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Magnetic Declination is simply a magnetic reading that points away from true north. and is given in decimal degrees (magnetic). Where it is not equal to 0.000°, magnetic declination will include a direction [ east (E) or west (W) ] away from true north. This indicates the direction of the magnetic field line followed by the compass needle at one specific point in the field.
Structural Declination is simply a directional reading of permanent structures (like walls) that points away from true north. and is given in decimal degrees (true). As with magnetic declination, where it is not equal to 0.000°, structural declination will include a direction [ east (E) or west (W) ] away from true north.
Declination is a value that permits us to convert, or tranlate, from magnetic or nonmagnetic direction to true direction. We apply declination as a reference to obtain true north in the form of a correction when we use magnetic sensors (like a compass) or when we must use permanent structures, like walls, as a directional reference. This is to properly orient our maps and charts and directional devices so we can navigate and communicate quickly and accurately. Declination helps to place our directional equipment on correct headings. Aerials, charts, directional displays, maps, masts, and rotators must be calibrated with the latest available declination. Combining declination with maps makes magnetic compass readings more accurate and meaningful. See STEM Challenge ONE for practical advice and practice with beam headings and declination.
Magnetic declination at all points must be redefined and updated over time in order to conform to natural variations in the magnetic field of the Earth. Because true north and magnetic true north (MTN) usually differ in value at any single point, declination is required to translate between them. Magnetic compass readings can be useless until a current declination is applied and MTN is defined for the point of interest.
True north is not magnetic , but its value is equal to 0.000° and 360.000° . One True Compass Rose easily represents nonmagnetic direction from all points on the earth. All directions depicted on this website are true, with the exception of magnetic declination.
Maps and charts are commonly printed with orientations to true north. They may also include some declination information as well. However, printed declination data may be too old for use if the map is aged by a few years or more.
To find true north with a magnetic compass, first find out the latest declination reading in effect for that point in the field. One authoritative source for updated declination at points worldwide is the National Geophysical Data Center (NGDC) . The National Atlas stated in recent years that the primary world model is the International Geomagnetic Reference Field (IGRF) . Here is a "health warning," or a description, of errors and limitations of the IGRF.
Never apply declination to true values (whether magnetic true or nonmagnetic true), as a directional error will result.
To apply a correction for east (positive, +) declination, subtract the declination value from 360.000°.
To apply a correction for west (negative, -) declination, add the declination value to 0.000°.
The result is Magnetic True North, or MTN. It is the correct magnetic direction for true north as displayed on the compass for that point in the field.
MTN is specific to a single point and cannot be applied to any other point. It is far easier and more practical to look up the current declination for each specific point of interest and compute the MTN directly than to attempt to carry over one MTN to several points. Forecasting one MTN for a range of points is unreliable at best.
It is rare for MTN to equal 0.000° and 360.000° prior to applying declination, but there are some historical examples of this happening with radio station WWV in Colorado. A current declination value of 0.000° will result in this rare exception.
After computing the new magnetic value for true north (MTN), you can orient maps and calibrate directional equipment that relate to that specific point. Simply line up your magnetic compass or sensor on the precise MTN. Then adjust your antennas, directional displays, maps, and rotators to match up with MTN either in line or in parallel fashion. At this point, stop using the MTN value for this direction. Rename and mark this direction as true north with numeric values 0.000° and 360.000° . You are now calibrated to true north and, by extension, to the entire True Compass Rose. It is best to calibrate directional equipment across its entire directional range.
Recalibrate compasses and directional equipment with a new declination and MTN at least once a year. Verify any structural declinations in use, especially when they are collected or measured in a new way. Expect structural declination to be more lasting and stable than magnetic measurements. Always recalibrate annually and whenever the placement and siting of directional equipment is disturbed by construction, demolition, earth movement, high winds, maintenance, severe weather, vandalism, and similar events.
