PART 6
Magnetic North
True North
Variation
Compass Variation. Your boat’s magnetic compass points to Magnetic North, but your charts are oriented to True North. The difference in degrees between your compass readings and True North is called variation. The amount and direction of variation will change depending on your location. The compass rose on your chart will indicate the amount and direction of the variation for your area.
Magnetic North
This difference is variation.
Variation can be either East or West. This example shows Magnetic North to the West of True North and is a West variation. The note inside the circles indicates a variation of 7° 30” W (West).
Geographic or True North
Magnetic North is not a fixed point like True North, but slowly wan- ders. The amount that it changes each year is also indicated on the com- pass rose as an annual increase or decrease.
Compass Tips • If a GPS gives your heading in degrees True, you will have to convert it to a magnetic heading for your compass by adding or subtracting the variation. Add for a West (W) variation and subtract for an East (E) variation.
• Know the position of the steering wheel for a straight course.
• Don’t chase the compass. • Motion can jostle a compass. Read it when the boat is steady.
• Don’t stare at the compass. Have a reference to steer to, such as a buoy, a point of land, or a distant cloud.
• Remember the boat turns around the compass – the compass doesn’t turn in the boat.
Compass Deviation. Your compass responds to iron or steel objects which have magnetic properties. The difference in compass readings created by their magnetic influence is called the deviation. For instance, a bag of tools accidentally placed next to the compass can cause a deviation so that the compass reads 075 degrees when it should read 080 degrees. Even metal eyeglass frames may do it with a hand bearing compass.
Compass Variation on a GPS. A GPS often tells you the direction to or from something. It can do this in degrees True or degrees Magnetic. It’s critical to know which it is, because in some places the discrepancy can be 20 degrees or more. A menu in most GPS sets and plotters allows the user to set the instrument to readout True or Magnetic. Either is fine, but you MUST know which it is.
THE WAYPOINT
A waypoint is a position of tactical interest chosen by the navigator. GPS will instantly give an activated waypoint’s distance and bearing relative to the boat.
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