Bearing (navigation)
Encyclopedia
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
. Bearings can be measured in two systems, Mils and Degrees.
The bearing consists of 2 characters and 1 number: first, the character representing the reference ray (N, or S); followed by the angle value; and finally the character representing the direction of the angle from the reference ray (E, or W). The angle value will always be less than 90 degrees. For example, if Point B is located exactly southeast of Point A, the bearing from Point A to Point B is S 45° E.
The US Army defines the azimuth between Point A and Point B as the angle, measured in the clockwise direction, between the north reference ray and Ray AB. For example, if the bearing between Point A and Point B is E 45° S, the azimuth between Point A and Point B is 135°.
The angle value in a bearing can be specified in the units of degrees
, mils
, or grad
. An azimuth is specified in the same angle units.
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.
A compass bearing, as in vehicle or marine navigation, 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).
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.
, 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.
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
, 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
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
. Bearings can be measured in two systems, Mils and Degrees.
US Army Definition Of Bearing
The US Army defines the bearing from Point A to Point B as the angle between: either a north, south, east, or west ray, whose origin is Point A and Ray AB, the ray whose origin is Point A and which contains Point B.The bearing consists of 2 characters and 1 number: first, the character representing the reference ray (N, or S); followed by the angle value; and finally the character representing the direction of the angle from the reference ray (E, or W). The angle value will always be less than 90 degrees. For example, if Point B is located exactly southeast of Point A, the bearing from Point A to Point B is S 45° E.
The US Army defines the azimuth between Point A and Point B as the angle, measured in the clockwise direction, between the north reference ray and Ray AB. For example, if the bearing between Point A and Point B is E 45° S, the azimuth between Point A and Point B is 135°.
The angle value in a bearing can be specified in the units of degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
, mils
Angular mil
An angular mil, also mil, is a unit of angle. All versions of the angular mil are approximately the same size as a trigonometric milliradian.-History:The milliradian was first identified in the mid nineteenth Century...
, or grad
Grad (angle)
The gradian is a unit of plane angle, equivalent to of a turn. It is also known as gon, grad, or grade . One grad equals of a degree or of a radian...
. An azimuth is specified in the same angle units.
Determining a position
A bearing is basically the position of A with respect to B and which is their angle with respect to north so the angle with B plus the angle with north is bearing.Piloting
A bearing can be taken on another vessel to aid piloting. If the two vessels are traveling 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. This is mainly used by ground troops when using an air-strike on the target.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
True north
True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid 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
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
) as a reference.
A grid bearing
Grid north
Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north and magnetic north...
is measured in relation to the fixed horizontal reference plane of grid north
Grid north
Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north and magnetic north...
, that is, using the direction northwards along the grid lines of the map projection
Map projection
A map projection is any method of representing the surface of a sphere or other three-dimensional body on a plane. Map projections are necessary for creating maps. All map projections distort the surface in some fashion...
as a reference point.
A compass bearing, as in vehicle or marine navigation, 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
Magnetic deviation
Magnetic deviation is the error induced in a compass by local magnetic fields, which must be allowed for, along with magnetic declination, if accurate bearings are to be calculated....
caused to the compass by ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
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
Clockwise
Circular motion can occur in two possible directions. A clockwise motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back to the top...
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
True north
True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid north...
, magnetic north, or grid north
Grid north
Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north and magnetic 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
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
, 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.
Usage
If navigating by gyrocompassGyrocompass
A gyrocompass is a type of non-magnetic compass which bases on a fast-spinning disc and rotation of our planet to automatically find geographical direction...
, the reference direction is true north
True north
True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid 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
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....
.
Generalizing this to two angular dimensions, a bearing is the combination of antenna azimuth and elevation
Elevation (ballistics)
In ballistics, the elevation is the angle between the horizontal plane and the direction of the barrel of a gun, mortar or heavy artillery. Originally, elevation was a linear measure of how high the gunners had to physically lift the muzzle of a gun up from the gun carriage to hit targets at a...
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
Great circle
A great circle, also known as a Riemannian circle, of a sphere is the intersection of the sphere and a plane which passes through the center point of the sphere, as opposed to a general circle of a sphere where the plane is not required to pass through the center...
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
Rhumb line
In navigation, a rhumb line is a line crossing all meridians of longitude at the same angle, i.e. a path derived from a defined initial bearing...
. 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.
See also
- Cardinal directionCardinal directionThe four cardinal directions or cardinal points are the directions of north, east, south, and west, commonly denoted by their initials: N, E, S, W. East and west are at right angles to north and south, with east being in the direction of rotation and west being directly opposite. Intermediate...
- Course or TrackCourse (navigation)In navigation, a vehicle's course is the angle that the intended path of the vehicle makes with a fixed reference object . Typically course is measured in degrees from 0° clockwise to 360° in compass convention . Course is customarily expressed in three digits, using preliminary zeros if needed,...
- Hand bearing compassHand compassA hand compass is a term for any compact magnetic compass capable of one-hand use and fitted with a sighting device to record a precise bearing or azimuth to a given target or to determine a location...
- Yaw, pitch and roll