magnetic declination

Magnetic Declination

Abridged from Chris M. Goulet, Montreal, Canada
 

What is magnetic declination?
How do I compensate for declination and inclination?
What factors influence declination?
How do I determine declination?
What can be learned from measured geomagnetism?



WHAT IS MAGNETIC DECLINATION?


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 cancelled, 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 early 1998, the average position of the modeled north magnetic dip pole (according to the IGRF-95 geomagnetic model) is 79.5° N, and 106.3° W, 40 kilometers northwest of Ellef Ringnes Island in the Canadian Arctic. This position is 1170 kilometers 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 common terms, declination is the angle between where a compass needle points and the true north 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.

 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.


HOW DO I COMPENSATE FOR DECLINATION AND INCLINATION?


 Declination adjustment

To perform accurate navigation, compass bearings must be adjusted to compensate for declination. The procedure varies from compass to compass.

 Arithmetic compensation

 Users who have graduated from primary 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.

 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. 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.

-a needle design featuring a low center of gravity.

-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.

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

 -for extreme dip, as encountered 500 to 1000 kilometers from magnetic poles, an electronic

flux-gate compass may be required


WHAT FACTORS INFLUENCE DECLINATION?


 Location

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 travelling 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. Navigators need periodically update the value to stay on course.

 Local magnetic anomalies

 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.

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. anomalies. Anomalous declination is the difference between the declination caused by the Earth's outer core and the declination at the surface.

 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 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 a vehicle in which a compass is mounted is called deviation or binnacle error, and some compasses can be calibrated to compensate for it. Pangolin in New Zealand discusses deviation.

 Altitude

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

 Secular change

 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 since Sir John Ross first reached it in 1831, as shown on this map at SARBC (extend the path to the northern tip of Ellef Ringes Island for 1998). Its rate of displacement has been accelerating in recent years and is currently moving about 24 kilometers per year, which is several times faster than the average of 6 kilometers per year since 1831.

 Where were/are/will be the magnetic poles?

 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 beenrecorded in the magnetic alignment of lava as it cooled. One theory is that large meteorite impacts 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 reversewhen 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

 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. This daily variation, or diurnal change, is negligible at tropical and temperate latitudes.

 Solar magnetic activity

 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. The term "geomagnetic storm" refers to the effect of a solar magnetic storm on the Earth (geo means Earth).

 The influence of solar magnetic activity on the compass can best be described as a probability. During severe magnetic storms, compass needles at high latitudes have been observed swinging wildly.

"Bermuda Triangle" type anomalies

 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 bubbles of natural gas suddenly escaping from the ocean floor. If the gas were ionized, erratic magnetic fields would be induced. The gas would cause a ship to lose buoyancy, or a plane flying through a rising pocket of natural gas could ignite it.


HOW DO I DETERMINE THE DECLINATION?


 The following methods require that you have a general idea of your position
 before you can determine your declination.

 Declination diagrams on maps

 Most topographic maps include a small diagram with three arrows: magnetic north, true north and Universal Transverse Mercator grid north. It is not clear whether the given value of declination, corresponding to the center of the map, takes local anomalies into account or not.

Some maps, such as the 1:50,000 scale topographic maps 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.

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.

 The 1:1,000,000 scale series of World Aeronautical Charts include isogonic lines.
 Hydrographic charts include known magnetic anomalies.

 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.

 A chart of Australia from AGRF95 for 1997.5: Australian Geological Survey Organisation (AGSO)

 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.webworkshop.com/pangolin/magvar.html

 Geological Survey of Canada: declination

http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html

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

http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl

 United States Geological Survey Branch of Earthquake and Geomagnetic Information: seven magnetic parameters and their rates of secular change.

telnet://neis.cr.usgs.gov (username: "QED", ctrl-z to exit)

 Global Positioning System (GPS) receivers

 Most GPS receivers have internal data and an algorithm to compute the declination after the position is established. However; this data cannot be updated from satellite transmission, therefore it is subject to become outdated.

 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 recognizable landmark. 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, and 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 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.


WHAT CAN BE LEARNED FROM MEASURING GEOMAGNETISM?

                                                                                                                                   
 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 data of solar events is one basis for the formulation of theories of solar processes.

 Commercial Mining

Magnetic anomalies betray ferromagnetic ores such as iron, nickel and cobalt; or diamond deposits associated with kimberlite minerals (magnesium rich ilmenite, olivine, chrome diopside and pyrope garnets); as well as precious metals.

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