Download declination - Troop 233, Bethesda, MD

DECLINATION
Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north.
Declination is positive east of true north and negative when west. Magnetic declination changes over time and
with location. As the compass points with local magnetic fields, declination value is needed to obtain true north.
The last time the magnetic field reversed was about 750,000 - 780,000 years ago
What happens to my compass in the southern hemisphere? For a compass to work properly, the compass needle
must be free to rotate and align with the magnetic field. The difference between compasses designed to work in the
northern and southern hemispheres is simply the location of the “balance”, a weight placed on the needle to ensure it
remains in a horizontal plane and hence free to rotate. In the northern hemisphere, the magnetic field dips down into the
Earth so the compass needle has a weight on the south end of the needle to keep the needle in the horizontal plane. In
the southern hemisphere, the weight needs to be on the north end of the needle. If you did not change the weight, the
needle would not rotate freely, and hence would not work properly.
The Earth's magnetic field, as measured by a magnetic sensor on or above the Earth's surface, is actually the sum of
magnetic fields generated by a variety of sources. These fields are superimposed on each other and through inductive
processes interact with each other. The most important of these geomagnetic fields are:
1. The Earth's main magnetic field generated in the conducting, fluid outer core.
2. The crustal field generated in Earth's crust and upper mantle.
3. The combined disturbance field from electrical currents flowing in the upper atmosphere and magnetosphere,
which induce electrical currents in the sea and ground.
To get current declination figures, use the declination finder on NOAA's National Geophysical Data Center
website. Just input the zip code or coordinates of your destination to get the latest estimate.
Declination calculator – Results for 22201-1011 on 2015-02-18
http://www.ngdc.noaa.gov/geomag/WMM/calculators.shtml
http://www.ngdc.noaa.gov/geomag-web/#declination
Adjustable compasses
Adjustable compass set to a declination of 0° and a bearing of 307°
A magnetic compass points to magnetic north, not geographic north. Compasses of the style commonly used for hiking
usually include a "baseplate" marked with a bezel that includes a graduated scale of degrees along with the four cardinal
directions. Most advanced or costlier compasses include a declination adjustment. Such an adjustment moves the red
"orienting arrow" (found on the base of the liquid filled cylinder that contains the needle) relative to the bezel and the
baseplate. Either the cylinder has a mark to read against the scale of degrees on the baseplate, or a separate scale
displays the current adjustment in degrees (i.e., the angle by which it has turned).
In either case, the underlying concept is that for a declination of 10° W, the red orienting arrow on the cylinder must lie 10°
W of 0°/N on the bezel. (Basically, in this case, you are permanently subtracting 10° from your future bearings to
compensate for the −10° declination. If your declination was 10°E you would rotate the baseplate's red orienting arrow 10°
E of 0°/N to compensate for the +10° declination.) In this sense, it can be said that the compass has been adjusted to
indicate true north instead of magnetic north (as long as the compass remains within an area on the same isogonic line).
Non-adjustable compasses
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: 26°- (-14°) = 40°.