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The first is magnetic declination or variation—the angular difference between magnetic North (the local direction of the Earth's magnetic field) and true North. [1] The second is magnetic deviation —the angular difference between magnetic North and the compass needle due to nearby sources of interference such as magnetically permeable ...
Deviation is positive if a compass bearing mark (e.g., compass north) is right of the related magnetic bearing (e.g., magnetic north) and vice versa. For example, if the boat is aligned to magnetic north and the compass' north mark points 3° more east, deviation is +3°.
4 - Compass north, including a two-part error; the magnetic variation (6) and the ship's own magnetic field (5) 5 - Magnetic deviation, caused by vessel's magnetic field. 6 - Magnetic variation, caused by variations in Earth's magnetic field. 7 - Compass heading or compass course, before correction for magnetic deviation or magnetic variation.
It should be very close to the magnetic bearing. The difference between a magnetic bearing and a compass bearing is the deviation caused to the compass by ferrous metals and local magnetic fields generated by any variety of vehicle or shipboard sources (steel vehicle bodies/frames or vessel hulls, ignition systems, etc.) [4]
If the Sun is observed at some time between sunrise and sunset, its altitude must also be recorded to calculate the true azimuth. The true azimuth may be compared to the magnetic azimuth to find the magnetic declination, the angle between the direction that the compass indicates as north and the true north direction. [3]
6 – Magnetic variation, caused by variations in Earth's magnetic field. 7 – Compass heading or compass course, before correction for magnetic deviation or magnetic variation. 8 – Magnetic heading, the compass heading corrected for magnetic deviation but not magnetic variation; thus, the heading reliative to magnetic north.
Scintillation occurs most frequently at tropical latitudes where it is a night time phenomenon. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to scintillation. [23] In addition to producing scintillation, magnetic storms can produce strong ionospheric gradients that degrade the accuracy of SBAS systems.
This card, or graph, lists the deviation for various compass courses and is referred to by the navigator when compass courses need to be corrected. It is named after the English navigator Matthew Flinders (1774-1814) who wrote a paper on ships' magnetism for the Royal Navy .