Declination. Caves are usually surveyed with a magnetic compass. Magnetic compasses don't point at the true North Pole. They point at the magnet North Pole. Today, the magnetic North Pole is about 250 miles south of the North Pole. That means that a compass reading will be offset from True North. The amount of the offset varies according the place on the Earth where the reading was taken. This offset is called "Declination."
The image to the right illustrates how declination works. In this example, the Magnet North Pole is 10 degrees to the right of the True North Pole. If the compass is aimed right at the Magnetic North Pole, the compass will read zero. However, since the declination is 10 degrees, the compass is really pointing 10 degrees to the right of the North Pole. That makes the real azimuth 10 degrees. From this we can work out the mathematics for handling declination. You simply add the declination to the compass reading to get the vlaue relative to True North. Here's the equation: TN = CR + Declination. TN = True North Reading CR = Compass Reading.
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As another example, if the compass reading was 25 degrees and the declination was 5, then the actual bearing is 30 degrees relative to True North.
30 = 25 + 5 |
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