What is a Geoid? Why do they use the geoid, and where does its shape come from?

Geoid illustration

Contrast of the Geoid model with an Ellipsoid and cross-section of the Earth's surface. (Public domain.)

What is a Geoid?  Why do they use the geoid, and where does its shape come from?

A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface.  Currently, we use the “WGS 84” version.

Why do they use the geoid, and where does its shape come from?

If the earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere.  However, the earth more closely approximates an ellipsoid, which is what a ball looks like if you sit on it.  Ellipsoid calculations aren’t as easy as spherical calculations, but they’re still well-known and do-able. But we all know that the earth is not really an ellipsoid because there are oceans, and mountains, and valleys, and many other features that are not part of an ellipsoid.

The geoid is an imaginary sea level surface that undulates (has a wavy surface) over all of the earth; it isn’t just for the oceanic areas, it also extends through the land masses.

You can generalize the relationship between the ellipsoid, the geoid, and the actual shape of the earth with this:

geoid + ellipsoid = Earth

Further details about the geoid can be found at:

NOAA - What is the Geoid?

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Geoid illustration

Geoid illustration

Contrast of the Geoid model with an Ellipsoid and cross-section of the Earth's surface. 

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Attribution: Natural Hazards