Terrestrial Gravity and Vertical Gravity Gradient Surveys

Gravity, or more precisely the acceleration due to the Earth's gravity field, g, is not constant on the surface of the Earth. As one moves up in elevation, one is further from the center of the Earth, the acceleration is reduced. In addition, local changes in surface density can cause more or less direct attraction.

Gravity is usually measured with an absolute gravity meter. These instruments track the free fall of an object in a vacuum using standard values of length and time, and are accurate to a few parts per billion. In addition, relative gravity meters can be used to determine the difference in gravity between two locations to about the same precision.

Absolute gravity was determined at each bench mark using either absolute instruments or a combination of absolute and relative instruments. In addition, the vertical gravity gradient (a measure of how quickly gravity gets weaker as one moves up, away from the center of the Earth) was determined at most bench marks. The gravity values, combined with the leveling data, is used to calculate the vertical undulations of the geoid.