A map is a two-dimensional representation of a curved object, namely the surface of the Earth. Basic maps, including road and simple trail maps, provide a legend, north reference, a scale, and important features such as roads, trails, and bodies of water. These maps are typically sufficient when remaining on marked paths, but do not reveal elevations. Topographic maps include descriptions of elevations through the use of contour lines. A contour line connects points of the same elevation allowing the identification of land features and the slope of elevation. Topographic maps are typically required when venturing off trail.
A compass consists of a freely rotating magnetized needle suspended in a fluid and encased in a protective housing. The needle is marked with a north end (usually colored red) and a south end. The needle rotates toward magnetic sources. It is intended to point toward the magnetic north pole of the Earth. However, magnets and ferrous metals can interfere with the compass and cause errors.
In land navigation a distinction is made between magnetic north, true north and grid north. Magnetic north is the location of the magnetic north pole of the Earth and is the direction indicated by a compass needle. Magnetic north has a different geographic location than true north. Compounding the issue is that the magnetic north pole moves at a varying rate from year to year. True north is the direction to the North Pole. Grid north is the orientation of grid lines on the map.
An azimuth is a direction measured from a reference point. In land navigation, azimuths are measured in degrees or mills clockwise from north. To find an azimuth, sight the compass in the direction of travel or toward an object. Some compasses include sights to aid in aiming the compass. Ensure the compass is level and the needle can rotate freely within the compass. Once the compass is aimed, rotate the bezel of the compass until the north marking on the bezel is aligned with the north end of the needle. A reference mark at the top of the compass will indicate the direction the compass is pointed. A back-azimuth is useful in determining the direction from an object. To calculate, back-azimuth in degrees, add 180 degrees if the azimuth is less than 180, and subtract 180 if the azimuth is more than 180. Put simply, if the azimuth to a destination is 32 degrees, then the back-azimuth, or way back, is 212 degrees (32 + 180 = 212).
Putting It Together
Dead reckoning uses a compass to continuously measure direction and a pace count to determine distance traveled when navigating. Terrain association uses the terrain features described on a map correlated to their real-world counterparts to navigate. Dead reckoning is employed for short distances whereas terrain association is frequently employed when navigating long distances and general directions. Thus, terrain association is often used to approach an objective before switching to dead reckoning to close the final short distance to the target. The transition point between terrain association and dead reckoning is often referred to as the attack point.