DEFINITION OF GEODESY

The study of the size and shape of the Earth, its

gravitational field, and the determination of the precise locations

of points on the surface. It also includes the study of the

temporal variations in the shape of the planet and the location

of points on the surface as a result of tides, rotation, and

plate tectonic movements. Geodetic measurements rely heavily

on positional measurements from satellite-based global

positioning systems (GPS), gravity measurements, and radar

altimetry measurements over the oceans. The science of measuring

the size of the Earth probably started with Erastothenes

in ancient Greece, who measured the distance from

Alexandria to Aswan and calculated the curvature of the

Earth from his measurements.

One branch of geodesy deals with the measurement of

the Earth’s gravity field and the geoid, the surface of equal

gravitational potential. The geoid has a roughly elliptical

shape that is slightly flattened at the poles as a result of the

planet’s rotation and is approximated by a reference ellipsoid.

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Variations in the height of the geoid from the reference ellipsoid

are expressed as the geoid height, in many cases reaching

tens of meters. These variations reflect variations in the mass

distribution within the Earth, and smaller, temporary variations

may result from tides or winds changing the mass distribution

of the oceans.

Geodetic measurements must use some reference frame,

typically an astronomical or celestial, or an inertial reference

frame. Many geodetic measurements are between different

points on the surface, and these terrestrial measurements are

useful for determinations of surface deformation such as

motion along faults. Regional geodetic measurements rely on

the art of triangulation, first developed by the Dutch scientist

Gemma Frisius in the 16th century. Triangulation uses

precise measurements of the angles and distances between

different points in a network or grid to determine the

changes in the shape of the grid with time, and hence the

deformation of the surface.

Space geodesy uses satellite positioning techniques where

GPS satellites emit microwave signals encoded with information

about the position of the satellite, and the precise time at

which the signal left the satellite. The distance to the GPS

receiver on the surface can therefore be determined, and by

using several GPS satellite signals the precise position on the

surface of the Earth can be determined. The accuracy of GPS

positions can be within a few meters (or less), and can be

improved by a technique known as differential GPS in which

a satellite receiver at a known position is coupled with and

emits signals to a roving receiver. Further, multi-receiver

interferometric and kinematic GPS techniques can improve

positional measurements to the submillimeter level. The

improved precision for these methods has greatly improved

observations of surface deformation needed to predict earthquakes

and volcanic eruptions and has aided precise navigation,

surveying, and guidance systems.

See also GEOID; GLOBAL POSITIONING SYSTEM.

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