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anchor watch

Bearing (navigation)

In navigation, a bearing is the direction one object is from another object. In navigation, it is usually the direction of an object from one's own vessel.

Purposes of bearings

Determining a position

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

However, a pre-calculated bearing to a charted object can be used as a warning. 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. This types of bearings are called 'limit 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

Some of the types of bearings are

  • true bearings
  • magnetic bearings and
  • compass bearings
  • relative bearings.

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

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 compass bearing is measured in relation to the vessel's magnetic compass. 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 vessel's compass by ferrous metals and magnetic fields of the vessel.

A relative bearing is measured in relation to the vessel's bow.

Bearing measurement

There are several methods used to measure navigation bearings:

1. A clockwise angle starting from the reference direction and increasing to 359.9 degrees. If the reference direction is north, the bearing is called "absolute." A true, magnetic and compass bearing is always measured in this way, with true north, magnetic north or compass north being 000°. If the reference direction is straight ahead, then the bearing is called "relative."

2. An angle measured from straight ahead on each side. Starboard bearings are 'green' and port bearings are 'red'. Thus, something directly off the starboard side would be 'Green090' or 'G090'. This method is used by the UK Royal Navy and Royal Australian Navy in accordance with the Admiralty Manual of Navigation, BR45. This method is only used for a relative bearing.

3. Traditionally, someone on lookout does not have a compass available with which to give an accurate bearing and might not be numerically literate. Therefore, every forty-five degrees of direction 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, non-nautical method of measuring relative bearing is 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.

In surveying, a bearing is the clockwise or counterclockwise angle between north or south and a direction. Bearings are written in the notation N57°E, S51°E, S21°W, N87°W, N15°W. An angle that is always measured clockwise from north is called an azimuth. 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.

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