While magnetic compasses are reliable, they need to be adjusted for two factors. Magnetic declination, known as variation, refers to the angle, on a flat plane, between the planet’s magnetic north pole and its true north pole. This angle varies depending on one’s latitude and longitude and oscillates over time, with areas closer to the poles experiencing more oscillation than equatorial zones. Since charts are presented in true-north-up orientation, navigators must correct for this difference when determining their best heading. The second factor, magnetic deviation, refers to locally introduced errors. These range from onboard ferrous metals, such as an engine block, to external magnetic anomalies, such as large iron deposits. To correct for small amounts of magnetic deviation (say, onboard tools), careful navigators swing their compass during slack tides and prepare deviation cards that allow them to make real-time corrections to the vessel’s heading. To correct for bigger problems, or for initial setup calibration, magnets are placed inside the compass housing by a professional to create equal but opposing magnetic fields that nullify deviation caused by in situ ferrous objects. This is known as compass adjustment or calibration.