Editing Magnetic declination (section)

== Declination change over time and location ==
Magnetic declination varies both from place to place and with the passage of time. 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. The declination at London, UK was one degree west (2014), reducing to zero as of early 2020.<ref>{{cite web|title=Find the magnetic declination at your location|url=http://www.magnetic-declination.com/ |publisher=Magnetic-Declination.com |access-date=6 December 2013}}</ref><ref>{{cite web|title=World Magnetic Model - Epoch 2020 -Declination|url=https://www.ngdc.noaa.gov/geomag/WMM/data/WMM2020/WMM2020_D_BoZ_MILL.pdf}}</ref> Reports of measured magnetic declination for distant locations became commonplace in the 17th century, and [[Edmond Halley|Edmund Halley]] made a map of declination for the [[Atlantic Ocean]] in 1700.<ref>{{Cite web|last=Government of Canada|first=Natural Resources Canada|title=Magnetic declination|url=https://www.geomag.nrcan.gc.ca/mag_fld/magdec-en.php|access-date=2021-09-30|website=www.geomag.nrcan.gc.ca|language=en}}</ref>

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. Similarly, [[Geomagnetic secular variation|secular]] changes to these flows result in slow changes to the field strength and direction at the same point on the Earth.

{|
|- valign = top
|[[File:World Magnetic Field Model 2025.jpg|thumb|350x350px|Level curves drawn on a declination map to denote the magnetic declination, described by signed degrees. Each level curve is an isogonic line.]]
|[[file:Magnetic Declination Chart for the International Geomagnetic Reference Field, 2005 (small).gif|thumb|350px|[[commons:file:Magnetic Declination Chart for the International Geomagnetic Reference Field, 2005.pdf|IGRF, 2005]]]]
|[[file:Mv-world.jpg|thumb|350px|NIMA Magnetic Variation Map 2000]]
|}

The magnetic declination in a given area  may (most likely will) change slowly over time, possibly as little as 2–2.5&nbsp;degrees every hundred years or so, depending on where it is measured. For a location close to the pole like [[Ivujivik]], the declination may change by 1&nbsp;degree every three years. This may be insignificant to most travellers, but can be important if using magnetic bearings from old charts or [[metes and bounds|metes]] (directions) in old deeds for locating places with any precision.

As 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.  The 1884 chart shows a variation of 8 degrees, 20 minutes West.  The 2008 chart shows 13 degrees, 15 minutes West.

{|
|- valign = top
|[[File:USCGS Chart number 361 (1884).jpg|thumb|150px|Western Long Island Sound, 1884]]
|[[File:NOAA Chart 12366 (2008).jpg|thumb|200px|Western Long Island Sound, 2008]]
|[[Image:Earth Magnetic Field Declination from 1590 to 1990.gif|right|Estimated declination contours by year, 1590 to 1990]]
|}