Geographically we refer to the imaginary axis of the Earth's rotation as going through the north and south poles. Maps are drawn using these points for north and south. They are referred to as true north and true south. Unfortunately, there are two other poles that we must consider when navigating with a magnetic compass. They are called the magnetic poles. These are the points where magnetic fields that surround the Earth originate. The compass reacts to these magnetic fields. When you draw a line on a map indicating where you want to go, you must take into consideration that your compass is telling you where magnetic north is, not true north. To make the necessary correction for this, pilots use what are called lines of variation. These are dashed lines on the map that are labeled either east or west and have a number on them. The number is the number of degrees you need to subtract or add to your true heading to get your magnetic heading. This number reflects the angular difference in degrees between the true north and south poles and the magnetic north and south poles. If the line is labeled "E" for east, you subtract the number of degrees indicated. If the line is labeled "W" for west, you add the number. A memory aid for this is: "east is least, west is best."

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Please send any comments to:
Curator: Tom.Benson@grc.nasa.gov
Responsible Official: Kathy.Zona@grc.nasa.gov