Compass variation and deviation; T-V-M-D-C

Bowditch Ch. 6 §601

The Magnetic Compass — Variation and Deviation Compass error has two parts. Variation is the angle between true north and magnetic north at a place; it is printed inside the chart's compass rose and changes with position and slowly over time. Deviation is the angle between magnetic north and the vessel's own compass north, caused by the vessel's magnetism; it changes with the vessel's heading and is read from the deviation card. Each is labelled east or west.

Bowditch Ch. 6 §602

The Magnetic Compass — Variation and Deviation Converting a course between true and compass follows the sequence True - Variation - Magnetic - Deviation - Compass (T-V-M-D-C). When correcting from compass toward true, add easterly errors and subtract westerly ("correcting, add east"); when uncorrecting from true toward compass, reverse it — add westerly and subtract easterly. Variation moves between true and magnetic; deviation moves between magnetic and compass.

Bowditch Pub. 9 Ch. 28 §2806

Direction Direction is one of the elements of dead reckoning. A Figure 2803b. Small area plotting sheet with selected latitude scale. Angle 0 18 31 41 49 56 63 69 75 81 87 90 Factor 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Table 2804. Simplified traverse table. EMERGENCY NAVIGATION 447 deviation table for each compass, including any lifeboat compasses, should already have been determined. In the event of destruction or failure of the gyrocompass and bridge magnetic compass, lifeboat compasses can be used. If an almanac, accurate Greenwich time, and the necessary tables are available, the azimuth of any celestial body can be computed and this value compared with an azimuth measured by the compass. If it is difficult to observe the compass azimuth, select a body dead ahead and note the …

Bowditch Pub. 9 Ch. 8 §800

Changes in Compass Technologies This chapter discusses the major types of compasses available to the navigator, their operating principles, their capabilities, and limitations of their use. As with other aspects of navigation, technology is rapidly revolutionizing the field of compasses. For much of maritime history the sole heading reference for navigators has been the magnetic compass. However, a great deal of effort and expense has gone into understanding the magnetic compass scientifically to make it as accurate as possible through research and development of elaborate compensation techniques. Over time, technological advances like the development of more sophisticated means for obtaining accurate compass readings, such as the electro-mechanical gyrocompass, diminished traditional rel…

Bowditch Pub. 9 Ch. 8 §801

The Magnetic Compass and Magnetism The principle of the present day magnetic compass is no different from that of the compasses used by ancient mariners. The magnetic compass consists of a magnetized needle, or an array of needles, allowed to rotate freely in the horizontal plane. The superiority of present-day magnetic compasses over ancient ones results from a better knowledge of the laws of magnetism and how it governs the behavior of the compass and from greater precision in design and construction. Any magnetized piece of metal will have regions of concentrated magnetism called poles. Any such magnet will have at least two poles of opposite polarity. Magnetic force (flux) lines connect one pole of such a magnet with the other pole. The number of such lines per unit area represents th…

Bowditch Pub. 9 Ch. 8 §802

Terrestrial Magnetism Consider the Earth as a huge magnet surrounded by lines of magnetic flux connecting its two magnetic poles. These magnetic poles are near, but not coincidental with, the Earth’s geographic poles. Since the north seeking end of a compass needle is conventionally called the north pole, or positive pole, it must therefore be attracted to a south pole, or negative pole. Figure 802a illustrates the Earth and its surrounding magnetic field. The flux lines enter the surface of the Earth at different angles to the horizontal at different magnetic latitudes. This angle is called the angle of magnetic dip,θ, and increases from 0° at the magnetic equator to 90° at the magnetic poles. The total magnetic field is generally considered as having two components: H, the horizontal co…