Course (navigation)

Course (navigation)

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In 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 vehicle's course is the angle that the intended path of the vehicle makes with a fixed reference object (typically 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...

). Typically course is measured in degrees from 0° clockwise to 360° in compass convention (0° being north, 90° being east). Course is customarily expressed in three digits, using preliminary zeros if needed, e.g. 058°.

Determining the true course of a vessel

  • Heading
    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...

    (2) is the angle of the vessel, aircraft or vehicle to an object (e.g. true north) (the heading of the ship shown in the image below is 058°).

  • 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 is given in nautical charts, and
    b) ship's own magnetic field may influence the compass by so-called magnetic 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....

    Deviation only depends on the ship'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 or compass course (7) has to be corrected first for deviation (the "nearer" error), wherefrom results the magnetic heading (8). Correcting this for variation yields true heading (2).

  • In case of a crosswind
    A crosswind is any wind that has a perpendicular component to the line or direction of travel. In aviation, a crosswind is the component of wind that is blowing across the runway making landings and take-offs more difficult than if the wind were blowing straight down the runway...

     (9), and/or tidal
    Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

     or other current (10), the heading will not meet the desired target, as the vessel will continuously drift sideways; it is necessary to point away from the intended course to counteract these effects.

Determining the track

The track or course over ground, is the actual path followed by the vessel from A to B. In the above given scheme, the corrections are shown that must be implemented to obtain the track of the vessel. Some ambiguity exists in the fact that the path a navigator intends to follow, after evaluating and counteracting possible effects of wind and current, is also called a track.

In real life, crosswinds and cross current occur which deflects the aircraft (or vessel) from its original heading (when this is not a headwind or a tailwind
A tailwind is a wind that blows in the direction of travel of an object, while a headwind blows against the direction of travel. A tailwind increases the object's speed and reduces the time required to reach its destination, while a headwind has the opposite effect...

). 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. To correct for these, the so-called wind correction angle and water flow correction angle is computed in advance and is frequently checked while "enroute". To correct for the water, 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 trial and error. In the above scheme, the track would be (9) for wind from port side.

GPX is an XML schema
XML schema
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself...

 for storing track logs.

Aircraft heading

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
Heading indicator
The heading indicator is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading. It is sometimes referred to by its older names, the directional gyro or DG, and also direction indicator or DI.- Use :The primary means of establishing the heading in most small...

, 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. East is 90, south is 180 and west is 270 degrees.

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 or crab angle.

See also

  • Acronyms and abbreviations in avionics
    Acronyms and abbreviations in avionics
    -A:*ACARS: Aircraft Communications Addressing and Reporting System.*ACAS: Airborne Collision Avoidance System.*ACP: Audio Control Panel.*ACS: Audio Control System.*ADAHRS: Air Data and Attitude Heading Reference System.*ADC: Air Data Computer....

  • 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...

  • E6B
    The E6B Flight Computer, or simply the "whiz wheel", is a form of circular slide rule used in aviation. They are mostly used in flight training, but many professional and even airline pilots still carry and use these flight computers...

  • Hodology
    Hodology is the study of pathways. The word derives from the Greek hodos, meaning "path". It is used in various contexts.*In neuroscience, it is the study of the interconnections of brain cells....

  • Bearing (navigation)
    Bearing (navigation)
    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 compass direction of the forward course of the aircraft...

  • Breton plotter
    Breton plotter
    A Breton plotter, also known as a Portland Course Plotter, is a navigational instrument used for nautical navigation with charts. It is a ruler with a rotating rose that enables to plot a course on charts, while aligning the North of the ruler with the North of the chart.The Breton Plotter was...