Want to lose weight fast? No need to adjust your diet – just move to higher ground. This weight change is the result of fluctuations in Earth's gravity, which a new high-resolution map shows are greater than thought.
Gravity is often assumed to be the same everywhere on Earth, but it varies because the planet is not perfectly spherical or uniformly dense. In addition, gravity is weaker at the equator due to centrifugal forces produced by the planet's rotation. It's also weaker at higher altitudes, further from Earth's centre, such as at the summit of Mount Everest.
NASA and the European Space Agency both have satellites with highly sensitive accelerometers that map the planet's gravitational field, but these are only accurate to within a few kilometres. Adding in topographical data, which adjusts for height variations in local terrain, can improve the maps' resolution. Accurately constructing tunnels, dams and even tall buildings requires knowledge of the local gravity to guide GPS measurements of height, so higher resolution maps are important for civil engineering.
Christian Hirt of Curtin University in Perth, Western Australia, and colleagues combined gravity data from satellites and topographic data to map gravity changes between latitudes 60° north and 60° south, covering 80 per cent of Earth's land masses.
The map consists of more than 3 billion points, with a resolution of about 250 metres. Computing gravity at five points would take 1 second on an ordinary PC, but the team used a supercomputer to do the whole lot in three weeks.