I.

Introduction

Geodesy, branch of earth science that deals with determining precise positions on the surface of Earth, the size and shape of Earth, the motions of Earth in space, and the gravity field of Earth. Scientists known as geodesists create networks of accurately measured points on Earth’s surface. They measure the angles, distance, and gravity differences between these points, and then compute their latitude, longitude, and height. In addition, they determine how processes such as movements of Earth’s crust cause the results of these measurements to vary over time.

The sets of accurately measured points that geodesists create are known as control networks. These control networks are used to construct accurate maps and charts. They also provide reference grids for surveys made in building roads, bridges, pipelines, tunnels, and many other structures (see Surveying).

Geodesy is closely related to geophysics. In fact, the two sciences overlap in several significant areas, such as in the study of Earth’s gravity. Both geodesists and geophysicists require precise measurements and accurate mathematical models in their work. In general, geodesists focus on measurements and coordinates (sets of numbers that together describe the exact positions of things), while geophysicists focus on how and why various physical processes and effects occur.

II.

Areas of Study

Geodesy is a specialized science. Even so, it is broad enough to be subdivided into a number of areas. These areas overlap, and the boundaries among them change over time. Today, the main subdivisions of geodesy are control surveys, space geodesy, global networks, gravity and geopotential, and the motions of Earth.

A.

Control Surveys

To begin a geodetic survey, geodesists measure several different points to serve as starting coordinates. For example, a point in a survey of the northeastern United States might be the top of Mount Washington in New Hampshire, the highest point in the area. Geodesists would record the latitude, longitude, and height of the peak. The initial survey would include several such points, called control points. These accurately measured points are marked on the ground. The markers usually consist of brass disks set in concrete or steel rods driven into bedrock. Together, the set of control points creates a control network. Measurements taken between control points are used to compute a datum, a consistent set of latitudes, longitudes, and heights that defines a control network. Once the control network is established, geodesists can measure angles, distances, and height differences and then compute new coordinates for the additional points relative to the original control points. Both the old coordinates and the new coordinates will be consistent, and they can be used for making maps and for many other purposes. In the past, geodesists developed separate networks for horizontal and vertical points. However, at the start of the 21st century, geodesists were developing a unified system that incorporates all three spatial dimensions (north-south direction, east-west direction, and up).

In a specialization known as geometric geodesy, geodesists use a simple model of Earth, in the form of a flattened sphere known as an ellipsoid, to compute latitudes, longitudes, and heights. They also determine angles and distances in relation to the surface of this ellipsoid.

B.

Space Geodesy and Global Networks

Geodesy underwent a revolution with the launch of the Sputnik satellite in 1957. Before satellites were used in geodesy, geodetic networks were typically no larger than an individual country or continent. Geodesists used slow, laborious methods to establish the continental networks. They took angle measurements through telescopes attached to steel circles with calibrations inscribed on them. The lines of measurement formed interlocking networks of triangles, 16 km (10 mi) or more on a side. Often, to get lines of sight above trees and hills, the scientists had to build towers, similar to oil-well derricks. Field parties frequently had to camp in the wilderness. Using these methods, a single observation project took months to complete. To survey just the United States required hundreds of such projects. And the resulting measurements were not very accurate: When comparing points across the United States, geodesists could find errors of over 3 m (10 ft).

After satellites were launched in the 1960s, scientists found that they could use measurements of radio or other electromagnetic signals from satellites to create a reference system (a set of starting coordinates) that was consistent on a global scale. American scientists and engineers developed the Global Positioning System (GPS), which uses 24 radio-navigation satellites to provide radio signals that are used worldwide. GPS establishes coordinates accurate to within a few centimeters. Using GPS satellites, transcontinental distances can be measured in just a few hours, within an accuracy of a little more than 1 cm (about 0.5 in).

In addition to GPS, geodesists use other measurement systems based on objects in space to create a global reference network. A technique called satellite laser ranging (SLR) measures the time it takes for a laser pulse to travel from a station on the ground to a specially designed satellite studded with reflectors and back to the ground station. Another technique, very long baseline interferometry (VLBI), uses radio signals emitted by strange objects far away in space called quasars. VLBI measures the small time differences between quasar emissions received at radio telescopes at different locations around the world (see Radio Astronomy). These space systems provide a global set of coordinates as well as information on the motions of Earth (for more information, see the Motions of Earth section of this article).

The global reference networks in use today are more accurate than any reference network used in earlier times. For purposes not requiring a global scale, geodesists use GPS surveying to develop regional and national networks of control points. A system of measurement stations known as Continuously Operating Reference Stations (CORS) now collects GPS satellite signals and provides this data on the Internet. This system often substitutes for the traditional geodetic survey markers in the ground.

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.

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).