Magnetic declination
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The magnetic declination (or magnetic variation) at any point on the earth is a property of the geomagnetic field defined as the angle that must be added or subtracted in converting between two kinds of directional information:
- the direction of the needle on a magnetic compass located there, and
- the direction of the earth's lines of longitude.
For points in the Northern Hemisphere, these are usually described as magnetic north and true north respectively. (In the Southern Hemisphere, visualizing the underlying physics and the practical calculations would be clearer with magnetic south and true south substituted.)
The agonic lines are the imaginary lines on the Earth surface along which the compass needle points to the geographic north and south, i.e. the magnetic declination is zero.
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"True" directions
True north and south are of course the local directions to the respective geographic poles. (More precisely, these are horizontal directions, along great circles, toward the poles; the real directions to the poles, along straight lines, point into the ground at angles to the earth's surface.) The geographic poles are defined by astronomical observations, and reflect the rotation of the earth (experienced roughly as the progress of day and night): the earth's axis is the line connecting the geographic poles, and every other point on the earth's surface traces, roughly daily, a circle whose center lies on that axis.
Where compasses point
Magnetic north and south, on the other hand, are widely misunderstood. The statement is often made that magnetic north is the direction to the North Magnetic Pole. This is, in most places, fairly close to being true. Of course, as with geographic poles, the direction of interest is almost always a horizontal direction. But the horizontal direction in question is that of the needle of a good compass, which nearly always differs measurably from the horizontal direction to the nearer magnetic pole.
The popular idea of "a huge body of magnetized material inside the earth" (and the picture of it as a symmetrical body) encourages this picture. In fact, flows of electrical charges in molten minerals produce the magnetic field. Any overall pattern of flow is secondary to flows that are largely vertical rather than horizontal. These local flows coordinate to a substantial extent so that there is an overall roughly north-south magnetic field, but the nearest flows contribute most strongly to the field sensed by a compass at the earth's surface; the horizontal component of these fields is usually near to the (horizontal) direction from one magnetic pole to another, but with some deviation to left or right. (What is true in the popular picture is that following a compass will eventually lead to a magnetic pole, but following a slightly meandering path.)
Theory
Change of declination in time and space
Magnetic declination varies both from place to place, and with the passage of time.
In most areas, the spatial variation reflects the irregularities of the flows deep in the earth; in some areas, deposits of iron ore or magnetite in the earth's crust may contribute strongly to the declination.
The time variation reflects changes in the deep flows: a flow becoming stronger or weaker, changing direction, or shifting its location. In each case, such a change is likely to contribute to a shift in the location of at least one of the magnetic poles, unless its effect on that pole is cancelled by the effect of a change in another part of the earth's interior.
Stating the declination
There are three main ways of stating the declination for a given location:
- In a diagram
- On some maps intended for wilderness or navigational use, including the topographic maps of the U.S. Geological Survey (USGS), 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, mils, or both. (On USGS maps, the diagram is near the lower left hand corner, and the information labelled "GN" in the same diagram is irrelevant to this discussion.)
- As the numeric size of the angle between magnetic and true north, and the direction from true north to magnetic north.
- For instance, "10° W" would indicate that magnetic north lies 10 degrees counter-clockwise from true north.
- As the signed number of degrees, where a positive angle indicates clockwise from true north and a negative counter-clockwise.
- For instance, "-10°" would indicate the same as the "10° W" just discussed.
Learning the declination for an area
Most use of declination is in conjunction with a map; as stated, that map may state the declination. If not,
- Lines of equal declination (isoclines) are given on aeronautical and shipping maps.
- A prediction of the current magnetic declination for a given location (based on an world-wide empirical model of the deep flows described above) can also be obtained on-line from a web page (http://www.ngdc.noaa.gov/seg/geomag/magfield.shtml) operated by the National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration of the United States. (One would of course rather have the real delination than a prediction. However, a map is sure to be months or years out of date, whereas the model is built with all the information available to the map makers at the start of the five-year period it is prepared for. The model reflects a highly predictable rate of change, and will usually be more accurate than a map, and almost never less accurate.)
Using the declination
Adjustable compasses
A magnetic compass points to magnetic north. Modern navigational compasses usually include a "baseplate" marked with a compass rose and a scale of degrees; some include a declination adjustment. Such an adjustment permits the baseplate to turn relative to an arrow, usually red, on the top of the cylinder that contains the compass needle, and measures the angle by which it has been turned. Either the cylinder will have a mark to be read against the scale of degrees on the baseplate, or a separate scale will display the current adjustment in degrees. In either case, the underlying concept is that for a declination of 10° W, the red arrow on the cylinder must lie 10° W of 0° and N on the baseplate, so when the compass as a whole is rotated so the needle lies under the red arrow, the N on the baseplate will be pointing toward true north. In this sense, it can be said that the compass has been adjusted to indicate true North instead of magnetic North (as long as it stays within an area where the declination is 10° W).
Non-adjustable compasses
With a compass lacking an adjustable baseplate, a careful, well-practiced, compass user can analyse the combination of declination and task, and decide whether the declination is to be added or subtracted from the known direction to determine an unknown direction.
Perhaps the most crucial point is recognizing the power of a trivial idea:
- In a place where the declination needs to be subtracted from an angle measured on a map from true north to a destination, to learn the compass reading to follow (on an unadjusted compass) to walk that course,
- the declination needs to be added to the compass reading that a landmark lies along, to learn the direction on the map to seek the name to match the landmark with.
See also
- USGS Geomagnetism Program (http://geomag.usgs.gov)
Geomagnetismde:Deklination (Geografie) nl:magnetische declinatie