The July 2013 Issue of Professional Surveyor Magazine has an article entitled “Defining Surfaces” By Charles Ghilani that teaches the common surveyor a bit of important geodesy. The article includes a great explanation of a basic reason why gravity planes, or equipotential surfaces vary as you move from the equator to the poles. The article says:
“The entire Earth is composed of an infinite number of gravitational surfaces. These surfaces are defined by their potential to do work and are known as equipotential surfaces; that is, these are surfaces that have an equal ability to do work. Work is defined as force times distance. In the case of the equipotential surfaces, the gravitational attraction applies the force, and the distance is defined as the distance of a point from the mass center of the Earth.
However, as we all know, the Earth rotates once a day. The centrifugal force caused by this rotation, shown as red arrows in Figure 2, works against the pull of gravity, which is shown as blue arrows. It is greatest at the equator and goes to zero at the pole, P. Additionally, as given by Newton’s universal law of gravitation, the force of gravity decreases with increasing distance between two objects.
Because the equatorial axis of the Earth is longer than the polar axis of the Earth, the force of gravity is greater at the poles than at the equator. This combined effect of the varying distances from the mass center of the Earth and the rotation of the Earth means that the force of gravity is less at the Equator than at either pole.
Because equipotential surfaces are defined by both force and distance, the distance between the equipotential surfaces must decrease as the force of gravity increases. Thus, the equipotential surfaces converge at the poles, which means they are closer at the poles than at the equator, as shown with the blue lines. These surfaces also undulate as the moon passes over the Earth and as densities of earth change in localities. (Instead of dieting you could simply go to the equator and stand on the highest mountain if you wish to weigh less! Mount Chimborazo in Ecuador might be a good choice.)”
The article also includes one of the simplest definitions of a geodetic datum that I’ve ever read:
“A datum is also called a reference frame because it is a framework of stations with coordinates related to the particular placement of the ellipsoid on the geoid. For example, modern reference frames are defined by the orientation of the polar axis and the position of the mass center of the Earth.”
There are other good geodetic tidbits in the article. Read it.