What is Geodesy?

Caitlin Kenney

Geodesy, sometimes called geodetics, is a science that applies mathematics to measure the size and shape of the Earth, the positions of points and regions on Earth, and the differences in its gravitational field. This science uses principles from physics, mathematics, and astronomy together with observation and modern technology to develop a spatial reference system. Geodesy also studies the movement of the Earth’s crust, polar motion, and tides. The work of geodesists includes giving points on Earth exact coordinates, precisely measuring the distances, angles, and heights between points, and looking at how and why the Earth’s surface has changed over different periods of time.

The military uses geodesy to track satellites.
The military uses geodesy to track satellites.

Many important fields of work depend on geodesy to function properly. Ships and planes use global positioning systems (GPS), maps, and other navigation systems based on geodetic data to ensure that they land in the correct place, avoid dangerous lanes of travel, and take the quickest and most fuel efficient routes. Scientists from other fields, such as oceanographers or paleontologists, use geodetic data to examine the forces that cause change in Earth’s surface features, or topography. The military has long used geodesy to determine exact points for pinpointing locations, controlling artillery, navigating, and, later, tracking satellites and directing missiles.

Geodesy has been used to direct missiles.
Geodesy has been used to direct missiles.

Some historians claim that the study of geodesy began with the ancient Greeks’ early attempts to measure the size of the Earth. The first earnest attempt at taking accurate measurements, as far as it is known, was pursued by the Greek scholar Eratosthenes in the third century BC. By measuring the angles of shadows at two points on Earth at a particular time of the year, measuring the distance between the points, and assuming that the two points lie on an exact north-south plane, Eratosthenes was able to make an astonishingly accurate approximation of the Earth’s circumference. Despite some faulty data, the scholar estimated the Earth to be 25,000 miles (40,233 km). Today, geodesists agree that the Earth is roughly 24,901 miles (40,074 km) at the Equator.

Several other ancient Greeks used similar methods, measuring the angles of stars to two points on Earth, to make their own estimates and maps. The field of geodesy continued to evolve throughout the centuries, meeting with rapid advancement in the 17th century AD. During this period, the telescope was developed, allowing greater accuracy in measuring angles to objects in space; logarithms were invented, allowing greater computational efficiency; and triangulation was discovered as a method of determining the location of a point. Using this new technology, French-Italian scholar Giovanni Domenico Cassini discovered that the Earth was not spherical, as it had previously been assumed, but elliptical, or egg-shaped.

Several techniques can be used to examine the three-dimensional shape and scope of the Earth, as well as the positioning of the gravitational field. Most can be put under the umbrella of land surveying, satellite imaging, and applied mathematics based on data gathered by the first two methods of observation. Because the Earth is highly irregular in shape, geodesists use a mathematical model of the Earth known as a reference ellipsoid to efficiently measure the Earth. The ellipsoid can be completely smooth, unlike the geoid, another model which represents the irregularity of the figure of the Earth and the changes in gravitational pull. While, the ellipsoid maintains the general shape of the planet, which is flatter at the poles and wider at the equator, the lack of topographical complexity makes computations far simpler.

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