Gravity surveying is one of two potential field methods, along with magnetic surveying. It builds on Newton's recognition that every mass in the universe attracts all other masses in the universe. The strength of attraction is most strongly felt when the masses are located close to one another. On Earth's surface, the main attraction objects experience is that to the planet itself. As the mass of the planet is much greater that of any near-surface mass it interacts with, the strength of attraction everywhere on Earth is felt to be the same; it causes objects to fall according to a well-known acceleration - g (9.8 ms-2). In reality, this g varies by a few hundred tens of thousandths of metres per second squared (unit of measurement known as the milliGal, after a famous early gravimetric scientist Galileo Galilei). The variations are related to the different ways that the masses in Earth's various rock and fluid layers - atmosphere, ocean, ice-sheet, sediments, crust, mantle, and core, are arranged with respect to each other. Accelerations from density contrasts at all depths are measureable, with those closest to the meter producing the sharpest variations. Understanding gravity data requires the isolation of signals by the calculation of so-called gravity anomalies.