Geoid

Definition

The geoid is a mathematical model of the Earth in which the geoid surface coincides exactly with the average elevation of the surface of the planet’s oceans. This model surface is extended over the globe, even through the continents, to form the geoid. This surface is smooth and is defined by mathematical calculations and numerous gravitational measurements. The geoid is a physical representation of the body of the Earth and is different from other representations of the Earth, for example, the Earth’s reference ellipsoid. Only since the advent of modern geomathematical computing has the geoid been calculated with high precision.

The geoid is sometimes referred to as an equipotential surface, which means that the force of gravity is everywhere the same on the geoid and that the force of gravity is everywhere perpendicular to the surface of the geoid. The form of the geoid is slightly irregular, but is everywhere very smooth. The geoid has a history dating to the nineteenth century, when several scientists worked on the problem of computing a mathematical description of the Earth, or computing the shape of the geoid. Since that time, more precise measurements and better means of computing have permitted progressively more precise definitions and understandings of the geoid and its complexity.

For the average person, the geoid’s most notable impact on daily life is the precision of handheld and vehicle-mounted global positioning satellite (GPS) receivers. The modern geoid allows these devices to provide extremely accurate physical locations on the Earth’s surface, sometimes including elevation. However, there are more applications for the geoid than personal GPS devices, including applications that are related to environmental and climate change. The word “geoid” is the root word for the term “geodesy,” which is the study of the shape of the Earth, including ways to describe that shape and practical applications incorporating it.

Significance for Climate Change

The significance of the geoid for climate change lies in its potential for very precise calculation of elevation and elevation changes on Earth. These precise calculations are made with the aid of GPS systems, which are used to compute elevations in reference to the geoid. These satellites are part of the global navigation satellite system (GNSS) operational around the world. GPS elevation readings are taken in reference to an Earth-centered reference ellipsoid, because satellites orbit about the center of gravity of the Earth. Using corrections, GPS elevations are computed within GPS receivers in reference to the geoid.

Knowing precise elevations is very important in studies of climate and environmental change. For example, measurements of sea-level rise or fall are important in such studies. Also, changes in elevation of ice-covered regions of Earth may help scientists understand the broader range of changes occurring there. The elevation of the surface of the ocean may vary from place to place as a result of changes in the temperature and volume of moving water masses. All this data is useful in assessing the degree, rate, and extent of climate and environmental change on Earth.

The Gravity Recovery and Climate Experiment (GRACE) is a geoid-referencing project that involves two orbiting spacecraft that were launched by the United States in 2002. In addition to detailed measurements of Earth’s gravity, GRACE has conducted several environmental and climate-related studies, including studies of the new, rapid melting of the Antarctic ice sheet. The is melting at a rate, according to GRACE, that is 75 percent faster than it was at the end of the twentieth century. These results show that Antarctic ice loss rivals the ice loss in Greenland, which was studied and documented by GRACE data in 2006.

GRACE has studied the freshwater storage patterns and human activity of many continental areas on Earth, including China’s Yangtze River, Australia, and parts of North America and Africa. In particular, GRACE data show that most continental areas are drying up or losing net stored water, findings that have long-term climatic implications affecting the future of human society. The location and distribution of water on Earth have a profound effect upon the distribution of life.

The more precisely defined the geoid becomes, the more Earth parameters can be measured precisely against it. For example, a more precisely defined geoid will help scientists understand better even very small changes in the elevation of the ocean surface, which can be useful in measuring changes in circulation patterns, tides, and heat transfer in the oceans. A precisely defined geoid is useful for hydrological and glaciological modeling, which in turn provides for enhanced climate modeling and analysis. Geoid definitions are dependent upon time-variable effects, which are an integral part of the definition. Modern geoid approximations are computer models based on numerous complex variables.

Bibliography

Hofmann-Wellenhof, Bernhard, and Helmut Moritz. Physical Geodesy. 2d ed. Berlin: Springer-Verlag, 2006.

Hooijberg, Maarten. Geometrical Geodesy: Using Information and Computer Technology. New York: Springer, 2008. Details

National Geodetic Survey. Geodesy: Imagine the Possibilities. Washington, D.C.: U.S. Department of Commerce, 2000.

"What Is the Geoid?" National Ocean Service, 16 June 2024, oceanservice.noaa.gov/facts/geoid.html. Accessed 20 Dec. 2024.