What is Geodesy?

geodesy. n. the branch of mathematics dealing with the figures and areas of the earth or large portions of it.

Geodesy ... a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal motion, tides, and polar motion. For this they design global and national control networks, using space and terrestrial techniques while relying on datums and coordinate systems.

Geodesy is a science, the oldest earth (geo-) science, in fact. It was born of fear and curiosity, driven by a desire to predict natural happenings and calls for the understanding of these happenings. The classical definition, according to one of the "fathers of geodesy" reads: "Geodesy is the science of measuring and portraying the earth's surface" [Helmert, 1880, p.3]. Nowadays, we understand the scope of geodesy to be somewhat wider. It is captured by the following definition [Vanicek and Krakiwsky, 1986, p.45]:

"Geodesy is the discipline that deals with the measurement and representation of the earth, including its gravity field, in a three-dimensional time varying space."

Geodesy is the study of the shape and size of the earth.

According to the classical definition of F.R. Helmert (A 1880), geodesy is the "science of the measurement and mapping of the earth's surface". This definition has to this day retained its validity; it includes the determination of the earth's external gravity field, as well as the surface of the ocean floor. With this definition, which as to be extended to include temporal variations of the earth and its gravity field, geodesy may be included in the geosciences, and also in engineering sciences, e.g., National Academy of Sciences (1978).

Triggered by the development of space exploration, geodesy turned in collaboration with other sciences toward the determination of the surfaces of other celestial bodies (moon, other planets). The corresponding disciplines are called selenodesy and planetary geodesy. (Bills and Synnot 1987).

Geodesy may be divided into the areas of global geodesy, national geodetic surveys, and plane surveying. Global geodesy is responsible for the determination of the figure of the earth and of the external gravity field. A geodetic survey establishes the fundamentals for the determination of the surface and gravity field of a country. This is realized by coordinates and gravity values of a sufficiently large number of control points, arranged in geodetic and gravimetric networks. In this fundamental work, curvature and the gravity field of the earth must be considered. In plane surveying (topographic surveying, cadastral surveying, engineering surveying), the details of the terrain are obtained. In plane surveying, the horizontal plane is generally sufficient.

There is close cooperation between global geodesy, geodetic surveying and plane surveying. The geodetic survey adopts the parameters determined by measurements of the earth, and its own results are available to those who measure the earth. The plane surveys, in turn, are generally tied to the control points of the geodetic surveys and serve then particularly in the development of national map series and in the formation of real estate cadastres. Measurement and evaluation methods are largely identical in global geodesy and national geodetic surveys. Particularly space methods (satellite geodesy) enter more and more into regional and even local surveys. This also implies more detailed gravity field determination on regional and local scale.

With the corresponding classifications in the realms of the English and French languages, the concept of "geodesy" (la geodesie, "hoher Geodasie" after Helmert) is to be referred only to global geodesy and geodetic surveying. The concept of "surveying" (la topometrie, Vermessungskunde or "niedere Geodasie" after Helmert) shall encompass plane surveying.

References

National Academy of Sciences, Commission on Geodesy -- Geodesy: Trend and prospects. National Academy of Sciences, Washington, DC, 1978.

Bills, B.G., S.P. Synnott. Planetary geodesy. Reviews of Geophysics, 25, pp. 833-839, 1987.

Last Revised 2009-05-13