Magnetic declination
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.
Change of declination in time and space
Magnetic declination varies both from place to place, and with the passage of time. As a traveller cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas), meaning a compass adjusted at the beginning of the journey would have a true north error of over 30 degrees if not adjusted for the changing declination.
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 2-25 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.
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" (grid north) 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.
- Lines of equal declination (isogonic lines) are shown on aeronautical and nautical charts.
- 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.
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.
Learning the declination for an area
Most use of declination is in conjunction with a map; as stated, that map may state (or even illustrate) the local declination. If not,
- A general isogonic chart of the world or continent can be consulted for a rough estimate of the local declination (within a few degrees).
- A prediction of the current magnetic declination for a given location (based on a world-wide empirical model of the deep flows described above) can also be obtained on-line from a web page 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 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.
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°
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.
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.
Navigation
In navigation the terminology of geomagnetism is used differently. In particular, magnetic declination is divided into two parts, namely Magnetic Variation and Magnetic Deviation. There are also three types of bearings--True, Magnetic, and Compass--which are related by the rules:
This relationship (finding what the compass should show when the true course is known) is frequently taught as:
- T = true course;
- V = variation (of the Earth's magnetic field);
- M = magnetic course (what the course would be in the absence of local declination);
- D = deviation caused by magnetic material (mostly iron and steel) on the vessel;
- C = compass course.
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.
Variation
Magnetic variation is the difference between True Bearings and Magnetic Bearings and is caused by the different locations of the Geographic North Pole and the Magnetic North Pole plus any local anomalies such as iron deposits. Variation is the same for all compasses in the same location and is usually stated on good quality maps and charts, along with the date it was measured.
Deviation
Magnetic Deviation is the difference between Magnetic Bearings and Compass Bearings. Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the boat, wrist-watches, etc. The value will also vary depending on the orientation of the boat. Magnets and/or iron masses can be used to correct for deviation so that a particular compass will accurately give Magnetic Bearings. More commonly, however, a correction card will be drawn up listing errors for the compass which can then be compensated for arithmetically.
Air navigation
Magnetic declination has a very important influence on air navigation, since most aircraft instruments are still designed to determine headings by locating magnetic north through the use of a compass or similar magnetic device. Charts and databases used for air navigation are usually based on magnetic bearings rather than true bearings, and the constant and significant change in the actual location of magnetic north and local irregularities in the planet's magnetic field require that charts and databases be continually updated to reflect changes in these bearings.
Radionavigation aids on the ground, such as VORs, must also be continually recalibrated to keep them aligned with magnetic north.
GPS systems used for air navigation use magnetic bearings by default, rather than true bearings, in order to make them more compatible with systems that depend on magnetic north. The GPS receiver simply calculates magnetic north based on its true position and data tables giving the current location of the North Magnetic Pole and (potentially) any local variations.