Transmitter hunters need to be concerned about Magnetic Declination, since this affects bearings taken with a compass. Many maps are oriented towards True North, but the bearings taken with a compass are oriented towards Magnetic North. Thus, you cannot just plot your compass bearings on your map without first applying a correction to make your bearings relative to True North.
This difference between True North and Magnetic North is called: magnetic declination. It may also be called magnetic variation, usually by mariners.
To use an example, the declination for the T-hunt start-point in Boston,MA. is currently -15.0 degrees. This means that when we take a compass bearing, we need to SUBTRACT 15.0 degrees to the reading, in order to get a true bearing, which can then be plotted on a map.
If you would like to get the exact magnetic declination for your area, you can dial into a phone bbs which is operated by the USGS Branch of Global Seismology and Geomagnetism. The phone number is 1-800-358-2663. You will need your Latitude, Longitude, and elevation, to plug into thier program.
Declination: don't take someone elses' word for it
If you would like to have a little fun measuring the actual Magnetic Declination in your area, the following easy experiment is one way to get a "hands-on" feel for how the earth's magnetic field affects your compass.
Go out on a clear night to an open area where you can observe the North Star (Polaris *). Drive a stake into the ground. Walk away from the stake towards the south, for at least fifty feet. Hold a plumb-bob (or small heavy object) by the string, in such a way as to line up the North star AND your first stake, using the string as a guide. Place a second stake in the ground exactly beneath the plumb-bob. The two stakes now define a line that points to True North. (You may want to wait until daylight to do the next step, merely because it will be easier to read your compass). Using your compass, take an accurate reading of the line between your two stakes. This will give you the magnetic bearing, and thus (since the stakes line up with "true north) the correction you will need to make for future bearings.
(* OK, there are some picky critics out there who will argue that Polaris isn't really on the exact axis of the earth's rotation, and thus not truly north, since it is actually off-axis by perhaps a degree. If this bothers you, too, you can get a current copy of a Solar Ephemeris, which will have tables of the precession of Polaris around the Pole axis, and you can look up the correction to apply at any given hour, any given day of the year. Sheesh).
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