The arrows show true north, grid north and magnetic north, and the caption explains that the average yearly change in the magnetic declination is 0☀3′ eastward. ![]() The 2008 chart shows 13 degrees, 15 minutes West.Įstimated declination contours by year, 1590 to 1990ĭetermination Magnetic declination indicated on an Israeli map. The 1884 chart shows a variation of 8 degrees, 20 minutes West. This may be insignificant to most travellers, but can be important if using magnetic bearings from old charts or metes (directions) in old deeds for locating places with any precision.Īs an example of how variation changes over time, see the two charts of the same area (western end of Long Island Sound), below, surveyed 124 years apart. For a location closer to the pole like Ivujivik, the declination may change by 1 degree every three years. The magnetic declination in a given area may (most likely will) change slowly over time, possibly as little as 2–2.5 degrees every hundred years or so, depending upon how far from the magnetic poles it is. Level curves drawn on a declination map to denote the magnetic declination, described by signed degrees. Similarly, secular changes to these flows result in slow changes to the field strength and direction at the same point on the Earth. 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. 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.ĭeclination change over time and location 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. Angle on the horizontal plane between magnetic north and true north Example of magnetic declination showing a compass needle with a "positive" (or "easterly") variation from geographic north.
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