1 Gal

Source Wikipedia: "The gal, sometimes called galileo, (symbol Gal) is a unit of acceleration used extensively in the science ofgravimetry. The gal is defined as 1 centimeter per second squared (1 cm/s²). The milligal (mGal) and microgal (µGal) refer respectively to one thousandth and one millionth of a gal. The gal is not part of the International System of Units (SI). However, in 1978 the CIPM decided that it was permissible to use the gal "with the SI until the CIPM considers that [its] use is no longer necessary." The gal is a derived unit, defined in terms of the centimeter-gram-second (CGS) base unit of length, the centimeter, and the second, which is the base unit of time in both the CGS as well as the modern SI system. In SI base units, 1 Gal is precisely equal to 0.01 m/s².The acceleration due to Earth’s gravity (see Standard gravity) at its surface is 976 to 983 Gal, the variation being due mainly to differences in latitude and elevation. Mountains and masses of lesser density within the Earth's crust typically cause variations in gravitational acceleration of tens to hundreds of milligals (mGal). The gravity gradient (variation with height) above Earth's surface is about 3.1 µGal per centimeter of height (3.1×10⁻⁶ s^–2), resulting in a maximum difference of about 2 Gal (0.02 m/s²) from the top of Mount Everest to sea level."

Gravitometer

A gravimeter is an instrument used  for measuring the local gravitational field of the Earth. "A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Though the essential principle of design is the same as in other accelerometers, gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or the shape of the Earth and by temporal tidal variations. This sensitivity means that gravimeters are susceptible to extraneous vibrations including noise that tend to cause oscillatory accelerations. In practice this is counteracted by integral vibration isolation and signal processing. The constraints on temporal resolution are usually less for gravimeters, so that resolution can be increased by processing the output with a longer "time constant". Gravimeters display their measurements in units of gals, instead of ordinary units of acceleration. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and othergeophysical research, and for metrology." More at http://en.wikipedia.org/wiki/Gravimeter

"The geoid, simply stated, is the shape that the surface of the oceans would take under the influence of gravity alone. All points on that surface have the same scalar potential - there is no difference in potential energy between any two. In that idealized situation, other influences such as winds due to solar heating, and tides have no effect. The surface of the geoid is farther away from the center of the earth where the gravity is weaker, and nearer where it is stronger. The differences in gravity, and hence the scalar potential field, arise from the uneven distribution of the density of matter in the earth.Specifically, the geoid is the equipotential surface that would coincide with the mean ocean surface of the Earth if the oceans and atmosphere were in equilibrium, at rest relative to the rotating Earth,[1] and extended through the continents (such as with very narrow canals). According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but highly irregular surface that corresponds not to the actual surface of the Earth's crust, but to a surface which can only be known through extensive gravitational measurements and calculations. Despite being an important concept for almost two hundred years in the history of geodesy and geophysics, it has only been defined to high precision in recent decades, for instance by works of Petr Vaníček, and others. It is often described as the true physical figure of the Earth,[1] in contrast to the idealized geometrical figure of a reference ellipsoid." More at Wikipedia: http://en.wikipedia.org/wiki/Geoid

GRACE

The Gravity Recovery And Climate Experiment (GRACE), a joint mission of NASA and the German Space Agency, has been making detailed measurements of Earth's gravity field since its launch in March 2002.

Gravity is determined by mass. By measuring gravity, GRACE shows how mass is distributed around the planet and how it varies over time. 

GRACE is a collaborative endeavor involving the Center for Space Research at the University of Texas, Austin; NASA's Jet Propulsion Laboratory, Pasadena, Calif.; the German Space Agency and Germany's National Research Center for Geosciences, Potsdam. The Jet Propulsion Laboratory is responsible for the overall mission management under the NASA ESSP program.

http://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment