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."