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 un-burnable 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 modelled north magnetic dip pole (according to the IGRF-95 geomagnetic
model) is 79.5° N, and 106.3° W, 40 kilometres northwest of Ellef Ringnes
Island in the Canadian Arctic. This position is 1170 kilometres 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?
To perform accurate navigation, compass
bearings must be adjusted to compensate for declination. The procedure
varies from compass to compass.
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 kilometres of distance, on typically 1:10,000
or 1:15,000 scale maps.
Inclination compensation for specific
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 centre
-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 kilometres from magnetic poles, an electronic
flux-gate compass may be required
WHAT FACTORS INFLUENCE DECLINATION?
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 kilometres. However; for one crossing
isogonic lines at high latitudes, or near magnetic anomalies, the
declination can change at over a degree per kilometre. 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 harbouring 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
travelling 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
modelled 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 modelled 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-centred 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.
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.
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 kilometres
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 kilometres per year, which is several times
faster than the average of 6 kilometres per year since 1831.
Where were/are/will be the magnetic
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 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 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
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.
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 kilometres 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
colours are caused by oxygen and nitrogen being ionized, and then
recapturing electrons at altitudes ranging from 100 to 1000 kilometres.
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 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 centre 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
Defence 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.
Geological Survey of Canada:
National (USA) Geophysical Data Center:
seven magnetic parameters and their rates of secular change.
United States Geological Survey Branch
of Earthquake and Geomagnetic Information: seven magnetic parameters and
their rates of secular change.
"QED", ctrl-z to exit)
Global Positioning System (GPS)
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 metres 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.
WHAT CAN BE LEARNED FROM MEASURING GEOMAGNETISM?
Composition and configuration of the
The relief of metamorphic or igneous
terrain buried under kilometres 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
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.
Magnetic observation data of solar
events is one basis for the formulation of theories of solar processes.
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.