Geodesy

Geodesy
Name	Geodesy
Caption	A geodetic control point marker, part of the physical infrastructure for precise measurement.
Field	Earth science
Related	Geophysics, Astronomy, Cartography, Surveying

Geodesy. It is the scientific discipline concerned with the precise measurement and understanding of the Earth's geometric shape, its orientation in space, and its gravity field, including their temporal variations. This field provides the fundamental framework for navigation, cartography, land surveying, and the monitoring of global geophysical processes. Its principles are essential for modern technologies like the Global Positioning System and for studying phenomena such as sea level rise and plate tectonics.

Definition and scope

The scope of this discipline encompasses the determination of the Earth's figure and geoid, the measurement of crustal motion, and the definition of terrestrial reference frames. It intersects significantly with geophysics in studying the Earth's rotation and polar motion. The work establishes the coordinate systems used by NASA for satellite orbits and by the National Geodetic Survey for national mapping. Its global scale is coordinated by bodies like the International Association of Geodesy.

Historical development

Early efforts were driven by the assumption of a spherical Earth, as suggested by philosophers like Eratosthenes, who made an early estimate of the Earth's circumference. The 17th and 18th centuries saw debates between proponents of an oblate spheroid, like Isaac Newton, and a prolate spheroid, like Giovanni Domenico Cassini; expeditions to Lapland and Peru by Pierre Louis Maupertuis and Charles Marie de La Condamine helped resolve this. The 19th century brought precise triangulation surveys, such as the Great Trigonometrical Survey of India and the Arc Measurement across Europe. The 20th century was revolutionized by the launch of Sputnik 1 and the development of satellite geodesy.

Fundamental concepts and principles

A core concept is the geoid, defined as an equipotential surface of the Earth's gravity field that best fits mean sea level. This is contrasted with the ellipsoid, a mathematically smooth surface used for computational purposes. The study of Earth's gravity field involves gravimetry and analysis of gravity anomalies. The theory of plate tectonics is supported by measurements of crustal deformation and post-glacial rebound. The Chandler wobble describes a small motion in the Earth's rotation axis.

Measurement techniques and technologies

Classical techniques include triangulation, trilateration, and leveling, using instruments like the theodolite. Satellite laser ranging to targets like LAGEOS and VLBI observations of quasars provide millimeter-level accuracy. The Global Positioning System, alongside other GNSS constellations like GLONASS and Galileo, is a ubiquitous tool. Altimetry missions such as TOPEX/Poseidon and GRACE have mapped ocean surfaces and gravity variations. InSAR from satellites like Sentinel-1 detects ground displacement.

Major applications

Applications are vast and critical. It enables precise navigation for aviation and maritime use. It underpins all modern cartography and GIS. It is essential for civil engineering projects, land surveying, and cadastre definition. Scientifically, it monitors sea level change observed by the Jason (satellite) series, studies earthquakes along the San Andreas Fault, and tracks ice sheet mass loss in Greenland and Antarctica. It also supports spacecraft tracking for missions by the European Space Agency.

Geodetic datums and reference systems

A geodetic datum is a reference for measuring locations, consisting of an ellipsoid and a fixed origin point. Historically, major datums included the North American Datum of 1927 and the European Datum 1950. Modern systems are based on ITRF, a realization of the International Terrestrial Reference System. The World Geodetic System 1984 is the default for the GPS. Vertical datums reference the geoid, such as the North American Vertical Datum of 1988. These systems are maintained by agencies like the National Oceanic and Atmospheric Administration.

Organizations and international cooperation

Global coordination is led by the International Association of Geodesy, a member of the International Union of Geodesy and Geophysics. The International Earth Rotation and Reference Systems Service maintains ITRF and monitors Universal Time. Key national agencies include the National Geodetic Survey in the United States, the Ordnance Survey in the United Kingdom, and the Bundesamt für Kartographie und Geodäsie in Germany. Major collaborative projects involve the Global Geodetic Observing System and missions from NASA and the European Space Agency.

Category:Earth science Category:Geophysics Category:Surveying