Experienced compass users will use isogonic charts, software, or measure declination (variation), and will understand the Earth's field and multiple influences on the compass.



by Chris M. Goulet, Alberta, Canada

Version 4.4, October, 2001.


Do compasses point to the north magnetic pole?
If unlike poles attract, then why doesn't the north tip of a compass point magnetic south?

Declination adjustment
  Brunton type adjustment
  Set screw adjustment
  Arithmetic compensation
  Maps with magnetic meridians
Inclination compensation for specific latitude zones

(What is the precision of a compass?)
Local magnetic anomalies
Secular change
  Where were/are/will be the magnetic poles?
Diurnal change
Solar magnetic activity
"Bermuda Triangle" type anomalies

Declination diagrams on maps
  Grid north and declination diagrams
Isogonic charts
Request for information
Global Positioning System (GPS) receivers
Direct measurement with map and compass

Composition and configuration of the crust
Dynamics of the inner Earth
Solar activity
Commercial mining
How animals use geomagnetism to navigate





    The elements iron, nickel and cobalt possess electrons in their outer electron shell, although the next inner shell is not filled. Their electron "spin" magnetic moments are not canceled, thus they are known as ferromagnetic. Iron is especially abundant in the universe, since it is the final unburnable stellar nuclear ash. These dense elements sank to the core of the molten Earth as it accreted from a nebula of exploded stars.

    Earth's core has remained molten due to heat from ongoing radioactive decay. Convection currents flowing in the outer core generate a magnetic field, but the poles of this field do not coincide with true north and south--the axis of rotation of the Earth. In mid 2002, the average position of the modeled north magnetic dip pole (according to the IGRF-2000 geomagnetic model) is 81.5° N, and 111.4° W, in the Canadian Arctic Ocean. This position is 950 kilometers (590 miles) from the true (geographic) north pole.

    The geomagnetic field can be quantified as total intensity, vertical intensity, horizontal intensity, inclination and declination. The total intensity is the magnetic strength, which ranges from about 23 microteslas (equivalent to 23000 nanoteslas or gammas, or 0.23 oersteds or gauss) around Sao Paulo, Brazil to 67 microteslas near the south magnetic pole near Antarctica. Vertical and horizontal intensity are components of the total intensity. The angle of the field relative to the level ground is the inclination, or dip, which is 90° at the north magnetic pole. Finally, the angle of the horizontal intensity with respect to the north geographic pole is the declination, also called variation in mariners' and aviators' jargon. In other words, declination is the angle between where a compass needle points and the true north pole.

    If the compass needle points west of true north, this offset is designated as west declination. The world standard, including in the southern hemisphere, is in reference to the magnetic north (MN) declination.

   In the context of astronomy or celestial navigation, declination has a different meaning. Along with right ascension, it describes the celestial coordinates of a star, etc.

Do compasses point to the north magnetic pole?

    Most people incorrectly believe that a compass needle points to the north magnetic pole. But since the Earth's field is the effect of complex convection currents in the magma, which must be described as several dipoles, each with a different intensity and orientation, the compass actually points to the sum of the effects of these dipoles at your location. In other words, it aligns itself with the magnetic lines of force. Other factors, of local and solar origin, further complicate the resulting field. It may be all right to say that a compass needle points "magnetic north" but it only roughly points to the north magnetic dip pole.

The table below compares examples of actual and incorrect declinations (using IGRF95 model for 1998.0, anomalies ignored).

Location      Lat.  Long.   Declination      Incorrect          Error   
                            (degrees)        Declination        (degrees)
                            (angle between   (degrees)                            where a compass  (angle between
                            needle points    north magnetic                            and true north   dip pole and
                            pole)            true north pole)

Sydney        34.0S 151.5E     13 E              13 E            00
Anchorage     61.5N 150.0W     23 E              20 E            03
Buenos Aires  34.5S 058.0W     06 W              09 W            03
Montreal      45.5N 073.5W     16 W              10 W            06
Los Angeles   34.0N 118.5W     14 E              03 E            11
Perth         32.0S 116.0E     02 W              09 E            11
Rio de Jan-   23.0S 043.0W     21 W              10 W            11
eiro Brazil
St. Peters-   60.0N 030.5E     08 E              12 W            20
burg, Russia
Ostrov        77.0N 148.0E     11 W              33 E            44
New Siberian

If unlike poles attract, then why doesn't the north tip of a compass point magnetic south?

    Should we be calling the north magnetic pole, the southern magnetic pole of the Earth, or should we be referring to the south magnetized needle of the compass as pointing magnetic north? Neither. A compass needle is a magnet and the north pole of any magnet is defined as the side which points magnetic north when the magnet is freely suspended; its correct title is "north seeking pole," but it has unfortunately been shortened to "north pole." Maps label the magnetic pole in the northern hemisphere as the "North Magnetic Pole".

    The cardinal points were defined long before the discovery that freely suspended magnets align to magnetic north. When some curious person placed lodestone (magnetite) on wood floating on water, or floated it directly on mercury, it was observed to align in a consistent direction, roughly pointing north. The side of the lodestone that pointed magnetic north was called, naturally, the "north pole". But that was before it was realized that like poles of magnets repel. So we must now make the distinction that the real north pole is the Earth's north magnetic pole, and the poles of all magnets that (roughly) point to it are north seeking poles.


Declination adjustment

    To perform accurate navigation, compass bearings must be adjusted to compensate for declination. The following definitions apply to the methods below.

Azimuth ring (also called azimuth dial, bezel dial, bezel ring, graduated dial, graduated ring): the ring on which the numbers and letters of directions are printed.

Capsule: (also called vial): the compartment that contains liquid and the needle.

Orienting arrow: (also called orienteering arrow): The arrow, printed on the bottom of the capsule, or on a separate disk under the capsule.

Meridian lines (also called orienting lines, north-south lines): The lines printed on the bottom of the capsule.

    Brunton type adjustment

    The capsule in low-end Brunton compasses, such as the 9020, can be rotated independently from the azimuth ring. The orienting arrow thus can be adjusted to a declination scale printed on the azimuth ring. This provides an inexpensive declination adjustment, but it makes it difficult to read a bearing from a map because the only reference that can be lined up with the map grid is the "N" and "S" or "W" and "E" on the azimuth ring, not the meridian lines.

    Set screw adjustment

    The capsule in these types of compasses, such as the Silva 15 (Ranger), Suunto MC-1, or the Nexus N15 is locked to the azimuth ring. It is rather the orienting arrow that can be rotated independently from the meridian lines by turning a small brass set screw. This screw actuates a gear under the azimuth ring, which turns a separate transparent disk on which the orienting arrow is printed. The orienting arrow thus can be adjusted to a declination scale printed on the bottom of the capsule. Using this type of adjustment, it is much easier to read a bearing from a map, because the meridian lines can be lined up with the map grid.

    Arithmetic compensation

    Users who have graduated from elementary school can use a compass without a declination adjustment feature by adding or subtracting their declination. From map to terrain: "declination west, turn dial west" (counterclockwise: add); "declination east, turn dial east" (clockwise: subtract). From terrain to map: vice versa. If you are afraid to forget, scribe "from map: decl W, turn W" with a sharp instrument on the baseplate or under the cover. The 0° declination (agonic) line passes west of Hudson's Bay, Lake Superior, Lake Michigan, and Florida, and visualizing it relative to your location helps to make the proper correction. Some compasses, such as the Silva 7 (Polaris), include a declination scale although they have no declination adjustment. The scale, printed on the bottom of the capsule, helps to remember which way to make the manual compensation.

    Maps with magnetic meridians

Another technique for dealing with declination is used in the sport of competitive orienteering. The meridians on all orienteering maps are drawn to magnetic north, not true north. The declination adjustment is done at the time the map is drawn, rather than during navigation. Magnetic meridians are considered straight and parallel within the confines of a given o-map. This technique wouldn't be suited for long-distance navigation, but orienteers are dealing with a few hundred meters to a few kilometers of distance, on typically 1:10,000 or 1:15,000 scale maps.

Inclination compensation for specific latitude zones

    Most compasses are compensated for magnetic inclination or dip by a counterweight on one end of the needle, to prevent it from dragging on the top or bottom of the capsule. Manufacturers make versions of compasses compensated for several magnetic latitude zones. For example, Silva offers:

MN    (Magnetic North) On this version of the Silva 7NL (Polaris), the south-pointing end of the needle is 17 millimeters (0.67") long and the north-pointing end is 16 millimeters (0.63").

NME   (North Magnetic Equatorial)

ME    (Magnetic Equatorial)

SME   (South Magnetic Equatorial)

MS    (Magnetic South)

Flinders in Australia features a map of these magnetic latitude zones. Observe that these zones only vaguely correspond to geographic latitude. The magnetic equator (where dip=0°) ranges from 12° N around Burkina Faso, Africa to 13° S around Cuzco, Peru.

Inclination also can be compensated by:

    -holding the compass at an angle (if using one compensated for another zone than where you are located). The mirror of a sighting compass, however, cannot be used at an angle relative to the horizon. I did not know about this effect, but noticed no problem on a Silva 7NL version MN used at 33° south latitude in Chile.

    -a needle design featuring a low center of gravity. The Silva 7NL has a washer attached to the underside of the needle hub.

    -a needle design that allows a cylindrical magnet to rotate and pivot on a jewel, while the needle pivots on the magnet so it stays horizontal.

Suunto Finland offers global compass models MC-2G Global Navigator, available at The Compass Store, USA.

Recta Swiss compasses with the Turbo-20 needle system, such as the DS-55, DS-56, DP-65, DO315, DO715, and DO765, are also global compasses, and are available at Perret Opticiens, Switzerland.

    -a "deep well" design such as used on American GI-issue military lensatic compass.

    -for extreme dip, as encountered 500 to 1000 kilometers (300-600 miles) from magnetic poles, an electronic fluxgate compass may be required, such as the Autohelm "personal compass", the Casio digital compass watch ATC1200-1V, or the Brunton Outback Electronic Compass.


(What is the precision of a compass?)


(one to thousands of kilometers/degree)

    Each position on the Earth has a particular declination. The change in its value as one travels is a complex function. If the navigator happens to be traveling along a rather straight line of equal declination, called an isogonic line, it can vary very little over thousands of kilometers. However; for one crossing isogonic lines at high latitudes, or near magnetic anomalies, the declination can change at over a degree per kilometer (6/10 mile). Navigators need periodically update the value to stay on course.

Local magnetic anomalies

(0-90 degrees; 3-4 degrees frequently)

    Predictive geomagnetic models such as the World Magnetic Model (WMM) and the International Geomagnetic Reference Field (IGRF) only predict the values of that portion of the field originating in the deep outer core. In this respect, they are accurate to within one degree for five years into the future, after which they need to be updated. The Definitive Geomagnetic Reference Field (DGRF) model describes how the field actually behaved.

    Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.

    Anomalous declination is the difference between the declination caused by the Earth's outer core and the declination at the surface. It is illustrated on 1:126,720 scale Canadian topographic maps published in the 1950's, which included a small inset isogonic map. On this series, it is common to observe a four-degree declination change over 10 kilometers (6 miles), clearly showing local anomalies. There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions.

A few areas with magnetic anomalies (there are thousands more):

    -North of Kingston, Ontario; 90° of anomalous declination.

    -Kingston Harbor, Ontario; 16.3° W to 15.5° E of anomalous declination over two kilometers (1.2 miles); magnetite and ilmenite deposits.

    -Near Timmins, Ontario, W of Porcupine.

    -Savoff, Ontario (50.0 N, 85.0 W). Over 60° of anomalous declination.

    -Michipicoten Island in Lake Superior (47.7 N, 85.8 W); iron deposits.

    -Near the summit of Mt. Hale, New Hampshire (one of the 4000-footers, near the Zealand Falls hut on the Appalachian Trail) ; old AMC Guides to the White Mountains used to warn against it.

    -Around Georgian Bay of Lake Huron.

    -Ramapo Mountains, northeastern New Jersey; iron ore; compass rendered useless in some areas.

    -Near Grants, New Mexico north of the Gila Wilderness area; Malpais lava flows; compass rendered useless.

The USGS declination chart of the USA (GP-1002-D) shows over a hundred anomalies. The following table lists the most extreme cases.

Anomalous declination(degrees) Lat.  Long.  Location                
                              46.4 W                         40.2  106.2  75 km.(45 mi.) W Boulder, Colorado
24.2 E                         40.7  75.3   20 km. (12 mi.) NE Allentown, Pennsylvania
16.6 E to 12.0 W over 10km(6mi)46.7  95.4   250 km. (150 mi.) NW Minneapolis, Minnesota
14.8 E                         33.9  92.4   85 km. (50 mi.) S Little Rock, Arkansas
14.2 E                         45.5  82.7   In Lake Huron, Ontario
13.8 W                         45.7  87.1   Escanaba, on shore of Lake Michigan
13.7 E                         48.4  86.6   In Lake Superior, Ontario
13.5 E                         48.5  122.5  80 km. (50 mi.) N Seattle, Washington
13.0 W                         42.2  118.4  In Alvord Desert, Oregon
12.2 W                         38.9  104.9  10 km. (6 mi.) W Colorado Springs, Colorado
11.5 E                         47.8  92.3   120 km. (75 mi.) N Duluth, Minnesota
    In 1994, the average location of the north magnetic dip pole was located in the field by the Geological Survey of Canada. This surveyed north magnetic dip pole was at 78.3° N, 104.0° W, and takes local anomalies into consideration. However; the DGRF-90 modeled magnetic dip pole for 1994 was at 78.7° N, 104.7° W. The 47-kilometer (29 mile) difference illustrates the extent of the anomalous influence. In addition to surveyed dip poles and modeled dip poles, a simplification of the field yields geomagnetic dipole poles, which are where the poles would be if the field was a simple Earth-centered dipole. Solar-terrestrial and magnetospheric scientists use these. In reality, the field is the sum of several dipoles, each with a different orientation and intensity.

    Distortion caused by cultural features is called deviation, and distortion in a vehicle in which a compass is mounted is also called binnacle error. Some compasses can be calibrated to compensate for binnacle error.


(negligible to 2 degrees)

    This factor is normally negligible. According to the IGRF, a 20,000 meter (66,000 foot) climb even at a magnetically precarious location as Resolute, 500 kilometers (300 miles) from the north magnetic pole, would result in a two-degree reduction in declination.

Secular change

(2-25 years/degree)

    Where were/are/will be the magnetic poles?

    As convection currents churn in apparent chaos in the Earth's core, all magnetic values change erratically over the years. The north magnetic pole has wandered over 1000 kilometers (600 miles) since Sir John Ross first reached it in 1831, as shown on this map at SARBC (extend the path to north west of Ellef Ringes Island for 1999), or this map at USGS. Its rate of displacement has been accelerating in recent years and in mid-2002, it is moving about 39 kilometers (24 miles) per year, which is several times faster than the average of 6 kilometers (4 miles) per year since 1831. The magnetic pole positions can be determined more precisely by using a calculator that returns magnetic inclination. Latitudes and longitudes can be entered by trial and error, until the inclination (I) is as close as possible to 90°.

    A given value of declination is only accurate for as long as it stays within the precision of the compass, preferably one degree. Typical secular change or variation (do not confuse with mariners' and aviators' variation) is 2-25 years per degree. A map that states: "annual change increasing 1.0' " would suggest 60 years per degree, but that rate of change just happened to be slow on the year of measurement, and will more than likely accelerate.

    The field has even completely collapsed and reversed innumerable times, which have been recorded in the magnetic alignment of lava as it cooled. One theory to explain magnetic pole reversals is related to large meteorite impacts, which could trigger ice ages. The movement of water from the oceans to high latitudes would accelerate the rotation of the Earth, which would disrupt magmatic convection cells into chaos. These may reverse when a new pattern is established. Another theory is that the reversals are triggered by a slight change the angular momentum of the earth as a direct result of the impacts. These theories are challenged by the controversial Reversing Earth Theory, which proposes that the entire crust could shift and reverse the true poles in a matter of days, but that the molten core would remain stationary, resulting in apparent magnetic reversal. The Sun would then rise in the opposite direction.

Diurnal change

(negligible to 9 degrees)

    The stream of ionized particles and electrons emanating from the Sun, known as solar wind, distorts Earth's magnetic field. As it rotates, any location will be subject alternately to the lee side, then the windward side of this stream of charged particles. This has the effect of moving the magnetic poles around an ellipse several tens of kilometers in diameter, even during periods of steady solar wind without gusts. The Geological Survey of Canada shows a map of this daily wander or diurnal motion in 1994.

    The resulting diurnal change in declination is negligible at tropical and temperate latitudes. For example, Ottawa is subject to plus or minus 0.1 degree of distortion. However; in Resolute, 500 kilometers (300 miles) from the north magnetic pole, the diurnal change cycles through at least plus or minus nine degrees of declination error. This error could conceivably be corrected, but both the time of day and the date would have to be considered, as this effect also varies with seasons.

Solar magnetic activity

(negligible to wild)

    The solar wind varies throughout an 11-year sunspot cycle, which itself varies from one cycle to the next. In periods of high solar magnetic activity, bursts of X-rays and charged particles are projected chaotically into space, which creates gusts of solar wind. These magnetic storms will interfere with radio and electric services, and will produce dazzling spectacles of auroras. The varied colors are caused by oxygen and nitrogen being ionized, and then recapturing electrons at altitudes ranging from 100 to 1000 kilometers (60 to 600 miles). The term "geomagnetic storm" refers to the effect of a solar magnetic storm on the Earth (geo means Earth).
This RealMedia documentary from the American Institute of Physics discusses geomagnetic storms.
This MPEG movie at the National (USA) Aeronautics and Space Administration shows a coronal mass ejection leaving the Sun and traveling towards Earth.
This animated GIF at the University of Michigan shows the response of the magnetosphere to the changing solar wind.

    The influence of solar magnetic activity on the compass can best be described as a probability. The chance that the declination will be deflected by two degrees in southern Canada over the entire 11-year cycle is 1% per day. This implies about four disturbed days per year, but in practice these days tend to be clustered in years of solar maxima. These probabilities drop off rapidly at lower latitudes. During severe magnetic storms, compass needles at high latitudes have been observed swinging wildly.

"Bermuda Triangle" type anomalies

(very rare)

    Legends of compasses spinning wildly in this area of the Atlantic, before sinking a ship, or blowing up an airplane, may be related to huge pockets of natural gas suddenly escaping from the ocean floor. As the gas bubbles up, it could induce a static charge or could ionize the gas, which would create erratic magnetic fields. The gas would cause a ship to lose buoyancy, or a plane flying through a rising pocket of natural gas could ignite it. The ionized gas may show as an eerie green glow at night. It could make people feel light headed and confused because the gas replaces the air, but it would not have the mercaptans that gas companies add to to gas to give it its distinctive odor.

    At enormous pressures and low temperatures (as at the bottom of the sea), water and gas molecules form gas hydrates. These compounds resemble ice but, unlike ordinary ice, the water molecules form cages that trap gas molecules such as methane. The solid hydrates retain their stability until conditions, such as higher temperatures or lower pressures, cause them to decompose. The gas may remain trapped under silt, until an earthquake triggers a release.

    This phenomenon is not restricted to the "Bermuda Triangle". The insurance statistics at the Lloyds of London have not revealed an unusual  number of sunken ships in the triangle.


Declination diagrams on maps

    Most topographic maps include a small diagram with three arrows: magnetic north, true north and Universal Transverse Mercator grid north. The given value of declination, corresponding to the center of the map, does not take local anomalies into account. The value is usually out of date, since it may have drifted several degrees due to secular change, especially on maps of remote regions with several decades between updates. Some maps, such as the 1:50,000 scale topographic maps by the Canadian Department of Energy, Mines and Resources include the rate of annual change, which is useful for predicting declination, but that rate of change is erratic and reliability of the forecast decreases with time. A rate of change over five years old is unreliable for one-degree precision. The United States Geological Survey's 1:24,000 scale maps do not even mention annual change.

For example, the approximate mean declination 1969 on the Trout River, Newfoundland map was 28° 33' west with annual change decreasing 3.0'. This implies a recent (1997) value of:

   28° 33'                            - ((1997-1969) * 3.0)
= 27° 93' (borrowed 60')   -               84'
= 27° 09'

but IGRF 1995 for 1997 yields 23° 44', which is 3° 25' less, showing that the 28-year prediction was in significant error.

    Grid north and declination diagrams

    (negligible to 2 degrees)

    Grid north is the direction of the north-south lines of the Universal Transverse Mercator (UTM) grid, imposed on topographic maps by the United States and NATO armed forces. UTM Provides a constant distance relationship anywhere on the map. In angular coordinate systems like latitude and longitude, the distance covered by a degree of longitude differs as you move towards the poles and only equals the distance covered by a degree of latitude at the equator. With the advent of inexpensive GPS receivers, many other map users are adopting the UTM grid system for coordinates that are simpler to use than latitude and longitude.

    The problem with grid north is that is coincident with true north only at the center line of each UTM zone, known as central meridians. The difference between grid north and true north can be up to three degrees. This might not be so bad if it were not for the different conventions with respect to declination diagrams adopted by different countries. A declination diagram on a topographic Canadian map or an Australian map shows magnetic north with respect to grid north, but a US map shows magnetic north with respect to true north. Therefore, if you use declination from a Canadian/Australian style declination diagram, be sure to take bearings to and from the map by making the meridian lines on the compass parallel with the UTM grid (grid north). However, if you use declination from a USGS style declination diagram or any of the other sources below, you must make the meridian lines on the compass parallel with the edges of the map (true north). Canadian maps show a blue fine-lined UTM grid, while some USGS 1:24,000 scale maps show black grid lines, but the others only show blue grid tick marks on the map margins. The choice of grid lines or tick marks on the US maps seems inconsistent by year or by region.

Printed Isogonic charts

    Isogonic or declination charts are plots of equal magnetic declination on a map, yielding its value by visually situating a location, and interpolating between isogonic lines. Some isogonic charts include lines of annual change in the magnetic declination (also called isoporic lines). Again, the older, the less valid. The world charts illustrate the complexity of the field.

    A Brunton 9020 compass included a 1985 isogonic chart of North America, on a sheet copyrighted in 1992. You would expect a leading manufacturer of compasses to be a little more up to date!

    The 1:1,000,000 scale series of World Aeronautical Charts include isogonic lines.

    Hydrographic charts include known magnetic anomalies.

    The McGraw-Hill Encyclopedia of Science and Technology (1992 edition) provides a small world chart under "geomagnetism."

    The best is the 1:39,000,000 Magnetic Variation chart of "The Earth's Magnetic Field" series published by the Defense Mapping Agency (USA). The 11th edition is based on magnetic epoch 1995.0 and includes lines of annual change and country borders. Ask for Geophysical Data Chart stock No. 42 (DMA stock No. WOBZC42) at a National (USA) Ocean Service navigation chart sales agent or order from the Office of Coast Survey, about US$10. Size: 1.26 X 0.9 meters (50" X 35"). It covers from 84° N to 70° S. North and south polar areas are on Geophysical Data Chart stock No. 43 (DMA stock No. WOBZC43).

    European marine chart distributors may have better availability for the 1:45,000,000 scale "The World Magnetic Variation 1995 and Annual Rates of Change" chart published by the British Geological Survey. However; it lacks country borders. Ask for No. 5374, about US$16.

    A 1:48,000,000 world declination chart of "The Magnetic Field of Earth" series is published by the United States Geological Survey's Earth Sciences Information Center. However; the most recent edition is still based on magnetic epoch 1990.0. It does include lines of annual change and country borders. Look it up at a university map library or order GP-1004-D from the United States Geological Survey. Only US$4.00 (+ US$3.50 for shipping and handling). Size 1.22 X 0.86 meters (48" X 34"). Includes polar regions at 1:68,000,000 scale.
A United States declination chart is also published. Scale 1:5,000,000 (Alaska and Hawaii 1:3,500,000), epoch 1990.0, GP-1002-D, US$4.00 + US$3.50 S&H, 1.14 X 0.8 meters (45" X 34"), includes over 100 magnetic anomalies.

On-line Isogonic charts

North America 1990, Others 1995: South America, Europe, Middle East, Southeast Asia, Australia/New Zealand, Global: Ricardo's Geo-Orbit Quick Look satellite dish site

World, small: United States Geological Survey

World, larger, color, 1995: National (USA) Geophysical Data Center

World, slightly more readable, 1995: National (USA) Geophysical Data Center

World, black and white, 2000, seven magnetic parameters, including polar projections: Kyoto University in Japan

World, color, 2000, seven magnetic parameters and their rates of secular change, click to zoom. USA Department of Defense

Canada, CGRF95: Geological Survey of Canada

Canada, more detailed (caution: outdated--1985): Search and Rescue Society of British Columbia

United States, 1995, small, three magnetic parameters (note: longitudes are in 360° format): United States Geological Survey

Mexico, IGRF95: Instituto de Geofísica, Universidad Nacional Autónoma de México. The blue lines are declination, and the red lines are annual change.

Australia, AGRF2000: Australian Geological Survey Organization (AGSO)

Finland, 2000: Finnish Meteorological Institute. It has wavy isogones in an attempt to include magnetic anomalies from the Earth's crust.

Generate your own: Kimmo Korhonen at the Helsinki University of Technology, Finland wrote this Java applet in which you specify a region and date. Great idea, but the maps lack detail.

Request for information

    National geological surveys or geographical institutes, local surveyors, and airport control towers should provide the current declination, given your location in latitude and longitude, or just the name of a town. For the National (USA) Geophysical Data Center, call (303) 497-6125.

On-line and downloadable declination data

Use an atlas to find your latitude and longitude before you can use the links below.

    Pangolin in New Zealand features a Java applet that continuously returns magnetic variation as the pointer is moved over a map of the world. Sorry, no zooms available, but it computes great circle bearings and distances.
http://www.pangolin.co.nz/downloads/magv_su.exe (download applet)

    Geological Survey of Canada: declination

    National (USA) Geophysical Data Center: seven magnetic parameters and their rates of secular change.

    Interpex Limited: GEOMAGIX freeware can be downloaded.

    Defense Mapping Agency: GEOMAG freeware can be downloaded.

    Ed William's Aviation page: Geomagnetic Field and Variation Calculator freeware can be downloaded in Mac, Linux, and DOS versions and are suitable for batch processing.

    CBU Software: MAGDEC shareware (30-day trial) provides a plot of declination vs. years, latitude or longitude and will transform bearings from one year to another. It covers USA only, from 1862 to present.

Global Positioning System (GPS) receivers

    Most GPS receivers have internal data and an algorithm to compute the declination after the position is established. For example, the Garmin GPS-45 displays the value on the Navigation Setup page, Heading: "Auto Mag," and uses it to supply magnetic bearings. However; this data cannot be updated from satellite transmission, therefore it is subject to become outdated.

When asked about this issue for the 12XL, Garmin responded:

"The magnetic variation model used within our products is a series of tables derived from the NOAA IGRF '90 model, which accounts for the movement of the magnetic poles over time.

Best regards,

Bill Stone
Product Support Manager"

To test this claim, I have done comparisons between the IGRF90 and IGRF95 models, and the Garmin GPS 45 ver. 2.43 (1995). I have come to the conclusion that the declination/variation values displayed on the GPS 45 are from the IGRF90 model that has been extrapolated to 1995. However, these declinations are in fact static and therefore are subject to become outdated. The unit has a provision to enter an updated value in "User Mag."

The table below shows data I used to arrive at this conclusion, and how the Garmin GPS 45/12XL declination error will increase in the future.

                                  IGRF  IGRF  Garmin  IGRF  IGRF  IGRF  IGRF
                                  90    95    GPS 45  95    95    95    95

Year..............................1990  1995  1998    1998  2000  2005  2010

Yellow-  | Declination            28.2E 26.9E 27 E    26.0E 25.4E 23.9E 22.4E
knife    | Garmin declin. error     -     -     -      1.0   1.6   3.1   4.6
NWT      | Annual rate of change  16.4W 18.3W   -     18.1W 18.0W 17.6W 17.3W
Canada   | (minutes of a degree)
63N 114W |
Glasgow, | Declination            07.4W 06.8W 07 W    06.4W 06.1W 05.4W 04.8W
Strath-  | Garmin declin. error     -     -     -      0.6   0.9   1.6   2.2
clyde    | Annual rate of change  07.9E 07.9E   -     07.9E 07.9E 07.9E 07.8E
Scotland | (minutes of a degree)
56N 004W |
Miami    | Declination            03.8W 04.5W 05 W    04.9W 05.2W 05.9W 06.6W
Florida  | Garmin declin. error     -     -     -      0.1   0.2   0.9   1.6
USA      | Annual rate of change  08.0W 08.5W   -     08.5W 08.6W 08.6W 08.6W
26N 080W | (minutes of a degree)
Manaus   | Declination            12.1W 12.8W 13 W    13.3W 13.7W 14.5W 15.4W
Amazonas | Garmin declin. error     -     -     -      0.3   0.7   1.5   2.4
Brazil   | Annual rate of change  08.7W 10.1W   -     10.2W 10.3W 10.4W 10.6W
03S 060W | (minutes of a degree)
The fact that the Garmin GPS 45 displays 27E instead of 26E for Yellowknife in 1998 clearly shows that it is using 1995 declination (26.9E), not 1998 declination (26.0E). The Glasgow data agree, although marginally. The Miami and Manaus data cannot distinguish, but show that even in some tropical locations, the declination values will be around 2 degrees in error around 2010. In Yellowknife the error will then approach 5 degrees, and greater for locations closer to the magnetic poles.

Of course by that time, there will be a whole lot of other reasons to get a new GPS receiver, at Walkman prices :-)

Direct measurement with map and compass

    Suppose you are using an old, foreign map and it gives no clue of declination. You didn't bother to pack an isogonic chart, you don't have a GPS (or its batteries died), and you don't happen to have a laptop with a satellite internet link or even GEOMAG software in your backpack? No problem.

    When your position is known, take a magnetic bearing to a landmark that is both visible on the terrain and represented on the map. Distinct and distant landmarks, such as a sharp mountain peak, a transmission tower, one edge of a lake or a river bend can be used. Next, measure the true bearing on the map using your compass as a protractor. The difference is simply the declination. To increase confidence, take bearings on different landmarks and average the declination results. If only one landmark is recognizable, take a few bearings on it, walking a few meters between readings, and average them before figuring the declination. If there are no landmarks on your terrain, but you can see Polaris, the North Star, you can use it as a bearing of 0°. Actually, its trace is a circle, currently 0.75° in radius around the north celestial pole (NCP), so the worst-case error would be that value when it is directly east or west of the celestial pole. It is directly north on July 1st around 9:00AM and 9:00PM Daylight Savings Time, and two hours earlier for each later month. At high latitudes, where the NCP is high, it is necessary to use a plumb-bob (a weight attached to a string) at arm's length and position yourself to align Polaris with a reference object at least 20 meters (70 feet) away, then take a bearing on the object. Use the plumb-bob in the least windy conditions as possible.

    Directly measured declination cannot be more up to date, and includes all anomalies. I am amazed that this very handy technique is described in very few hiking or orienteering books. David Seidman's "The Essential Wilderness Navigator" does include it.


    A declinometer/inclinometer is sophisticated instrument makes precision measurements of declination and inclination. It is used to calibrate compasses or to periodically calibrate continuously recording variometers in magnetic observatories. The angle at which its electronic fluxgate magnetometer reads a minimum value, is compared to a sighting through its optical theodolite. True north is determined by sighting a true north reference target mounted some distance away, or is derived from celestial navigation calculations on a sighting of the sun or another star.


Composition and configuration of the crust

    The relief of metamorphic or igneous terrain buried under kilometers of sediments can be mapped from magnetic anomalies, exploiting the knowledge that sedimentary rocks are generally non-magnetic. Paleomagnetism gives clues to the past rate and direction of continental drift.

Dynamics of the inner Earth

    The configuration of the field and its secular change, along with paleomagnetic data, builds our understanding of the colossal forces at work in the deep Earth.

Solar activity

    Magnetic observation of solar events is one basis for the formulation of theories of solar processes. The observations can be used to predict disruptions to radio communications and electric power grids, or to forecast exquisite aurora.

Commercial mining

    Magnetic anomalies betray ferromagnetic ores such as iron, nickel and cobalt; or diamond deposits associated with kimberlite minerals; as well as precious metals. The anomalies can be either stronger or weaker than the average magnetic strength of an area, and are usually found by examining magnetic surveys taken from airplanes.

How animals use geomagnetism to navigate

    Many animals, such as whales, dolphins, tuna, salmon, honeybees, pigeons, and sea turtles, navigate with the help of geomagnetism. They possess magnetite crystals in contact with neurons in their brains, which sense the inclination and the intensity of the field. Since these magnetic parameters vary with latitude and longitude, the animals can sense their position.


    I bought my compass while preparing a peddleboat expedition on Lake Huron in 1986. I was paranoid that fog would roll in while in the middle of a *suicidal* windy crossing to a distant island. Had we missed it and could not backtrack against the wind, we would have found ourselves a hundred kilometers (sixty miles) from the opposite shore. The problem turned out not to be visibility, but waves that kept splashing into the boat! My trusty compass has been guiding me ever since: trails that fizzle out on Mount Elbert, Colorado, getting back to the right path in the White Mountains of New Hampshire, finding my way through the jungle of Mexico City, or choosing between unmarked tracks that weave between volcanos, while cycling on the Bolivian Altiplano.

    I devised the technique of direct measurement of declination out of sheer necessity, while preparing to navigate a 140-kilometer (85-mile) bicycle crossing of Bolivia's Salar de Uyuni. None of my maps gave any clue of the declination. It was an exhilarating experience riding on the blinding white pristine surface of this world's largest salt flat at a lofty altitude of 3653 meters (11,985 feet). And yes...I finally did navigate through fog, while kayaking on Penobscot Bay off the coast of Maine.

    While bushwhacking up remote Mount Babel, the central peak at ground zero of the Manicouagan Crater in Quebec, I was paranoid of the compass because of a report of severe anomalies in the nearby Monts Groulx. This was plausible, considering that the seven-kilometer (four-mile)-wide hunk of rock that rammed into the Earth at this spot, was possibly loaded with iron and nickel. Refusing to use the compass, we slogged through thick bush along streams that fizzled out, and never found the summit. The next day, I took my chances and the compass worked perfectly after all! This episode compelled me to study anomalies.

Link to references and acknowledgments.

Questions, comments, corrections, and additions are welcome. Please E-Mail me in French or English at: chris_goulet@yahoo.ca.

Copyright 1997-2001 by Chris M. Goulet. (reprinted with permission)

Updates of this FAQ will be posted at Geocities:

Disclaimer (Lawyer Repellent): Permission is hereby granted to apply the information in this document on the condition that be author not be held responsible nor liable for any damages.

Last updated: 10 October, 2001

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