Magnetic declination

Antique declinometer

The magnetic declination at any particular place can be measured directly by reference to the celestial poles—the points in the heavens around which the stars appear to revolve, which mark the direction of true north and true south. The instrument used to perform this measurement is known as a declinometer.

The approximate position of the north celestial pole is indicated by Polaris (the North Star). In the northern hemisphere, declination can therefore be approximately determined as the difference between the magnetic bearing and a visual bearing on Polaris. Polaris currently traces a circle 0.73° in radius around the north celestial pole, so this technique is accurate to within a degree. At high latitudes a plumb-bob is helpful to sight Polaris against a reference object close to the horizon, from which its bearing can be taken.[4]

A rough estimate of the local declination (within a few degrees) can be determined from a general isogonic chart of the world or a continent, such as those illustrated above. Isogonic lines are also shown on aeronautical and nautical charts.

Larger-scale local maps may indicate current local declination, often with the aid of a schematic diagram. Unless the area depicted is very small, declination may vary measurably over the extent of the map, so the data may be referred to a specific location on the map. The current rate and direction of change may also be shown, for example in arcminutes per year. The same diagram may show the angle of grid north (the direction of the map's north–south grid lines), which may differ from true north.

On the topographic maps of the U.S. Geological Survey (USGS), for example, 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.

A prediction of the current magnetic declination for a given location (based on a worldwide empirical model of the deep flows described above) can be obtained online from a web page operated by the National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration of the United States.[5] This model is built with all the information available to the map-makers at the start of the five-year period it is prepared for. It reflects a highly predictable rate of change, and is usually more accurate than a map—which is likely months or years out of date—and almost never less accurate.

The National Geospatial-Intelligence Agency (NGA) provides source code written in C that is based on the World Magnetic Model (WMM). The source code is free to download and includes a data file updated every five years to account for movement of the magnetic north pole.

Adjustable compass set to a declination of 0° and a bearing of 312°

A magnetic compass points to magnetic north, not geographic north. Compasses of the style commonly used for hiking include a declination adjustment in the form of a bezel which swivels relative to the base plate. To establish a declination the bezel is rotated until the desired number of degrees plus or minus lie between the bezel's designation N (for North) and the direction indicated by the magnetic end of the needle (usually painted red). This allows the user to establish a true bearing for travel or orientation by aligning the embossed red indicator arrow on the base plate with a landmark or heading on a map. A compass thus adjusted can be said to be reading “true north” instead of magnetic north (as long as it remains within an area on the same isogonic line).

In the image to the left, the bezel's N has been aligned with the direction indicated by the magnetic end of the compass needle, reflecting a magnetic declination of 0 degrees. The arrow on the base plate indicates a bearing of 312 degrees.

How to compensate for magnetic declination when reading a compass. In this example, the declination is 14°E (+14°), so the compass card points to a "north" 14 degrees to the East of true North. To obtain a true bearing, add 14 degrees to the bearing shown by the compass.

To work with both true and magnetic bearings, the user of a non-adjustable compass needs to make simple calculations that take into account the local magnetic declination. The example on the left shows how you would convert a magnetic bearing (one taken in the field using a non-adjustable compass) to a true bearing (one that you could plot on a map) by adding the magnetic declination. The declination in the example is 14°E (+14°). If, instead, the declination was 14°W (−14°), you would still “add” it to the magnetic bearing to obtain the true bearing: 40°+ (−14°) = 26°.

The opposite procedure is used in converting a true bearing to a magnetic bearing. With a local declination of 14°E, a true bearing (perhaps taken from a map) of 54° is converted to a magnetic bearing (for use in the field) by subtracting the declination: 54° – 14° = 40°. If, instead, the declination was 14°W (−14°), you would still “subtract” it from the true bearing to obtain the magnetic bearing: 54°- (−14°) = 68°.

Magnetic deviation is the angle from a given magnetic bearing to the related bearing mark of the compass. Deviation is positive if a compass bearing mark (e.g., compass north) is right of the related magnetic bearing (e.g., magnetic north) and vice versa. For example, if the boat is aligned to magnetic north and the compass' north mark points 3° more east, deviation is +3°. Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the vessel, wristwatches, etc. The value also varies depending on the orientation of the boat. Magnets and/or iron masses can correct for deviation, so that a particular compass accurately displays magnetic bearings. More commonly, however, a correction card lists errors for the compass, which can then be compensated for arithmetically. Deviation must be added to compass bearing to obtain magnetic bearing.

Magnetic declination has a very important influence on air navigation, since the most simple aircraft navigation instruments 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 twice each year to reflect the current magnetic variation correction from true north.

For example, as of March 2010, near San Francisco, magnetic north is about 14.3 degrees east of true north, with the difference decreasing by about 6 minutes of arc per year.[7]

When plotting a course, most small aircraft pilots plot a trip using true north on a sectional (map), then convert the true north bearings to magnetic north for in-plane navigation using the magnetic compass. These bearings are then converted on a pre-flight plan by adding or subtracting the local variation displayed on a sectional.

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.

Runways are designated by a number between 01 and 36, which is generally one tenth of the magnetic azimuth of the runway's heading: a runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°).[8] However, due to magnetic declination, changes in runway designators have to occur at times to keep their designation in line with the runway's magnetic heading. An exception is made for runways within the Northern Domestic Airspace of Canada; these are numbered relative to true north because proximity to the magnetic North Pole makes the magnetic declination large.

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; the unit can then 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.