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In navigation, a vehicle's course is the direction over the ground the vehicle is currently moving in. The line connecting the vehicle's consecutive positions on the ground is referred to as the track. The track the vehicle was intended to follow is called the route. The route is represented by the great circle line that connects the previous waypoint with the next waypoint. The responsibility of a navigator is to make the track coincide as much as possible with the route. The direction of the route is called the route course. "Course" exceptionally, and arguably erroneously, may also refer to the route, such as in a course deviation indicator, in which case it no longer constitutes an angle but rather a line. The direction of the great circle line that runs from the current position to the next waypoint is called the course to steer, or the bearing to that waypoint. The tracking angle is the angle between the course to steer and the course. The heading is the direction to which the "nose" of the vehicle is pointing. Directions (course to steer, course, heading and route course) are typically measured clockwise from north, either true or magnetic, in degrees from 0° to 360°, following compass convention (0° being north, 90° being east, etc.). For land based vehicles (like cars) heading and course are typically identical, but for aircraft and vessels the action of wind and current may cause the two to differ significantly.
Relationship between true and magnetic direction
- Heading (2) is the angle between the direction in which the vehicle's nose is pointing and a reference direction (e.g. true north (1)) (the heading of the ship shown in the image below is about 060°).
- Any reading from a magnetic compass refers to compass north (4), which is supposed to contain a two-part compass error:
- a) The Earth's magnetic field's north direction, or magnetic north (3), almost always differs from true north by magnetic variation (6), the local amount of which may be found in nautical or aeronautical charts, and
- b) The vehicle's own magnetic field may influence the compass by so-called magnetic deviation (5). Deviation only depends on the vehicle's own magnetic field and the heading, and therefore can be checked out and given as a deviation table or, graphically, as a Napier's diagram.
- The compass heading (7) has to be corrected first for deviation (the "nearer" error), which yields the magnetic heading (8). Correcting this for variation yields true heading (2).
- In case of a crosswind (9), and/or tidal or other current (10), the heading will not meet the desired target, as the vehicle will continuously drift sideways; it becomes necessary to point the heading away from the course to counteract these effects and make the track coincide with the great circle.
Relationship between course and heading
The heading will differ from the course depending on (1) the forward speed (speed parallel to the heading) of the vehicle in its medium (air for an aircraft, water for a vessel), (2) drift speed (speed orthogonal to the heading) in its medium (only for vessels, especially for sailing boats at close points of sale), and (3) wind speed and wind direction (only for aircraft) or current speed and current direction (only for vessels). In the event of a headwind or tailwind, heading and course in an aircraft are theoretically identical. If also the current is running parallel to the heading, heading and course in a vessel are also theoretically identical.
To correct for these difference between heading and course, a so-called wind correction angle or water flow correction angle is computed in advance and is frequently checked while "en route". To correct for drift speeds and currents, often a correction of 20° is foreseen, while the correction for wind is generally around 10°. Although these are the general values, the values for the correction are, of course, dependent on each individual vehicle; as such, the actual values are often found on a case-by-case method using either computation or trial and error.
An aircraft's heading is the direction that the aircraft's nose is pointing.
It is referenced by using either the magnetic compass or heading indicator, two instruments that most aircraft have as standard. Using standard instrumentation, it is in reference to the local magnetic north direction. True heading is in relation to the lines of meridian (north–south lines). The units are degrees from north in a clockwise direction. North is 0°, east is 90°, south is 180°, and west is 270°.
Note that, due to wind forces, the direction of movement of the aircraft, or track, is not the same as the heading. The nose of the aircraft may be pointing due west, for example, but a strong northerly wind will change its track south of west. The angle between heading and track is known as the drift angle. Crab angle is the amount of correction an aircraft must be turned into the wind in order to maintain the desired course. It is equal in magnitude but opposite in direction to the drift angle.
An aircraft can have instruments on board that show to the pilot the aircraft heading. The autopilot can be programmed to maintain either the aircraft heading or its course (when set in a proper mode and with correct navigational data inputs).
- The "heading and track" diagram above shows a magnetic declination to the east, as is commonly encountered in most of the Pacific Ocean, and a somewhat exaggerated (relative to most real-life examples) deviation (of about 10°). By conventional degaussing, deviation could usually be kept below 10°, and fluxgate compasses can be degaussed to close to 0°.
- The possible influences of wind and current are maximized by presupposing a very slow boat in heavy wind and current.
- To increase readability of the diagram, all possible influences were given as positive values, e.g. variation to the east, positive deviation, wind and current from port side. The principle is the same regardless of the sign/direction of any of the components.