Geodesy or geodetics is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. Geodesy is an earth science and many consider the study of Earth's shape and gravity to be central to that science. It is also a discipline of applied mathematics.
The metre is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
A Kater's pendulum is a reversible free swinging pendulum invented by British physicist and army captain Henry Kater in 1817, published 29th January 1818 for use as a gravimeter instrument to measure the local acceleration of gravity. Its advantage is that, unlike previous pendulum gravimeters, the pendulum's centre of gravity and center of oscillation do not have to be determined, allowing a greater accuracy. For about a century, until the 1930s, Kater's pendulum and its various refinements remained the standard method for measuring the strength of the Earth's gravity during geodetic surveys. It is now used only for demonstrating pendulum principles.
The geoid is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended through the continents. According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth. It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.
The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The standard is published and maintained by the United States National Geospatial-Intelligence Agency.
The history of geodesy (/dʒiːˈɒdɪsi/) began during antiquity and ultimately blossomed during the Age of Enlightenment.
In geodesy, the figure of the Earth is the size and shape used to model planet Earth. The kind of figure depends on application, including the precision needed for the model. A spherical Earth is a well-known historical approximation that is satisfactory for geography, astronomy and many other purposes. Several models with greater accuracy have been developed so that coordinate systems can serve the precise needs of navigation, surveying, cadastre, land use, and various other concerns.
In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data. The laser pulse can also be reflected by the surface of a satellite without a retroreflector, which is used for tracking space debris.
The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of that standard. The ITRS defines a geocentric system of coordinates using the SI system of measurement.
Satellite geodesy is geodesy by means of artificial satellites—the measurement of the form and dimensions of Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques. It belongs to the broader field of space geodesy. Traditional astronomical geodesy is not commonly considered a part of satellite geodesy, although there is considerable overlap between the techniques.
Arc measurement, sometimes degree measurement, is the astrogeodetic technique of determining the radius of Earth – more specifically, the local Earth radius of curvature of the figure of the Earth – by relating the latitude difference and the geographic distance surveyed between two locations on Earth's surface. The most common variant involves only astronomical latitudes and the meridian arc length and is called meridian arc measurement; other variants may involve only astronomical longitude or both geographic coordinates . Arc measurement campaigns in Europe were the precursors to the International Association of Geodesy (IAG).
The Paris meridian is a meridian line running through the Paris Observatory in Paris, France – now longitude 2°20′14.02500″ East. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world. The "Paris meridian arc" or "French meridian arc" is the name of the meridian arc measured along the Paris meridian.
The International Union of Geodesy and Geophysics is an international non-governmental organization dedicated to the scientific study of Earth and its space environment using geophysical and geodetic techniques.
An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations.
European Combined Geodetic Network (ECGN) is a research project aimed at high accuracy geoid determination. The purpose of ECGN is to connect the height systems obtained via geometric positioning by GNSS with gravity-referenced heights with a cm-level accuracy. The effects of the atmosphere, the oceans and time-dependent parameters of the solid Earth on the gravity field are investigated. ECGN uses the data of satellite gravity missions CHAMP, GRACE and GOCE to model the Earth's gravity field and is linked to other gravity-related projects. The ECGN is considered as a European contribution to the International Association of Geodesy (IAG) project Global Geodetic Observation System (GGOS). ECGN is managed by EUREF.
The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision then in use, they also preferred a decimal system to ease their calculations.
Carlos Ibáñez e Ibáñez de Ibero, 1st Marquis of Mulhacén, was a Spanish divisional general and geodesist. He represented Spain at the 1875 Conference of the Metre Convention and was the first president of the International Committee for Weights and Measures. As a forerunner geodesist and president of the International Geodetic Association, he played a leading role in the worldwide dissemination of the metric system. His activities resulted in the distribution of a platinum and iridium prototype of the metre to all States parties to the Metre Convention during the first meeting of the General Conference on Weights and Measures in 1889. These prototypes defined the metre right up until 1960.
The Geodetic Observatory Wettzell is located atop the 616 meter-high mountain Wagnerberg, west of the village Wettzell in the German district Cham in the Bavarian Forest.
Thomas A. Herring is a geophysicist, known for developing and applying systems of space geodesy to high-precision geophysical measurements and geodynamic research.
Ivan Istvan Mueller was a Hungarian-American geodesist and professor at Ohio State University, a leading training center for geodesy in the USA.
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