Bearing (navigation) - Bearing Measurement

Bearing Measurement

There are several methods used to measure navigation bearings:

1. In land navigation, a bearing is ordinarily calculated in a clockwise direction starting from a reference direction of 0° and increasing to 359.9 degrees. Measured in this way, a bearing is referred to as an azimuth by the US Army but not by armies in other English speaking nations, which use the term bearing. If the reference direction is north (either true north, magnetic north, or grid north), the bearing is termed an absolute bearing. In a contemporary land navigation context, true, magnetic, and grid bearings are always measured in this way, with true north, magnetic north, or grid north being 0° in a 360-degree system.
2. In aircraft navigation, an angle is normally measured from the aircraft's track or heading, in a clockwise direction. If the aircraft encounters a target that is not ahead of the aircraft and not on an identical track, then the angular bearing to that target is called a relative bearing. (needs clarification: track and heading are not usually the same number due to wind)
3. In marine navigation, starboard bearings are 'green' and port bearings are 'red'. Thus, in ship navigation, a target directly off the starboard side would be 'Green090' or 'G090'. This method is only used for a relative bearing. A navigator on watch does not always have a corrected compass available with which to give an accurate bearing. If available, the bearing might not be numerate. Therefore, every forty-five degrees of direction from north on the compass was divided into four 'points'. Thus, 32 points of 11.25° each makes a circle of 360°. An object at 022.5° relative would be 'two points off the starboard bow', an object at 101.25° relative would be 'one point abaft the starboard beam' and an object at 213.75° relative would be 'three points on the port quarter'. This method is only used for a relative bearing.
4. An informal method of measuring a relative bearing is by using the 'clock method'. In this method, the direction a vessel, aircraft or object is measured as if a clock face is laid over the vessel or aircraft, with the number twelve pointing forward. Something straight ahead is at 'twelve o'clock', while something directly off to the right is at 'three o'clock'. This method is only used for a relative bearing.
5. In land surveying, a bearing is the clockwise or counterclockwise angle between north or south and a direction. For example, bearings are recorded as N57°E, S51°E, S21°W, N87°W, or N15°W. In surveying, bearings can be referenced to true north, magnetic north, grid north (the Y axis of a map projection), or a previous map, which is often a historical magnetic north.