Geodesy

C

Gravity and Geopotential

Accurate knowledge of Earth’s gravity field is essential for processing geodetic measurements and establishing heights above mean sea level. For example, variations in the force of Earth’s gravity can perturb, or slightly distort, the paths of satellites in their orbits. Such variations result from differences in the density of Earth’s crust and the existence of high mountains or deep ocean trenches. Geodesists use precise measurements of the motions of satellites to improve their mathematical models of Earth’s gravity. These improved models are used, in turn, to get more accurate coordinates from satellite measurements.

Variations in gravity also affect surveying instruments and other devices that use level bubbles, such as theodolites and geodetic levels. These instruments and devices have a small, slightly curved tube filled with liquid and containing an air bubble. When the slightly curved tube is held level, the liquid inside conforms to a level surface and makes the air bubble rise to the top. However, small variations in gravity can affect these instruments. To make sure that the instruments read correctly, scientists and surveyors use gravity measurements in operations and surveys where high precision is necessary.

Variations in gravity can affect sea level on Earth because water tends to conform to a level surface. A level surface is defined as being everywhere perpendicular to the direction of the force of gravity. To measure height above sea level, geodesists use geopotential, a series of horizontal layers that are perpendicular to the direction of this force. The layer of the geopotential used as the sea-level reference for heights is called the geoid. The geoid is defined over both sea and land. Geodesists use models of the geoid to convert heights from GPS measurements into heights above mean sea level. Radar altimeters (special instruments used to measure altitude) on satellites measure variations of the ocean surface over space and time. Their measurements are used to obtain gravity data over the oceans. Satellites are being used to measure gravity ever more accurately over both land and sea.

D

Motions of Earth

By studying extremely accurate data from GPS, SLR, VLBI, and other techniques, scientists have observed Earth’s numerous motions in space. For example, while Earth rotates on its axis once every 24 hours or so, variations in this rotation have been detected. Scientists have traced these variations to seasonal wind patterns and to the oceanic and atmospheric phenomenon known as El Niño. In addition, Earth’s rotation is slowing gradually as a result of tidal effects (see Tide). Earth’s orientation in space (the direction in which its axis of rotation points) displays the gyroscopic motions of precession and nutation, much like a spinning top (see Gyroscope). In addition, Earth itself moves relative to its axis of rotation, a phenomenon known as polar motion.

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Geodesists measure the tidal motions of Earth. These motions include not only the oceanic tides but also tides in the solid Earth, which amount to about 0.5 m (1.6 ft) of vertical motion of Earth’s surface. The precise measurements that geodesists make reveal small effects, such as deformation of Earth’s crust caused by the weight of ocean water when it piles up in tides. They also provide information about Earth’s internal structure and composition. Accurate geodetic positioning makes it possible to track the relative motions of Earth’s crust caused by plate tectonics. Geodesists also measure crustal motion that results from earthquakes.

E

Other Specializations

Geodesy encompasses other specialized topics. Some geodesists work to ensure the accuracy of map projections. Others analyze measured data and make mathematical adjustments to account for uncertainties in measurements, fluctuations in instruments, and approximations of mathematical models. Still others prepare standards and guidelines for the observation and processing of surveying data.

III

Selected Organizations

In the United States, the National Geodetic Survey (NGS), under the Department of Commerce, provides the geodetic control network for national mapping, charting, and positioning activities. In addition, the National Imagery and Mapping Agency (NIMA) supplies geodetic products for the U.S. Department of Defense. In Canada, the Geodetic Survey Division of Natural Resources Canada provides the national geodetic control network, while the Directorate of Geographic Operations serves the needs of the Department of National Defence. Many other governments maintain geodetic agencies and institutes. Geodesists worldwide cooperate through the International Association of Geodesy (IAG), which is a component of the International Union of Geodesy and Geophysics (IUGG). In the United States, geodesists participate in scientific meetings through the Geodesy section of the American Geophysical Union (AGU) and through the American Association of Geodetic Surveying (AAGS), a component of the American Congress on Surveying and Mapping (ACSM).

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