Editing Magnetic declination (section)

== Determination ==
[[Image:IsraelCVFRmag.jpg|thumb|130px|Magnetic declination indicated on an Israeli map. 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°03′ eastward.]]

=== Field measurement ===
[[Image:Boussole de déclinaison Lenoir.png|thumb|left|200px|Antique declinometer]]

The magnetic declination at any particular place can be measured directly by reference to the [[celestial pole]]s—the points in the heavens around which the stars appear to revolve, which mark the direction of true north and true south. The [[Measuring instrument|instrument]] used to perform this measurement is known as a ''declinometer''.

The approximate position of the north celestial pole is indicated by [[Polaris]] (the North Star). In the [[Northern Hemisphere|northern hemisphere]], declination can therefore be approximately determined as the difference between the magnetic bearing and a visual bearing on Polaris.  Polaris currently traces a circle 0.73° in radius around the north celestial pole, so this technique is accurate to within a degree.  At high latitudes a [[plumb-bob]] is helpful to sight Polaris against a reference object close to the horizon, from which its bearing can be taken.<ref>{{citation|url=http://earthsci.org/education/fieldsk/declin.htm|title=Magnetic declination, what it is, how to compensate.|access-date=2010-03-03|archive-url=https://web.archive.org/web/20100107205605/http://earthsci.org/education/fieldsk/declin.htm|archive-date=2010-01-07|url-status=dead}}</ref>
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===Determination from maps  ===
A 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. Isogonic lines are also shown on [[aeronautical chart|aeronautic]]al and [[nautical chart]]s.

Larger-scale local maps may indicate current local declination, often with the aid of a schematic diagram. 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. The current rate and direction of change may also be shown, for example in [[arcminute]]s per year. 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.

On the [[topographic map]]s of the [[U.S. 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, [[angular mil|mil]]s, or both.  However, the diagram itself is not an accurate depiction of the stated numerical declination angle, but is intentionally exaggerated by the cartographer for purposes of legibility.

=== Models and software ===
{{see also|Earth's magnetic field#Statistical models}}
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. One such model is [[World Magnetic Model]] (WMM) of the US and UK. It is built with all the information available to the map-makers at the start of the five-year period it is prepared for. It reflects a highly predictable rate of change,{{efn|This rate of change is known as the [[geomagnetic secular variation]], and current models using a constant variation over five-year periods are on average ([[root mean square]]) off by 15 arcminutes at the end of each forecast.<ref>{{cite journal |last1=Fournier |first1=Alexandre |last2=Aubert |first2=Julien |last3=Lesur |first3=Vincent |last4=Thébault |first4=Erwan |title=Physics-based secular variation candidate models for the IGRF |journal=Earth, Planets and Space |date=December 2021 |volume=73 |issue=1 |pages=190 |doi=10.1186/s40623-021-01507-z|bibcode=2021EP&S...73..190F |s2cid=239022300 |doi-access=free }}</ref>}} and is usually more accurate than a map—which is likely months or years out of date.{{citation needed|date=February 2014}} For historical data, the IGRF and GUFM models may be used. Tools for using such models include:

* Web apps hosted by the [[National Geophysical Data Center]], a division of the [[National Oceanic and Atmospheric Administration]] of the United States.<ref>{{cite web|work=Geomagnetism |url=https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml |title=Estimated Value of Magnetic Declination |publisher=NOAA National Geophysical Data Center |access-date=6 December 2013}}</ref>
* C demo program that for WMM by the [[National Geospatial-Intelligence Agency]], along with various other third-party implementations.<ref>{{cite web |last1=Meyer |first1=Brian |title=World Magnetic Model - Software Download |url=https://www.ncei.noaa.gov/products/world-magnetic-model |website=www.ngdc.noaa.gov|date=8 February 2022 }}</ref>

The WMM, IGRF, and GUFM models only describe the magnetic field as emitted at the core-mantle boundary. In practice, the magnetic field is also distorted by the Earth crust, the distortion being [[magnetic anomaly]]. For more precise estimates, a larger crust-aware model such as the [[Enhanced Magnetic Model]] may be used. (See cited page for a comparison of declination contours.)<ref>{{cite web |last1=National Centers for Environmental Information (NCEI) |title=Enhanced Magnetic Model (EMM) |url=https://www.ncei.noaa.gov/products/enhanced-magnetic-model |website=www.ngdc.noaa.gov |date=10 March 2022 |language=EN-US}}</ref>