For historical data, the IGRF and GUFM models may be used. It reflects a highly predictable rate of change, and is usually more accurate than a map-which is likely months or years out of date. It is built with all the information available to the map-makers at the start of the five-year period it is prepared for. One such model is World Magnetic Model (WMM) of the US and UK. Worldwide empirical model of the deep flows described above are available for describing and predicting features of the Earth's magnetic field, including the magnetic declination for any given location at any time in a given timespan. See also: Earth's magnetic field § Statistical models Geological Survey (USGS), for example, a diagram shows the relationship between magnetic north in the area concerned (with an arrow marked "MN") and true north (a vertical line with a five-pointed star at its top), with a label near the angle between the MN arrow and the vertical line, stating the size of the declination and of that angle, in degrees, mils, or both. The same diagram may show the angle of grid north (the direction of the map's north–south grid lines), which may differ from true north. The current rate and direction of change may also be shown, for example in arcminutes per year. Unless the area depicted is very small, declination may vary measurably over the extent of the map, so the data may be referred to a specific location on the map. Larger-scale local maps may indicate current local declination, often with the aid of a schematic diagram. Isogonic lines are also shown on aeronautical and nautical charts. Similarly, secular changes to these flows result in slow changes to the field strength and direction at the same point on the Earth.Ī rough estimate of the local declination (within a few degrees) can be determined from a general isogonic chart of the world or a continent, such as those illustrated above. In most areas, the spatial variation reflects the irregularities of the flows deep in the Earth in some areas, deposits of iron ore or magnetite in the Earth's crust may contribute strongly to the declination. Reports of measured magnetic declination for distant locations became commonplace in the 17th century, and Edmund Halley made a map of declination for the Atlantic Ocean in 1700. The declination at London, UK was one degree west (2014), reducing to zero as of early 2020. As a traveller cruises the east coast of the United States, for example, the declination varies from 16 degrees west in Maine, to 6 in Florida, to 0 degrees in Louisiana, to 4 degrees east in Texas. Magnetic declination varies both from place to place and with the passage of time. 1 Declination change over time and locationĭeclination change over time and location.Magnetic declination should not be confused with magnetic inclination, also known as magnetic dip, which is the angle that the Earth's magnetic field lines make with the downward side of the horizontal plane. The term magnetic deviation is sometimes used loosely to mean the same as magnetic declination, but more correctly it refers to the error in a compass reading induced by nearby metallic objects, such as iron on board a ship or aircraft.
The lowercase Greek letter δ (delta) is frequently used as the symbol for magnetic declination. Isogonic lines are lines on the Earth's surface along which the declination has the same constant value, and lines along which the declination is zero are called agonic lines. The angle between magnetic and grid meridians is called grid magnetic angle, grid variation, or grivation.” īy convention, declination is positive when magnetic north is east of true north, and negative when it is to the west. Somewhat more formally, Bowditch defines variation as “the angle between the magnetic and geographic meridians at any place, expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north. This angle varies depending on position on the Earth's surface and changes over time. Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) and true north (the direction along a meridian towards the geographic North Pole). N g is geographic or true north, N m is magnetic north, and δ is magnetic declination Example of magnetic declination showing a compass needle with a "positive" (or "easterly") variation from geographic north.