Course (navigation)
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Course, in navigation, is a general term describing a direction of travel.
In navigation, any direction is given by clockwise measurement of its angle from a reference direction. In order to be used in a chart or for dead reckoning, this reference has to be true north, the direction of the north geographical pole (1). This angle is customarily expressed in three digits, using preliminary zeros if needed.
- Heading (2) is the direction the vessel, aircraft or vehicle is truly "pointing towards" (the heading of the ship shown in the image 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 (5).
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 cross wind (9), and/or tidal 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.
[edit] Track
A track, also course over gound, is the actual path followed by a moving body, e.g. the vessel's track from A to B in the above given scheme. 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 track.
The track is equivalent to the heading (a bearing "right ahead"), if no cross wind and cross current occur (2), but this would hardly ever happen in aviation.
When wind is present, and is not a directly facing or tail wind, the wind deflects the aircraft (or vessel) from its heading.
To correct for the wind, the aircraft (or vessel) points more or less into the wind. The amount depends on the vehicle's speed, the wind's speed, and the angle of the wind in relation to the vehicle. This so-called wind correction angle is computed in advance and is frequently checked while "en route". In the above scheme, the track would be (9) for wind from port side.
GPX is a format which can store track logs.
Notes:
- The above scheme shows a variation of 6° East, as might happen in areas of Pacific ocean, for instance, and
- a more-than-somewhat exaggerated deviation (taken from a fictitious deviation table for educational purpose) of +12°, for a compass heading of 040°. By conventional degaussing, deviation could usually be kept beyond 10°, and fluxgate compasses can be degaussed to almost D=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 scheme, all possible influences were given as „positive“, i.e. variation=East, „+“ 12° deviation, wind and current from port side. The principle is the same for the opposite of any of the components.
