Parts of the Compass
BASE PLATE: A rectangular, transparent plate upon which the compass housing is mounted. It is inscribed with the direction-of-travel arrow and often has a scale to measure distance on a map.
COMPASS HOUSING: The round, usually liquid-filled casing that rotates and contains the orienting arrow and the magnetic needle. The outer ring, or dial, of the compass housing contains the 360 degrees of direction and the cardinal points.
DIRECTION-OF-TRAVEL ARROW: Located on the base plate, it is used to indicate travel direction once the magnetic needle has been aligned with the orienting arrow. A bearing is read, in degrees, where the line meets the compass ring.
ORIENTING ARROW: Located beneath the magnetic needle, this arrow is rotated to align with the magnetic needle in order to orient the map and to establish a bearing.
MAGNETIC NEEDLE: The needle inside the movable compass housing that always points to magnetic north.
CONTOUR LINES/CONTOUR INTERVAL: These brown lines connect points of equal elevation. The interval, found in the map legend, is the distance in elevation between two contour lines.
DECLINATION (Magnetic variation): The difference in degrees or the angle between true north and magnetic north. It is found in the map legend.
ELEVATION: Often used interchangeably with altitude, its correct use refers to height above sea level with relation to the ground, while altitude is used in reference to the air. Used predominantly with topographical maps.
MAGNETIC NORTH: The geographical point on the Earth toward which all magnetic needles point.
TRUE NORTH: The North Pole.
TOPOGRAPHICAL MAP: A two-dimensional representation of three-dimensional terrain by use of contour lines, shading and color. Backpackers commonly use a USGS 7.5' quadrangle.
SCALE: The fraction or ratio that expresses the relationship between distance on the map and distance on the ground. For example 1:24,000 indicates 1 inch on the map is equal to 24,000 inches (or 2000 feet) on the ground.
Getting Your Bearings
AZIMUTH or BEARING: The direction of travel from one position to another position expressed in degrees from 1 to 360, read at the direction-of-travel arrow when the compass is oriented.
BACK BEARING: Also known as back azimuth, this is the opposite (180 degrees) of your bearing (direction of travel). It is used to confirm your bearing with a known landmark to your rear.
ORIENTING THE MAP: With a compass, this is the procedure of using declination and magnetic north to point to true north on the compass so the map can be aligned with true north and thereby accurately represent the surrounding terrain.
TRIANGULATION: Use of two known points to determine a third location.