Bearing (navigation)

In navigation, bearing or azimuth is the horizontal angle between the direction of an object and north or another object. The angle value can be specified in various angular units, such as degrees, mils, or grad. More specifically:
Alternatively, the US Army defines the bearing from point A to point B as the smallest angle between the ray AB and either north or south, whichever is closest. The bearing is expressed in terms of 2 characters and 1 number: first, the character is either N or S; next is the angle numerical value; third, the character representing the perpendicular direction, either E or W. The bearing 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". For example, if the bearing between Point A and Point B is S 45° E, the azimuth between Point A and Point B is 135°.
In nautical navigation the absolute bearing is the clockwise angle between north and an object observed from the vessel.
If the north used as reference is the true geographical north then the bearing is a true bearing whereas if the reference used is magnetic north then the bearing is a magnetic bearing. An absolute bearing is measured with a bearing compass.
The measurement of absolute bearings of fixed landmarks and other navigation aids is useful for the navigator because this information can be used on the nautical chart together with simple geometrical techniques to aid in determining the position of the vessel.
A grid bearing (also known as grid azimuth) 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.)
In nautical navigation the relative bearing of an object is the clockwise angle from the heading of the vessel to a straight line drawn from the observation station on the vessel to the object.
The relative bearing is measured with a pelorus or other optical and electronic aids to navigation such as a periscope, sonar system, and radar systems. Since World War II, relative bearings of such diverse point sources have been and are calibrated carefully to one another. The United States Navy operates a special range off Puerto Rico and another on the west coast to perform such systems integration. Relative bearings then serve as the baseline data for converting relative directional data into true bearings (N-S-E-W, relative to the Earth's true geography). By contrast, Compass bearings have a varying error factor at differing locations about the globe, and are less reliable than the compensated or true bearings.
The measurement of relative bearings of fixed landmarks and other navigational aids is useful for the navigator because this information can be used on the nautical chart together with simple geometrical techniques to aid in determining the vessel's position, speed, course, etc.
The measurement of relative bearings of other vessels and objects in movement is useful to the navigator in avoiding the danger of collision. For example:
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 planning on using an air-strike on the target.
There are several methods used to measure navigation bearings including:
Moving from A to B along a great circle can be considered as always going in the same direction (the direction of B), such as when holding fixed a steering wheel or ship's wheel. However, following a great circle does not keep 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); see inverse geodetic problem. For example, assume A and B in the northern hemisphere have the same latitude, and at A the direction to B is east-northeast. Then going from A to B, one arrives at B with the direction east-southeast, and conversely, the direction at B of A is west-northwest.
To "keep to a bearing" is not, in general, the same as going in a fixed direction along a great circle. Conversely, one can keep to a great circle and the bearing may change. Thus the bearing of a straight path crossing the North Pole changes abruptly at the Pole from North to South. When travelling East or West, it is only on the Equator that one can keep East or West and be going straight (without the need to adjust). Anywhere else, maintaining latitude requires a change in direction, requires adjustment. This change in direction becomes increasingly negligible as one moves to a lower latitude.