165:
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compass' north mark points 3° more east, deviation is +3°. Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the vessel, wristwatches, etc. The value also varies depending on the orientation of the boat. Magnets and/or iron masses can correct for deviation, so that a particular compass accurately displays magnetic bearings. More commonly, however, a correction card lists errors for the compass, which can then be compensated for arithmetically. Deviation must be added to compass bearing to obtain magnetic bearing.
155:
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212:
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polarized tip of the needle (usually painted red). The entire compass is then rotated until the magnetic needle lies within the outlined orienting arrow or box on the bottom of the capsule, and the course heading (in degrees) is displayed at the base of the direction-of-travel arrow on the baseplate. A compass thus adjusted provides a course bearing in relation to true north instead of magnetic north as long as it remains within an area on the same isogonic line.
222:
31:
360:(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reflects a highly predictable rate of change, and is usually more accurate than a map—which is likely months or years out of date. For historical data, the IGRF and GUFM models may be used. Tools for using such models include:
249:
991:: a runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°). However, due to magnetic declination, changes in runway designators have to occur at times to keep their designation in line with the runway's magnetic heading. An exception is made for runways within the
177:
333:(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,
289:(the North Star). In the 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
450:
sighting system. A floating card compass always gives bearings in relation to magnetic north and cannot be adjusted for declination. True north must be computed by adding or subtracting local magnetic declination. The example on the left demonstrates a typical conversion of a magnetic bearing from
424:
After determining local declination, a rotating dial compass may be altered to give true north readings by taping or painting a small delta-point or arrowhead on the compass baseplate west or east of magnetic north pointing to true north on the compass bezel. Other compasses of this design utilize
313:
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
124:
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
1017:
When plotting a course, some small aircraft pilots may plot a trip using true north on a sectional chart (map), then convert the true north bearings to magnetic north for in-plane navigation using the magnetic compass. These bearings are then converted on a pre-flight plan by adding or subtracting
948:
is the angle from a given magnetic bearing to the related bearing mark of the compass. Deviation is positive if a compass bearing mark (e.g., compass north) is right of the related magnetic bearing (e.g., magnetic north) and vice versa. For example, if the boat is aligned to magnetic north and the
416:
which rotates 360 degrees and is independent of the magnetic needle. To manually establish a declination for true north, the bezel is rotated until the desired number of degrees lie between the bezel's designation N (for North) and the direction (east or west) of magnetic north indicated by the
83:
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. The angle between magnetic and grid meridians is called grid magnetic angle, grid variation, or
1005:
For simplicity aviation sectional charts are drawn using true north so the entire chart need not be rotated as magnetic declination changes. Instead individual printed elements on the chart (such as VOR compass roses) are updated with each revision of the chart to reflect changes in magnetic
463:
from a true bearing to obtain a magnetic bearing. With a local declination of 14°E, a true bearing (i.e. obtained from a map) of 54° is converted to a magnetic bearing (for use in the field) by subtracting declination: 54° – 14° = 40°. If the local declination was 14°W (−14°), it is again
420:
In the image at the right, the bezel's N has been aligned with the direction indicated by the magnetic end of the compass needle, adjusted for local declination (10 degrees east of magnetic north). The direction-of-travel arrow on the baseplate thus reflects a true north heading.
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systems used for aircraft navigation also display directions in terms of magnetic north even though their intrinsic coordinate system is based on true north. This is accomplished by means of lookup tables inside the GPS which account for magnetic declination. If flying under
405:
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How to compensate for magnetic declination when reading a compass. In this example, the declination is 14°E (+14°), so the compass card points to a "north" 14 degrees to the East of true North. To obtain a true bearing, add 14 degrees to the bearing shown by the
168:
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As a fail-safe even the most advanced airliner will still have a magnetic compass in the cockpit. When onboard electronics fail, pilots can still rely on paper charts and the ancient and highly reliable device—the magnetic compass.
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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 on where it is measured. For a location close to the pole like
232:
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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
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the magnetic declination. The declination in the example is 14°E (+14°). If, instead, the declination was 14°W (−14°), you would still “add” it to the magnetic bearing to obtain the true bearing: 40°+ (−14°) = 26°.
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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.
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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
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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′
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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
501:
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Air navigation is based on magnetic directions thus it is necessary to periodically revise navigational aids to reflect the drift in magnetic declination over time. This requirement applies to
337:, 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.
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These rules are often combined with the mnemonic "West is best, East is least"; that is to say, add W declinations when going from True bearings to
Magnetic bearings, and subtract E ones.
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A magnetic compass points to magnetic north, not geographic (true) north. Compasses of the style commonly used for hiking (i.e., baseplate or protractor compass) utilize a dial or
125:
London, UK was one degree west (2014), reducing to zero as of early 2020. Reports of measured magnetic declination for distant locations became commonplace in the 17th century, and
193:, the declination may change by 1 degree every three years. This may be insignificant to most travellers, but can be important if using magnetic bearings from old charts or
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of Canada; these are numbered relative to true north because proximity to the magnetic North Pole makes the magnetic declination large and changes in it happen at a high pace.
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1002:, are also checked and updated to keep them aligned with magnetic north to allow pilots to use their magnetic compasses for accurate and reliable in-plane navigation.
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480:, the latter originating from magnetic properties of the vessel or aircraft. Variation and deviation are signed quantities. As discussed above, positive (easterly)
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an adjustable declination mechanism integrated with the compass bezel, resulting in true north readings each time the needle is aligned with the orienting arrow.
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Compasses that utilize a floating magnetized dial or card are commonly found in marine compasses and in certain models used for land navigation that feature a
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needle points, which corresponds to the direction of the Earth's magnetic field lines. True north is the direction along a meridian towards the geographic
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For example, if the compass reads 32°, the local magnetic variation is −5.5° (i.e. West) and the deviation is 0.5° (i.e. East), the true bearing will be:
496:
204:), 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.
80:
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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
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Level curves drawn on a declination map to denote the magnetic declination, described by signed degrees. Each level curve is an isogonic line.
116:, 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.
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For locations west of the agonic line (zero declination), roughly west of the
Mississippi: the magnetic bearing is always smaller.
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For locations east of the agonic line (zero declination), roughly east of the
Mississippi: the magnetic bearing is always bigger.
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274:—the points in the heavens around which the stars appear to revolve, which mark the direction of true north and true south. The
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Example of magnetic declination showing a compass needle with a "positive" (or "easterly") variation from geographic north. N
1331:
Federal
Aviation Administration Aeronautical Information Manual, Chapter 2, Section 3 Airport Marking Aids and Signs part 3b
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In most areas, the spatial variation reflects the irregularities of the flows deep in the Earth; in some areas, deposits of
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By convention, declination is positive when magnetic north is east of true north, and negative when it is to the west.
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is helpful to sight
Polaris against a reference object close to the horizon, from which its bearing can be taken.
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changes to these flows result in slow changes to the field strength and direction at the same point on the Earth.
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476:: true, magnetic, and compass bearing. Compass error is divided into two parts, namely magnetic variation and
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As an example of how variation changes over time, see the two charts of the same area (western end of
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The magnetic declination at any particular place can be measured directly by reference to the
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are designated by a number between 01 and 36, which is generally one tenth of the magnetic
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Another simple way to remember which way to apply the correction for continental USA is:
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indicates that magnetic north is east of geographic north. Likewise, positive (easterly)
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Baseplate compass adjusted for 10° east declination and a true north direction of travel
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322:(the direction of the map's north–south grid lines), which may differ from true north.
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at a particular location on the Earth's surface. The angle can change over time due to
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1192:
Fournier, Alexandre; Aubert, Julien; Lesur, Vincent; Thébault, Erwan (December 2021).
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it is acceptable to fly with an outdated GPS declination database however if flying
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To calculate compass bearing from true bearing (and known deviation and variation):
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To calculate true bearing from compass bearing (and known deviation and variation):
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subtracted from the true bearing to obtain a magnetic bearing: 54°- (−14°) = 68°.
17:
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MC = magnetic course (what the course would be in the absence of local deviation);
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D = deviation caused by magnetic material (mostly iron and steel) on the vessel;
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Online declination calculator at the
National Geophysical Data Center (NGDC)
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in the Earth's crust may contribute strongly to the declination. Similarly,
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declination. For an example refer to the sectional chart slightly west of
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may be used. (See cited page for a comparison of declination contours.)
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A Google spreadsheet application to bulk calculate magnetic declination
1286:
National
Centers for Environmental Information (NCEI) (10 March 2022).
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875:{\displaystyle T=32^{\circ }+(-5.5^{\circ })+0.5^{\circ }=27^{\circ }}
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387:. For more precise estimates, a larger crust-aware model such as the
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The approximate position of the north celestial pole is indicated by
553:{\displaystyle {\begin{aligned}T&=M+V\\M&=C+D\end{aligned}}}
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68:
Magnetic north is the direction that the north end of a magnetized
27:
Angle on the horizontal plane between magnetic north and true north
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197:(directions) in old deeds for locating places with any precision.
29:
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Looks up your IP address location and tells you your declination.
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1194:"Physics-based secular variation candidate models for the IGRF"
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the database must be updated every 28 days per FAA regulation.
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in March 2021, magnetic north is 8 degrees west of true north (
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indicates that the compass needle is east of magnetic north.
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Online declination and field strength calculator at the NGDC
230:
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Magnetic declination calculator at
Natural Resources Canada
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The general equation relating compass and true bearings is
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made a map of declination for the Atlantic Ocean in 1700.
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Magnetic declination, what it is, how to compensate.
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the local variation displayed on a sectional chart.
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Radionavigation aids located on the ground, such as
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Compass, magnetic and true bearings are related by:
235:
Estimated declination contours by year, 1590 to 1990
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Mobile web-app for magnetic declination at the NGDC
1061:) off by 15 arcminutes at the end of each forecast.
1389:Historical magnetic declination viewer at the NGDC
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318:per year. The same diagram may show the angle of
110:Magnetic declination should not be confused with
42:is magnetic north, and δ is magnetic declination.
1141:Government of Canada, Natural Resources Canada.
1125:"World Magnetic Model - Epoch 2020 -Declination"
1103:"Find the magnetic declination at your location"
977:, all of which are based on magnetic direction.
472:On aircraft or vessels there are three types of
347:Earth's magnetic field § Statistical models
927:V = variation (of the Earth's magnetic field);
370:National Oceanic and Atmospheric Administration
278:used to perform this measurement is known as a
1404:World Magnetic Model source code download site
902:Magnetic bearing - deviation = Compass bearing
888:Compass bearing + deviation = magnetic bearing
451:a floating card compass to a true bearing by
8:
314:of change may also be shown, for example in
899:True bearing - variation = Magnetic bearing
891:Magnetic bearing + variation = true bearing
1269:"World Magnetic Model - Software Download"
1081:. Paradise Cay Publications. p. 849.
1242:"Estimated Value of Magnetic Declination"
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1248:. NOAA National Geophysical Data Center
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377:National Geospatial-Intelligence Agency
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1053:This rate of change is known as the
795:for easterly Variation and Deviation
755:for westerly Variation and Deviation
375:C demo program that for WMM by the
25:
459:Conversely, local declination is
429:Floating magnetic card compasses
1267:Meyer, Brian (8 February 2022).
366:National Geophysical Data Center
180:NIMA Magnetic Variation Map 2000
1288:"Enhanced Magnetic Model (EMM)"
1012:Note the dashed line marked 8°W
225:Western Long Island Sound, 2008
215:Western Long Island Sound, 1884
843:
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395:Compass Declination Adjustment
38:is geographic or true north, N
1:
1055:geomagnetic secular variation
1008:Winston-Salem, North Carolina
788:{\displaystyle V>0,D>0}
748:{\displaystyle V<0,D<0}
1079:American Practical Navigator
1077:Bowditch, Nathaniel (2002).
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1219:10.1186/s40623-021-01507-z
1105:. Magnetic-Declination.com
993:Northern Domestic Airspace
920:Common abbreviations are:
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208:
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1364:USGS Geomagnetism Program
1198:Earth, Planets and Space
79:Somewhat more formally,
1210:2021EP&S...73..190F
595:{\displaystyle T=C+D+V}
400:Rotating dial compasses
389:Enhanced Magnetic Model
364:Web apps hosted by the
297:Determination from maps
53:) is the angle between
1419:Orientation (geometry)
1147:www.geomag.nrcan.gc.ca
1143:"Magnetic declination"
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936:CC = compass course.
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358:World Magnetic Model
266:Antique declinometer
113:magnetic inclination
47:Magnetic declination
1027:visual flight rules
975:air traffic control
649:is Magnetic bearing
341:Models and software
18:Grid magnetic angle
1336:2012-01-18 at the
971:aircraft vectoring
946:Magnetic deviation
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1292:www.ngdc.noaa.gov
1273:www.ngdc.noaa.gov
924:TC = true course;
708:{\displaystyle D}
686:{\displaystyle V}
664:{\displaystyle T}
642:{\displaystyle M}
620:{\displaystyle C}
258:Field measurement
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244:Determination
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49:(also called
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1424:Geomagnetism
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1314:. Retrieved
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1250:. Retrieved
1246:Geomagnetism
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1177:, retrieved
1173:the original
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1161:
1150:. Retrieved
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1107:. Retrieved
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95:agonic lines
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84:grivation."
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965:numbering,
1413:Categories
1252:6 December
1204:(1): 190.
1179:2010-03-03
1152:2021-09-30
1109:6 December
1041:References
468:Navigation
461:subtracted
351:Worldwide
345:See also:
320:grid north
316:arcminutes
276:instrument
169:IGRF, 2005
74:North Pole
59:true north
1316:4 January
1228:239022300
961:beacons,
941:Deviation
868:∘
855:∘
839:∘
831:−
820:∘
486:deviation
482:variation
448:prismatic
291:plumb-bob
253:eastward.
141:magnetite
100:The term
1334:Archived
444:lensatic
438:compass.
191:Ivujivik
81:Bowditch
1206:Bibcode
989:heading
985:azimuth
981:Runways
604:Where:
474:bearing
329:of the
325:On the
287:Polaris
145:secular
70:compass
1226:
1085:
967:airway
963:runway
453:adding
1429:Angle
1224:S2CID
1128:(PDF)
414:bezel
195:metes
1318:2020
1254:2013
1111:2013
1083:ISBN
1000:VORs
780:>
768:>
740:<
728:<
335:mils
306:and
134:iron
57:and
1214:doi
1031:IFR
1022:GPS
1014:).
959:VOR
851:0.5
835:5.5
446:or
139:or
137:ore
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860:=
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828:(
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809:T
783:0
777:D
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771:0
765:V
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703:D
681:V
659:T
637:M
615:C
590:V
587:+
584:D
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578:C
575:=
572:T
544:D
541:+
538:C
535:=
528:M
521:V
518:+
515:M
512:=
505:T
40:m
36:g
20:)
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