The Elements of Geodesy: Gravity |
|
|
|
|
|
Imagine if all the mountains and valleys
were scoured off
the planet
leaving a continuous
world ocean completely
at rest. The effects
of the Earth's gravity
on this hypothetical
world mean sea level
is represented by
the geoid.
Click on the image for a larger view and
more images.
|
|
|
Gravity is the force that pulls all objects in
the universe toward each other. On Earth, gravity
pulls all objects "downward" toward
the center of the planet. According to Sir Isaac
Newton's Universal Law of Gravitation, the gravitational
attraction between two bodies is stronger when
the masses of the objects are greater and closer
together. This rule applies to the Earth's gravitational
field as well. Because the Earth rotates and its
mass and density vary at different locations on the planet, gravity
also varies.
One reason that geodesists measure variations in the Earth's gravity is because gravity plays a major role in determining mean sea level. Geodesists calculate the elevation of locations on the Earth's surface based on the mean sea level. So knowing how gravity changes sea level helps geodesists make more accurate measurements. In general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. In areas where the Earth's gravitational forces are weaker, the mean sea level will be lower.
To
measure the Earth's gravity field, geodesists use
instruments in space and on land. In space, satellites
gather data on gravitational changes as they pass over points on the
Earth's surface. On land, devices called gravimeters measure the Earth's
gravitational pull on a suspended mass. With this
data, geodesists can create detailed maps of gravitational fields and
adjust elevations on existing maps. Gravity principally affects the
vertical datum because it changes the elevation of the land surface
(Geodesy for the Layman, 1984).
(top)