The Elements of Geodesy: The
Figure of the Earth |
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Because the surface of the Earth is so
complex, geodesists
use simplified, mathematical
models of the Earth
for many applications. Click
on the image for larger view.
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The Earth's shape is nearly spherical, with a radius of about 3,963 miles (6,378 km), and its surface is very irregular. Mountains and valleys make actually measuring this surface impossible because an infinite amount of data would be needed. For example, if you wanted to find the actual surface area of the Grand Canyon, you would have to cover every inch of land. It would take you many lifetimes to measure every crevice, valley, and rise. You could never complete the project because it would take too long.
To measure
the Earth and avoid the problems that places like
the Grand Canyon present, geodesists use a theoretical
mathematical surface called the ellipsoid. Because
the ellipsoid exists only in theory and not in real life, it can be
completely smooth and does not take any irregularities - such
as mountains or valleys -- into account. The ellipsoid
is created by rotating an ellipse around its shorter
axis. This matches the real Earth's shape, because
the earth is slightly flattened at the poles and
bulges at the equator.
While the ellipsoid gives a common reference
to geodesists, it is still only a mathematical
concept. Geodesists often need to account for the
reality of the Earth's surface. To meet this need,
the geoid, a shape that refers to global mean sea
level, was created. If the geoid really existed,
the surface of the Earth would be equal to a level
in between the high-tide and low-tide marks.
Although a geoid may seem to be a smooth, regular
shape, it isn't. The Earth's mass is unevenly distributed,
meaning that certain areas of the planet experience
more gravitational "pull" than
others. Because of these variations in gravitational
force, the "height" of
different parts of the geoid is always changing,
moving up and down in response to gravity. The geoidal
surface is an irregular shape with a wavy appearance;
there are rises in some areas and dips in others
(Geodesy for the Layman, 1984).
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