Navigation and Seamanship

1.2.12. Directions in Navigation

                                                                           

                 Direction is the position of one point relative to another. Navigators express direction as the angular difference in degrees from a reference direction, usually north or the ship’s head. Course (C, Cn) is the horizontal direction in which a vessel is steered or intended to be steered, expressed as angular distance from north clockwise through 360°.

                 The course is often designated as true, magnetic, compass or grid according to the reference direction.

                 In navigation, true direction is the direction from one point on the earth’s surface to another, without regard for the distance between them; it is expressed as an angle in degrees from 000 to 360, referenced to true north. Direction is always expressed in three digits; for example, a direction 8 east of north is expressed as 008 , and one 34 east of north as 034. True north may be considered as either 000 or 360.

                It is axiomatic that every line has two directions; hence the direction intended should be clearly indicated by arrow heads or some system of labeling. Direction can be shown as clearly by the order of letters used; thus AB is the direction from A to B; the reciprocal direction from B to A is BA.

True Direction

                If direction is expressed in reference to the geographic pole, then it is said to be True Direction, and is symbolised by 'T'. True North is therefore written as 000°T (360°T) or 000T (360T).

Magnetic Direction

                The surface of the Earth is covered by a weak magnetic field. The entry and exit points of the field through the earth's surface (which is where the field lines are perpendicular to the surface) are called the Magnetic Poles. Because of their proximity to the North and South Geographic Poles, the magnetic poles are referred to as the North Magnetic Pole and the South Magnetic Pole.

                 It is a useful property of a magnetic compass that it aligns to the magnetic field lines and therefore points in the direction of Magnetic North. Unfortunately for navigation, the magnetic poles and the geographic poles are not in the same location and consequently there is a small but significant difference between the True (i.e. geographic) Direction as shown in the chart and the Magnetic Direction as indicated by the compass.