Some people think that a compass points North. It doesn't really. It points at Magnetic North, which is not at the same location on the globe as the North pole.
Base beams should be calibrated in True North.
When hunting in the field, it is more convenient to take the beam bearing using a hand held compass, and then convert that reading to True Bearing. In the figure provided we have a simplified compass rose. Note that there is a TN (True North) marking and a MN (Magnetic North) marking. In Southern California, where the writer lives, MN is at 14.7 degrees True heading. We will use this number in further discussion.
Display figure as a separate page. Click to down load as a GIF Image. Its name is Compass.GIF.
Assume a compass wherein the compass rose is mounted in the case, and the needle floats above it. If the compass were to be lined up perfectly with zero pointing True North, the needle would be resting at the MN position. This represents the reality of the situation.
Assume a compass wherein the compass rose is floating in the case and has the needle in it. The internal magnetic needle is always and permanently fixed at the (MN) zero reading of the compass. They have a pointing device which you aim at the object to take the bearing, and the heading appears under the pointer. With this compass, if you took a heading towards True North in So. Calif., you would get a reading of 360 - 14.7 = 345.3 degrees (-14.7 degrees). Obviously, we need to add 14.7 to get the (correct; 360 = 0) heading of True North.
If you were to take a bearing on a signal arriving from 14.7 degrees, zero in your RDF, and then took a compass sighting, the compass would indicate 0 degrees. Thus to obtain the True bearing you must add 14.7 degrees. This is the procedure for any bearing and for any type of compass.
To obtain the correction factor for your area, check on maps you may own. If these don't give this information, try contacting a local airport. They should know it, or can obtain it from air navigation maps. The correction factor is usually given as so many degrees East (as in my area) or West Magnetic Declination.
George R. Andrews
President
Contact
George R. Andrews (Russ, K6BMG)
Temple City, CA
91780, USA
Voice: 1(626)285-6963
FAX: 1(626)285-1684 (24 Hour, automatic)
America OnLine: Grandrews
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(31 Jan 1996)
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