The geoid is a representation of the Earth’s surface, created by imagining an ideal sea level that would cover the planet. It shows peaks and troughs that correspond to changes in land mass and illustrates how the gravitational field behaves in different regions. It is more accurate than the reference ellipsoid and mean sea level for elevation measurements.
The geoid is a visual representation of the outline of the Earth. It is accomplished by creating a hypothetical ideal sea level that would cover the entire planet, extending under most land masses. It is not a representation of mean or mean sea level where the surface of the Earth would be totally flat and covered in water, because the planet itself is not flat. Viewed in cross section, the geoid has a number of troughs and peaks that correspond to changes in land mass.
There are several ways to display the earth’s surface for convenience when creating maps and charts. The Earth is not perfectly round, but has a more elliptical shape. Some resources use the reference ellipsoid, which expects the surface of the Earth to be flat. Others may look at the topography of the Earth or mean sea level. None of these representations give a completely accurate picture of what the planet’s surface and shape look like.
Sea level is not constant worldwide; individual oceans can be higher and lower and have variations on their surfaces. This is exaggerated with the influence of tides and other factors. To determine the shape of the geoid, the researchers pretend the Earth is covered in water and note where it would be highest or lowest. On land, this falls short of topography, as the ocean could not extend to cover a mountain range. Areas with higher mass, such as mountains, would force the water to swell, while deep valleys have less gravitational pull and cause a plunge.
Viewed in cross section, the geoid shows a series of gentle peaks and troughs that illustrate how Earth’s gravitational field behaves in different regions. It doesn’t represent the gravitational field itself, but what would happen if water covered the Earth or, more realistically, if deep ditches were dug through the continents to allow the ocean to flow inward. The water in these trenches would change elevation as it passed through the geoid.
Graphs are available that illustrate the difference between topography, reference ellipse, and geoid. These show that the reference ellipse can be highly inadequate for things like elevation measurements, as it could be considerably higher or lower than the actual topography. The geoid roughly corresponds to features such as mountains, but does not take on such extreme elevations and valleys. This information can be important for tasks such as using a global positioning satellite to fix a location.
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