Course (navigation)

From Wikipedia, the free encyclopedia
  (Redirected from Crab angle)
Jump to: navigation, search
The world's largest compass rose, drawn on the desert floor at Edwards Air Force Base in California, shows the use of degrees measured clockwise from north, (magnetic north in this case).

In navigation, a vehicle's course (angle) is the angle between the direction to a fixed reference object (typically true north) and the tangent line to the path over the ground the vehicle intends to follow. "Course" may also refer to the path itself (as in a course deviation indicator). In the first meaning, course is typically measured clockwise from north (in degrees from 0° to 360°) following compass convention (0° being north, 90° being east, etc.). The related concept of track (angle) is the direction of movement of the vehicle over the ground relative to the same fixed reference direction. "Track" is sometimes used to refer to the actual path over the ground ("track path"). Keeping track path and course path identical can only be achieved if the track and course angles are identical at any time. However, identical track and course angles do not necessarily guarantee identical track and course paths, as the paths may run parallel to each other. Heading is the direction to which the vehicle is pointing with its "nose", regardless of the vehicle's track angle; for vehicles like cars heading and track angle are typically the same, but for aircraft and ships the action of wind and current may cause the two to differ significantly.

Relationship between heading and track[edit]

Heading and track (A to B)
  • 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 course.

Determining course and track[edit]

The course path is the linear path over the ground a vehicle needs to follow to go from A to B, whereas the track path or course over ground, is the actual path over the ground it follows. In the diagram above, the corrections are shown that must be implemented to obtain the track of the vessel. Some ambiguity in usage of the terms may exist based on the fact that the path a navigator intends to follow, after evaluating and counteracting possible effects of wind and current, is sometimes called "a track", whereas it should correctly be called "course".

In real life, crosswinds and cross current occur which deflects the aircraft or vessel from its course (when this is not a direct headwind or tailwind). The aircraft or vessel points more or less into the wind. The amount of this depends on the vehicle's speed, the wind's speed, and the angle of the wind in relation to the vehicle's course. To correct for these, the so-called wind correction angle and water flow correction angle is computed in advance and is frequently checked while "en route". To correct for the water currents, often a correction of 20° is foreseen, while the correction for the 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.

Aircraft heading[edit]

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°.

thrust vector
Aircraft performing a crosswind landing

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.

See also[edit]