled by non-directional loop ramps. Assuming right-handed traffic, to go left vehicles first cross over or under the target route bear right onto a curved ramp that turns 270 degrees, merging onto the target route from the right, crossing the route just departed; these loop ramps produce the namesake cloverleaf shape. Two major advantages of cloverleaves are that they require only one bridge which makes such junctions inexpensive as long as land is plentiful, that they do not require any traffic signals to operate. However, weaving is a major shortcoming of cloverleaves, as the four total offramps and onramps are present, merge on the main routes; the capacity of this design is comparatively low. Cloverleaves use a considerable area of land, are more found along older highways, in rural areas and within cities with low population densities.
A variant design separates all turning traffic into a parallel carriageway to minimize the problem of weaving. Collector and distributor roads are similar, but are separated from the main carriageway by a divider, such as a guard rail or Jersey barrier. A stack interchange is a four-way interchange whereby a semi-directional left turn and a directional right turn are both available. Access to both turns is provided by a single offramp. Assuming right-handed driving, in order to cross over incoming traffic and go left, vehicles first exit onto an off-ramp from the rightmost lane. After demerging from right-turning traffic, they complete their left turn by crossing both highways on a flyover ramp or underpass; the penultimate step is a merge with the right-turn on-ramp traffic from the opposite quadrant of the interchange. An onramp merges both streams o
Kevlar is a heat-resistant and strong synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, this high-strength material was first commercially used in the early 1970s as a replacement for steel in racing tires, it is spun into ropes or fabric sheets that can be used as such or as an ingredient in composite material components. Kevlar has many applications, ranging from bicycle tires and racing sails to bulletproof vests, because of its high tensile strength-to-weight ratio, it is used to make modern marching drumheads that withstand high impact. When used as a woven material, it is suitable for mooring other underwater applications. A similar fiber called. Poly-paraphenylene terephthalamide – branded Kevlar – was invented by Polish-American chemist Stephanie Kwolek while working for DuPont, in anticipation of a gasoline shortage. In 1964, her group began searching for a new lightweight strong fiber to use for light but strong tires.
The polymers she had been working with at the time, poly-p-phenylene-terephthalate and polybenzamide, formed liquid crystal while in solution, something unique to those polymers at the time. The solution was "cloudy, opalescent upon being stirred, of low viscosity" and was thrown away. However, Kwolek persuaded the technician, Charles Smullen, who ran the spinneret, to test her solution, was amazed to find that the fiber did not break, unlike nylon, her supervisor and her laboratory director understood the significance of her accidental discovery and a new field of polymer chemistry arose. By 1971, modern Kevlar was introduced. However, Kwolek was not involved in developing the applications of Kevlar. Kevlar is synthesized in solution from the monomers 1,4-phenylene-diamine and terephthaloyl chloride in a condensation reaction yielding hydrochloric acid as a byproduct; the result has liquid-crystalline behavior, mechanical drawing orients the polymer chains in the fiber's direction. Hexamethylphosphoramide was the solvent used for the polymerization, but for safety reasons, DuPont replaced it by a solution of N-methyl-pyrrolidone and calcium chloride.
As this process had been patented by Akzo in the production of Twaron, a patent war ensued. Kevlar production is expensive because of the difficulties arising from using concentrated sulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis and spinning. Several grades of Kevlar are available: Kevlar K-29 – in industrial applications, such as cables, asbestos replacement, brake linings, body/vehicle armor. Kevlar K49 – high modulus used in cable and rope products. Kevlar K100 – colored version of Kevlar Kevlar K119 – higher-elongation and more fatigue resistant Kevlar K129 – higher tenacity for ballistic applications Kevlar AP – 15% higher tensile strength than K-29 Kevlar XP – lighter weight resin and KM2 plus fiber combination Kevlar KM2 – enhanced ballistic resistance for armor applicationsThe ultraviolet component of sunlight degrades and decomposes Kevlar, a problem known as UV degradation, so it is used outdoors without protection against sunlight; when Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa, a relative density of 1.44.
The polymer owes its high strength to the many inter-chain bonds. These inter-molecular hydrogen bonds form between NH centers. Additional strength is derived from aromatic stacking interactions between adjacent strands; these interactions have a greater influence on Kevlar than the van der Waals interactions and chain length that influence the properties of other synthetic polymers and fibers such as Dyneema. The presence of salts and certain other impurities calcium, could interfere with the strand interactions and care is taken to avoid inclusion in its production. Kevlar's structure consists of rigid molecules which tend to form planar sheet-like structures rather like silk protein. Kevlar maintains its resilience down to cryogenic temperatures. At higher temperatures the tensile strength is reduced by about 10–20%, after some hours the strength progressively reduces further. For example: enduring 160 °C for 500 hours, reduces strength by about 10%. Kevlar is used in the field of cryogenics for its low thermal conductivity and high strength relative to other materials for suspension purposes.
It is most used to suspend a paramagnetic salt enclosure from a superconducting magnet mandrel in order to minimize any heat leaks to the paramagnetic material. It is used as a thermal standoff or structural support where low heat leaks are desired. Kevlar is a well-known component of personal armor such as combat helmets, ballistic face masks, ballistic vests; the PASGT helmet and vest used by United States military forces, use Kevlar as a key component in their construction. Other military uses include bulletproof face masks and spall liners used to protect the crews of armoured fighting vehicles. Nimitz-class aircraft carriers use Kevlar reinforcement in vital areas. Civilian applications include: high heat resistance uniforms worn by firefighters, body armour worn by police officers and police tactical teams such as SWAT. Kevlar is used to manufacture gloves, jackets and other articles of clothin
Aircraft principal axes
An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down. The axes are alternatively designated as vertical and longitudinal respectively; these axes rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first manned spacecraft were designed in the late 1950s; these rotations are produced by torques about the principal axes. On an aircraft, these are intentionally produced by means of moving control surfaces, which vary the distribution of the net aerodynamic force about the vehicle's center of gravity. Elevators produce pitch, a rudder on the vertical tail produces yaw, ailerons produce roll. On a spacecraft, the moments are produced by a reaction control system consisting of small rocket thrusters used to apply asymmetrical thrust on the vehicle. Normal axis, or yaw axis — an axis drawn from top to bottom, perpendicular to the other two axes. Parallel to the fuselage station.
Transverse axis, lateral axis, or pitch axis — an axis running from the pilot's left to right in piloted aircraft, parallel to the wings of a winged aircraft. Parallel to the buttock line. Longitudinal axis, or roll axis — an axis drawn through the body of the vehicle from tail to nose in the normal direction of flight, or the direction the pilot faces. Parallel to the waterline; these axes are represented by the letters X, Y and Z in order to compare them with some reference frame named x, y, z. This is made in such a way that the X is used for the longitudinal axis, but there are other possibilities to do it; the yaw axis has its origin at the center of gravity and is directed towards the bottom of the aircraft, perpendicular to the wings and to the fuselage reference line. Motion about this axis is called yaw. A positive yawing motion moves the nose of the aircraft to the right; the rudder is the primary control of yaw. The term yaw was applied in sailing, referred to the motion of an unsteady ship rotating about its vertical axis.
Its etymology is uncertain. The pitch axis has its origin at the center of gravity and is directed to the right, parallel to a line drawn from wingtip to wingtip. Motion about this axis is called pitch. A positive pitching motion lowers the tail; the elevators are the primary control of pitch. The roll axis has its origin at the center of gravity and is directed forward, parallel to the fuselage reference line. Motion about this axis is called roll. An angular displacement about this axis is called bank. A positive rolling motion lowers the right wing; the pilot rolls by decreasing it on the other. This changes the bank angle; the ailerons are the primary control of bank. The rudder has a secondary effect on bank; these axes are not the same. They are geometrical symmetry axes, regardless of the mass distribution of the aircraft. In aeronautical and aerospace engineering intrinsic rotations around these axes are called Euler angles, but this conflicts with existing usage elsewhere; the calculus behind them is similar to the Frenet–Serret formulas.
Performing a rotation in an intrinsic reference frame is equivalent to right-multiplying its characteristic matrix by the matrix of the rotation. The first aircraft to demonstrate active control about all three axes was the Wright brothers' 1902 glider. Aerodynamics Aircraft flight control system Euler angles Fixed-wing aircraft Flight control surfaces Flight dynamics Moving frame Panning Six degrees of freedom Screw theory Triad method Yaw Axis Control as a Means of Improving V/STOL Aircraft Performance. 3D fast walking simulation of biped robot by yaw axis moment compensation Flight control system for a hybrid aircraft in the yaw axis
Unmanned aerial vehicle
An unmanned aerial vehicle known as a drone, is an aircraft without a human pilot onboard. UAVs are a component of an unmanned aircraft system; the flight of UAVs may operate with various degrees of autonomy: either under remote control by a human operator or autonomously by onboard computers. Compared to manned aircraft, UAVs were used for missions too "dull, dirty or dangerous" for humans. While they originated in military applications, their use is expanding to commercial, recreational and other applications, such as policing and surveillance, product deliveries, aerial photography and drone racing. Civilian UAVs now vastly outnumber military UAVs, with estimates of over a million sold by 2015. Multiple terms are used for unmanned aerial vehicles, which refer to the same concept; the term drone, more used by the public, was coined in reference to the early remotely-flown target aircraft used for practice firing of a battleship's guns, the term was first used with the 1920s Fairey Queen and 1930's de Havilland Queen Bee target aircraft.
These two were followed in service by the similarly-named Airspeed Queen Wasp and Miles Queen Martinet, before ultimate replacement by the GAF Jindivik. The term unmanned aircraft system was adopted by the United States Department of Defense and the United States Federal Aviation Administration in 2005 according to their Unmanned Aircraft System Roadmap 2005–2030; the International Civil Aviation Organization and the British Civil Aviation Authority adopted this term used in the European Union's Single-European-Sky Air-Traffic-Management Research roadmap for 2020. This term emphasizes the importance of elements other than the aircraft, it includes elements such as data links and other support equipment. A similar term is an unmanned-aircraft vehicle system, remotely piloted aerial vehicle, remotely piloted aircraft system. Many similar terms are in use. A UAV is defined as a "powered, aerial vehicle that does not carry a human operator, uses aerodynamic forces to provide vehicle lift, can fly autonomously or be piloted remotely, can be expendable or recoverable, can carry a lethal or nonlethal payload".
Therefore, missiles are not considered UAVs because the vehicle itself is a weapon, not reused, though it is unmanned and in some cases remotely guided. The relation of UAVs to remote controlled model aircraft is unclear. UAVs may not include model aircraft; some jurisdictions base their definition on weight. For recreational uses, a drone is a model aircraft that has first-person video, autonomous capabilities, or both; the earliest recorded use of an unmanned aerial vehicle for warfighting occurred on July 1849, serving as a balloon carrier in the first offensive use of air power in naval aviation. Austrian forces besieging Venice attempted to launch some 200 incendiary balloons at besieged city; the balloons were launched from land. At least one bomb fell in the city. UAV innovations started in the early 1900s and focused on providing practice targets for training military personnel. UAV development continued during World War I, when the Dayton-Wright Airplane Company invented a pilotless aerial torpedo that would explode at a preset time.
The earliest attempt at a powered UAV was A. M. Low's "Aerial Target" in 1916. Nikola Tesla described a fleet of unmanned aerial combat vehicles in 1915. Advances followed including the Hewitt-Sperry Automatic Airplane; this developments inspired the development of the Kettering Bug by Charles Kettering from Dayton, Ohio. This was meant as an unmanned plane that would carry an explosive payload to a predetermined target; the first scaled remote piloted vehicle was developed by film star and model-airplane enthusiast Reginald Denny in 1935. More emerged during World War II – used both to train antiaircraft gunners and to fly attack missions. Nazi Germany used various UAV aircraft during the war. Jet engines entered service after World War II in vehicles such as the Australian GAF Jindivik, Teledyne Ryan Firebee I of 1951, while companies like Beechcraft offered their Model 1001 for the U. S. Navy in 1955, they were little more than remote-controlled airplanes until the Vietnam War. In 1959, the U.
S. Air Force, concerned about losing pilots over hostile territory, began planning for the use of unmanned aircraft. Planning intensified after the Soviet Union shot down a U-2 in 1960. Within days, a classified UAV program started under the code name of "Red Wagon"; the August 1964 clash in the Tonkin Gulf between naval units of the U. S. and North Vietnamese Navy initiated America's classified UAVs into their first combat missions of the Vietnam War. When the Chinese government showed photographs of downed U. S. UAVs via Wide World Photos, the official U. S. response was "no comment". During the War of Attrition the first tactical UAVs installed with reconnaissance cameras were first tested by the Israeli intelligence bringing photos from across the Suez canal; this was the first time that tacti
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between those layers. Turbulence is observed in everyday phenomena such as surf, fast flowing rivers, billowing storm clouds, or smoke from a chimney, most fluid flows occurring in nature or created in engineering applications are turbulent. Turbulence is caused by excessive kinetic energy in parts of a fluid flow, which overcomes the damping effect of the fluid's viscosity. For this reason turbulence is realized in low viscosity fluids. In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other drag due to friction effects increases; this increases the energy needed to pump fluid through a pipe. Turbulence can be exploited, for example, by devices such as aerodynamic spoilers on aircraft that "spoil" the laminar flow to increase drag and reduce lift.
The onset of turbulence can be predicted by the dimensionless Reynolds number, the ratio of kinetic energy to viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, the interactions within turbulence create a complex phenomenon. Richard Feynman has described turbulence as the most important unsolved problem in classical physics. Smoke rising from a cigarette. For the first few centimeters, the smoke is laminar; the smoke plume becomes turbulent as its Reynolds number increases with increases in flow velocity and characteristic lengthscale. Flow over a golf ball. If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable to unfavorable, creating a large region of low pressure behind the ball that creates high form drag. To prevent this, the surface is dimpled to promote turbulence; this results in higher skin friction, but it moves the point of boundary layer separation further along, resulting in lower drag.
Clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing. Most of the terrestrial atmospheric circulation; the oceanic and atmospheric mixed intense oceanic currents. The flow conditions in many industrial equipment and machines; the external flow over all kinds of vehicles such as cars, airplanes and submarines. The motions of matter in stellar atmospheres. A jet exhausting from a nozzle into a quiescent fluid; as the flow emerges into this external fluid, shear layers originating at the lips of the nozzle are created. These layers separate the fast moving jet from the external fluid, at a certain critical Reynolds number they become unstable and break down to turbulence. Biologically generated. Snow fences work by inducing turbulence in the wind, forcing it to drop much of its snow load near the fence. Bridge supports in water. In the late summer and fall, when river flow is slow, water flows smoothly around the support legs. In the spring, when the flow is faster, a higher Reynolds number is associated with the flow.
The flow may start off laminar but is separated from the leg and becomes turbulent. In many geophysical flows, the flow turbulence is dominated by the coherent structures and turbulent events. A turbulent event is a series of turbulent fluctuations that contain more energy than the average flow turbulence; the turbulent events are associated with coherent flow structures such as eddies and turbulent bursting, they play a critical role in terms of sediment scour and transport in rivers as well as contaminant mixing and dispersion in rivers and estuaries, in the atmosphere. In the medical field of cardiology, a stethoscope is used to detect heart sounds and bruits, which are due to turbulent blood flow. In normal individuals, heart sounds are a product of turbulent flow as heart valves close. However, in some conditions turbulent flow can be audible due to other reasons, some of them pathological. For example, in advanced atherosclerosis, bruits can be heard in some vessels that have been narrowed by the disease process.
Turbulence in porous media became a debated subject. Turbulence is characterized by the following features: Irregularity Turbulent flows are always irregular. For this reason, turbulence problems are treated statistically rather than deterministically. Turbulent flow is chaotic. However, not all chaotic flows are turbulent. Diffusivity The available supply of energy in turbulent flows tends to accelerate the homogenization of fluid mixtures; the characteristic, responsible for the enhanced mixing and increased rates of mass and energy transports in a flow is called "diffusivity". Turbulent diffusion is described by a turbulent diffusion coefficient; this turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the molecular diffusivities, but it does not have a true physical meaning, being dependent on the flow conditions, not a property of the fluid itself. In addition, the turbulent diffusivity concept assumes a con
Poland the Republic of Poland, is a country located in Central Europe. It is divided into 16 administrative subdivisions, covering an area of 312,696 square kilometres, has a temperate seasonal climate. With a population of 38.5 million people, Poland is the sixth most populous member state of the European Union. Poland's capital and largest metropolis is Warsaw. Other major cities include Kraków, Łódź, Wrocław, Poznań, Gdańsk, Szczecin. Poland is bordered by the Baltic Sea, Russia's Kaliningrad Oblast and Lithuania to the north and Ukraine to the east and Czech Republic, to the south, Germany to the west; the establishment of the Polish state can be traced back to AD 966, when Mieszko I, ruler of the realm coextensive with the territory of present-day Poland, converted to Christianity. The Kingdom of Poland was founded in 1025, in 1569 it cemented its longstanding political association with the Grand Duchy of Lithuania by signing the Union of Lublin; this union formed the Polish–Lithuanian Commonwealth, one of the largest and most populous countries of 16th and 17th century Europe, with a uniquely liberal political system which adopted Europe's first written national constitution, the Constitution of 3 May 1791.
More than a century after the Partitions of Poland at the end of the 18th century, Poland regained its independence in 1918 with the Treaty of Versailles. In September 1939, World War II started with the invasion of Poland by Germany, followed by the Soviet Union invading Poland in accordance with the Molotov–Ribbentrop Pact. More than six million Polish citizens, including 90% of the country's Jews, perished in the war. In 1947, the Polish People's Republic was established as a satellite state under Soviet influence. In the aftermath of the Revolutions of 1989, most notably through the emergence of the Solidarity movement, Poland reestablished itself as a presidential democratic republic. Poland is regional power, it has the fifth largest economy by GDP in the European Union and one of the most dynamic economies in the world achieving a high rank on the Human Development Index. Additionally, the Polish Stock Exchange in Warsaw is the largest and most important in Central Europe. Poland is a developed country, which maintains a high-income economy along with high standards of living, life quality, safety and economic freedom.
Having a developed school educational system, the country provides free university education, state-funded social security, a universal health care system for all citizens. Poland has 15 UNESCO World Heritage Sites. Poland is a member state of the European Union, the Schengen Area, the United Nations, NATO, the OECD, the Three Seas Initiative, the Visegrád Group; the origin of the name "Poland" derives from the West Slavic tribe of Polans that inhabited the Warta river basin of the historic Greater Poland region starting in the 6th century. The origin of the name "Polanie" itself derives from the early Slavic word "pole". In some languages, such as Hungarian, Lithuanian and Turkish, the exonym for Poland is Lechites, which derives from the name of a semi-legendary ruler of Polans, Lech I. Early Bronze Age in Poland begun around 2400 BC, while the Iron Age commenced in 750 BC. During this time, the Lusatian culture, spanning both the Bronze and Iron Ages, became prominent; the most famous archaeological find from the prehistory and protohistory of Poland is the Biskupin fortified settlement, dating from the Lusatian culture of the early Iron Age, around 700 BC.
Throughout the Antiquity period, many distinct ancient ethnic groups populated the regions of what is now Poland in an era that dates from about 400 BC to 500 AD. These groups are identified as Celtic, Slavic and Germanic tribes. Recent archeological findings in the Kujawy region, confirmed the presence of the Roman Legions on the territory of Poland; these were most expeditionary missions sent out to protect the amber trade. The exact time and routes of the original migration and settlement of Slavic peoples lacks written records and can only be defined as fragmented; the Slavic tribes who would form Poland migrated to these areas in the second half of the 5th century AD. Up until the creation of Mieszko's state and his subsequent conversion to Christianity in 966 AD, the main religion of Slavic tribes that inhabited the geographical area of present-day Poland was Slavic paganism. With the Baptism of Poland the Polish rulers accepted Christianity and the religious authority of the Roman Church.
However, the transition from paganism was not a smooth and instantaneous process for the rest of the population as evident from the pagan reaction of the 1030s. Poland began to form into a recognizable unitary and territorial entity around the middle of the 10th century under the Piast dynasty. Poland's first documented ruler, Mieszko I, accepted Christianity with the Baptism of Poland in 966, as the new official religion of his subjects; the bulk of the population converted in the course of the next few centuries. In 1000, Boleslaw the Brave, continuing the policy of his father Mieszko, held a Congress of Gniezno and created the metropolis of Gniezno and the dioceses of Kraków, Kołobrzeg, Wrocław. However, the pagan unrest led to the transfer of the capital to Kraków in 1038 by Casimir I the Restorer. In 1109, Prince Bolesław III Wrymouth defeated the King of Germany Henry V at the Battle of Hundsfeld, stopping the Ge