TM 55-1905-223-24-14
(1) Deviation. The compass needle, free to turn horizontally, tends to align itself with the earth's magnetic lines of
force. It is not free to do so in a steel vessel; such vessels have marked magnetic properties of their own, and these tend
to deflect the compass from the magnetic meridian. The divergence caused between the north-south axis of the
compass card and the magnetic meridian is called deviation. Even in a vessel made of wood or fiberglass there is
enough magnetic material on board (engines, fuel and water tanks, and rigging) to cause deviation.
(a) Electrical. The possibility of deviation from electrical circuits cannot be overlooked. Direct currents (dc) flowing in
straight wires establish magnetic fields. Care must be taken that all wiring in the vicinity of a magnetic compass is
properly installed to eliminate or reduce any effect on the compass.
(b) Variation. Although deviation differs from variation in that variation is caused by the earth's magnetism, the two
are designated in the same manner. If no deviation is present, the compass card lies with its axis in the magnetic
meridian and its north point indicates the direction-of magnetic north. If deviation is present and the north point of the
compass points eastward of magnetic north, the deviation is named easterly and marked E. If it points westward of
magnetic north, the deviation is named westerly and marked W. The navigator can easily find the correct variation by
referring to the compass rose on the chart of the vessel's position. Deviation, however, is not as easy to recognize. It
varies not only on different vessels, but on any particular ship it varies with changes in the vessel's heading. It also
changes with large changes in the vessel's magnetic latitude.
(2) Compass Error. The algebraic sum of variation and deviation is compass error. The navigator must understand
thoroughly how to apply variation, deviation, and compass error, in order to use them in converting one kind of direction
to another. There are three ways in which a direction can be expressed:
(a) As true, when the true (geographic) meridian is the reference of measurement.
(b) As magnetic, when the magnetic meridian is the reference of measurement.
(c) As compass, when the axis of the compass card is the reference of measurement.
(d) Any given direction may be expressed in all three of these ways, if it is understood that true differs from magnetic
by variation, magnetic differs from compass by deviation, and compass differs from true by compass error.
b. Coefficients of Deviation. The total local magnetic effects which cause deviation of a vessel's compass can be
broken down into a series of components termed coefficients. Soft iron is material in which magnetism is induced by the
earth's magnetic field. This magnetism changes as its orientation with respect to the earth's magnetic field changes. On
the other hand, the vessel's hard iron has the relative permanent magnetism acquired during construction and fitting-out.
Soft and hard iron are also classified as horizontal or vertical, according to the orientation of their magnetic axes when
induced by components of the earth's field.
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