Lecture 13 Maps, Direction, and Compass . Remember a magnetic compass does not depend on an external power source. It senses the earth's magnetic field. Because it does not require external power, it remains as the basic back-up navigation device on aircraft and ships.
Compasses vary considerably in price and design.
Be sure if you purchase one, it meets your requirements. On some compasses the card rotates, whereas on others the needle rotates. Directional numbering systems also differ between compass types. For example, 0-360 degrees, quadrant, mils, or cardinal directions. Always remember, that unless you locally compensate for magnetic declination, your compass reading is in magnetic degrees.