Bearing (navigation)

In marine navigation, a bearing is the direction one object is from another object, usually, the direction of an object from one's own vessel. In aircraft navigation, a bearing is the actual (corrected) compass direction of the forward course of the aircraft. In land navigation, a bearing is the angle between a line connecting two points and a north-south line, or meridian.

Land navigation
In land navigation, a bearing was traditionally defined in land surveying terms as a fixed number line which gave the smallest arc (never to exceed 90 degrees). More specifically, a bearing was measured both east and west from north and south, divided into four quadrants. In contrast, an azimuth was a clockwise measurement of a circle from a zero point at a fixed horizontal plane of reference (such as a north meridian), expressed in degrees, mils, or other unit of angular measurement. Under this definition, for example, a given azimuth in degrees would be expressed as 60°, while the equivalent bearing would be expressed as N 60° E.

Determining a position
A bearing can be taken relative to a charted or mapped object at a specific time. This bearing can be used to determine the position of the vessel or landmark when used in conjunction additional bearing(s) or other information (e.g. depth, declination, etc.).

However, a pre-calculated bearing to a charted object can be also be used as a precautionary measure. For instance, if anchored in a harbor with a shore to the north, it can be decided that the vessel must stay to the south of an object to its east. If it does not, an anchor watch must inform someone to take action. Or, if moving through a channel with obstructions to the west and which is not well-marked with buoys, it can be decided that the vessel might have to stay to east of a particular charted object. Similarly, a land navigator might walk a bearing several degrees to the east of his actual course to avoid difficult ground, or to intercept an intersecting position (such as a road or trail) leading to his/her destination. Upon reaching the road, the navigator simply turns west to ensure he will reach the destination. These types of bearings are sometimes termed limit bearings, aim-off bearings, danger bearings, or index bearings.

Piloting
A bearing can be taken on another vessel to aid piloting. If the two vessels are travelling toward each other and the relative bearing remains the same over time, there is likelihood of collision and action needs to be taken by one or both vessels to prevent this.

Warfare
A bearing can be taken to a fixed or moving object in order to target it with gunfire or missiles.

Search and rescue
A bearing can be taken to a person or vessel in distress in order to go to their aid or, when that is not possible, to report the person or vessel to authorities or someone who can go to their aid.

Types of bearings
Types of bearings include:
 * true bearings
 * magnetic bearings
 * grid bearings


 * compass bearings
 * relative bearings.

A true bearing is measured in relation to the fixed horizontal reference plane of true north, that is, using the direction toward the geographic north pole as a reference point.

A magnetic bearing is measured in relation to magnetic north, that is, using the direction toward the magnetic north pole (in northeastern Canada) as a reference.

A grid bearing is measured in relation to the fixed horizontal reference plane of grid north, that is, using the direction northwards along the grid lines of the map projection as a reference point.

In vehicle or marine navigation, a compass bearing is measured in relation to the magnetic compass of the navigator's vehicle or vessel (if aboard ship). It should be very close to the magnetic bearing. The difference between a magnetic bearing and a compass bearing is the deviation caused to the compass by ferrous metals and local magnetic fields generated by any variety of vehicle or shipboard sources (steel vehicle bodies/frames or vessel hulls, ignition systems, etc.)

A relative bearing is one in which the reference direction is straight ahead, where the bearing is measured relative to the direction the navigator is facing (on land) or in relation to the vessel's bow (aboard ship).

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. 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.

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 have 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.

Other information
If navigating by gyrocompass, the reference direction is true north, in which case the terms true bearing and  geodetic bearing are used. In stellar navigation, the reference direction is that of the North Star, Polaris.

Generalizing this to two angular dimensions, a bearing is the combination of antenna azimuth and elevation required to point (aim) an antenna in a given direction. The bearing for geostationary satellites is constant. The bearing for polar-orbiting satellites varies continuously.

Moving from A to B along a great circle can be considered as always going in the same direction (the direction of B), but not in the sense of keeping the same bearing, which applies when following a rhumb line. Accordingly, the direction at A of B, expressed as a bearing, is not in general the opposite of the direction at B of A (when traveling on the great circle formed by A and B). For example, assume A and B in the northern hemisphere have the same latitude, and at A the direction to B is eastnortheast. Then going from A to B, one arrives at B with the direction eastsoutheast, and conversely, the direction at B of A is westnorthwest.