The magnetic declination (also known as grid magnetic angle in military circles) at any point on the Earth is the angle between the local magnetic field -- the direction the north end of a compass points -- and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is equivalent, and is more often used in aeronautical and other forms of navigation. Isogonic lines are where the declination has the same value, and the lines where the declination is zero are called agonic lines.
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. Similarly, secular changes to these flows result in slow changes to the field strength and direction at the same point on the Earth.
The magnetic declination in a given area will change slowly over time, possibly as much as 2-2.5 degrees every hundred years or so, depending upon how far from the magnetic poles it is. This may be insignificant to most travellers, but can be important if using magnetic bearings from old charts or metes (directions) in old deeds for locating places with any precision. Simply speaking, true north is the direction in which the north pole is located along the Earth's rotational axis.
On the other hand, magnetic north is the direction at which the compass needle points.
There are three main ways of stating the declination for a given location:
Declination converts between true and magnetic bearings: True Bearing equals Magnetic Bearing plus Magnetic Declination. (See http://www.ngdc.noaa.gov/seg/geomag/faqgeom.shtml#q5d). A useful mnemonic for converting between magnetic and true bearings is: "east is least, west is best". Using this, the magnetic heading is less than the true heading if the declination is east, and greater if it is west. Put differently, add West declinations to, or subtract East declinations from, true to get magnetic.
One would of course rather have the real declination 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 However, a map is sure to be months or years out of date, whereas the model is built with all the information available to the map makers at the start of the five-year period it is prepared for.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.
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°)
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.
This relationship (finding what the compass should show when the true course is known) is frequently taught as:
If one knows the course shown by the compass and wishes to find the course relative to true north, the steps are inverted and the signs of deviation and variation inverted.
A simple way of remembering which way to apply the correction is as follows: (in the Continental USA) For locations East of the Agonic Line (zero declination), roughly East of the Mississippi: The Magnetic Bearing is ALWAYS Bigger. For locations West of the Agonic Line (zero declination), roughly West of the Mississippi: The Magnetic Bearing is ALWAYS Smaller.
Magnetic declination has a very important influence on air navigation, since the most common aircraft instruments (VOR's) are designed to determine headings by locating magnetic north through the use of a compass or similar magnetic device.
Aviation sectionals (maps / charts) and databases used for air navigation are based on True north rather than Magnetic north, and the constant and significant slight changes in the actual location of magnetic north and local irregularities in the planet's magnetic field require that charts and databases be updated at least 2 times per year to reflect the current magnetic variation correction from True north. For example, near San Francisco, True north is about 15.5 degrees less than magnetic north.
When plotting a course, a pilot in most small planes will plot a trip using True north on a sectional (map), then, convert the True north bearings to Magnetic north for in-plane navigation use (which rely on cockpit instruments that read Magnetic north).
Radionavigation aids located on the ground, such as VORs, 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
GPS systems used for air navigation can use Magnetic north or True north. In order to make them more compatible with systems that depend on magnetic north, Magnetic north is often chosen, at the pilot's preference. The GPS receiver natively reads in True north, but can elegantly calculate Magnetic north based on its true position and data tables calculate the current location and direction of the North Magnetic Pole and (potentially) any local variations, if the GPS is set to use Magnetic compass readings.
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