Magnetic deviation
Magnetic deviation refers to the angular error in a magnetic compass reading arising from local magnetic influences produced by the vessel, aircraft, or nearby equipment, which causes the compass needle to deviate from the true magnetic north direction.[1] Unlike magnetic variation (or declination), which is the fixed angular difference between true geographic north and magnetic north due to the Earth's uneven magnetic field, deviation is a variable error dependent on the heading and magnetic properties of the surrounding structure.[2] This phenomenon is particularly relevant in navigation, where accurate compass readings are essential for determining course over ground, and it affects both maritime and aviation contexts by introducing inaccuracies that can lead to navigational errors if uncorrected.[3] The primary causes of magnetic deviation stem from ferromagnetic materials, such as steel hulls, engines, or instruments, that become magnetized either permanently or temporarily, as well as from electrical currents in wiring or devices that generate electromagnetic fields.[1] In ships, for instance, components like anchors, propellers, or cargo can induce deviation that varies as the vessel changes heading, with permanent magnetism from the hull's steel creating consistent offsets and induced magnetism fluctuating with external fields or motion.[3] Similarly, in aircraft, avionics, ferrous parts, or even modifications like new installations can distort the compass, exacerbated by factors such as acceleration or turns that amplify magnetic dip effects.[1] These local disturbances are typically small but can reach several degrees, necessitating regular monitoring to ensure safety. To mitigate magnetic deviation, navigators perform a process known as "swinging the compass," where the vessel or aircraft is aligned to known headings—often using visual references like leading lights or GPS—and adjustments are made using compensating magnets or soft iron correctors to minimize errors across all directions.[3] The resulting corrections are recorded in a deviation card or table, which lists offsets for specific headings (e.g., 4° west at 220°), allowing pilots or mariners to apply them manually during operation.[3] In modern systems, gyrocompasses or electronic fluxgate compasses may reduce reliance on traditional magnetic ones, but deviation remains a critical consideration for backup navigation and regulatory compliance in both aviation and maritime standards.Fundamentals
Definition and Principles
Magnetic deviation refers to the angular error in a magnetic compass reading caused by local magnetic fields generated by the vehicle itself, such as a ship or aircraft, which interfere with the Earth's magnetic field.[4][5] This local interference distorts the compass needle's alignment, leading to a discrepancy between the indicated heading and the actual magnetic heading relative to the Earth's magnetic north.[6] Unlike magnetic variation, which arises from the angular difference between true north and magnetic north due to the Earth's geomagnetic field, deviation is a vehicle-specific effect that must be accounted for in navigation.[4] The fundamental principle underlying magnetic deviation is that a magnetic compass needle aligns with the direction of the total horizontal magnetic field at its location, which is the vector sum of the Earth's geomagnetic field (B_Earth) and the disturbing field (B_local) produced by nearby ferromagnetic materials or electrical systems in the vehicle.[4][6] When B_local is present, it alters the resultant field direction, causing the needle to point away from the true magnetic meridian by an angle known as the deviation angle (δ).[5] This deviation varies with the vehicle's heading because the orientation changes the relative components of B_local—typically reaching maximum values on east-west headings where transverse disturbing fields are most pronounced relative to the Earth's field.[4] To quantify this, the deviation angle δ is defined as the difference between the magnetic heading (H_m, the direction relative to magnetic north) and the compass heading (H_c, the reading on the compass): \delta = H_m - H_c This relation derives from the vector addition of magnetic fields: the compass heading H_c corresponds to the azimuth of the resultant field B_total = B_Earth + B_local, while H_m aligns with B_Earth alone; for small deviations, δ ≈ (component of B_local perpendicular to B_Earth) / |B_Earth|.[6][4] In navigation, the true heading (H_t, relative to true north) is then obtained by adding magnetic variation (V) to the magnetic heading: H_t = H_m + V, or equivalently, H_t = H_c + δ + V, emphasizing deviation's role as a local correction alongside the global variation.[5]Distinction from Magnetic Variation
Magnetic variation, also known as magnetic declination, refers to the angular difference between true north (geographic north) and magnetic north at a given location on Earth's surface, arising from irregularities in the planet's magnetic field due to its molten core and crustal anomalies. This angle varies by geographic position and over time, with changes occurring gradually due to shifts in Earth's magnetic poles, typically on the order of a few degrees per decade. In contrast, magnetic deviation is the error introduced in a compass reading by the local magnetic influences of the vehicle or craft in which it is installed, such as ferrous materials or electrical equipment, and it depends on the specific heading and configuration of that vehicle. Unlike variation, which is a fixed value for a particular location (though it evolves slowly), deviation fluctuates with the vehicle's orientation—for instance, it might be zero degrees when heading north but reach up to 10 degrees when heading east due to onboard magnetic fields. The distinctions between these two phenomena are critical for accurate navigation, as they affect compass reliability in different ways:| Aspect | Magnetic Variation | Magnetic Deviation |
|---|---|---|
| Cause | Geographic and temporal variations in Earth's magnetic field. | Local effects from the vehicle's ferrous materials, electrical currents, or nearby magnets. |
| Variability | Fixed for a specific location but changes slowly over years due to geomagnetic shifts. | Changes with vehicle heading, load distribution, and modifications; can be zero or up to 20 degrees depending on circumstances. |
| Correction Method | Determined from charts, models, or isogonic lines (lines of equal declination); applied globally for the area. | Measured onboard via compass swinging and compensated using deviation cards or adjustable magnets. |
| Examples | In London, variation is about 1° east as of 2025.[7] | On a ship, deviation could be +5° on a northerly heading but -3° on a southerly one, corrected by swinging the vessel through all headings. |
Causes
Ferromagnetic Influences
Ferromagnetic materials in vehicles such as ships and aircraft generate local magnetic fields that distort the Earth's magnetic field, leading to compass deviation. These influences arise primarily from permanent and induced magnetism in ferrous components, which alter the direction of the compass needle relative to the magnetic north. Permanent magnetism originates from hard iron elements that retain magnetization after exposure to external fields, while induced magnetism occurs in soft iron that becomes temporarily magnetized in proportion to the ambient field strength.[6][8] Permanent magnetism is caused by hard iron components, such as steel hulls in ships or engine blocks in aircraft, which acquire stable magnetic poles during construction or manufacturing when aligned with the Earth's field. This results in a fixed magnetic field that produces consistent deviation patterns, often semicircular in nature. For instance, the fore-aft component (denoted as B) creates north-seeking or south-seeking effects along the vehicle's longitudinal axis, while the athwartship component (C) generates east-west deviations perpendicular to it. Vertical permanent magnetism can also contribute to errors when the vehicle tilts.[9][6][8] Induced magnetism, in contrast, affects soft iron items like cranes, pipes, or structural beams, which do not retain magnetism but align with the Earth's field, producing magnetization that varies with the vehicle's heading and location. This leads to dynamic deviation, including semicircular effects from vertical soft iron and quadrantal deviations from horizontal placements, where errors are maximum in northeast/southwest or southeast/northwest quadrants. The intensity depends on the Earth's horizontal (H) and vertical (Z) field components, with stronger effects at higher latitudes due to increased dip angle.[9][6][8] The field components from ferromagnetic influences include north-seeking (fore-aft), east-west (athwartship), and vertical deviations. Vertical components become prominent during vehicle tilt, causing heeling error in ships, where rolling induces a horizontal force on the compass needle, maximum on north-south headings and varying with latitude. In aircraft, similar vertical effects arise from pitch or bank, exacerbating deviations from structural magnetism.[9][6] Specific examples illustrate these effects: In aircraft, engine mounts and steel fittings induce fields from both permanent and soft iron properties, leading to significant compass errors that can reach several degrees without adjustment.[9][6][8][10] Stray magnetic fields from sources such as ferrous tools, magnetic cargo, or unshielded permanent magnets in devices like loudspeakers can also induce significant deviations. For example, permanent magnets in speaker systems can alter compass readings by several degrees if placed nearby without shielding, as their static fields interact directly with the compass needle. Such stray influences are common in operational environments where portable equipment or cargo is moved, leading to unpredictable errors.Non-Ferromagnetic Sources
Non-ferromagnetic sources of magnetic deviation arise primarily from electromagnetic fields generated by electrical currents and equipment, which produce localized distortions in the Earth's magnetic field without involving ferrous materials. These effects are distinct from those caused by ferromagnetic influences, as they stem from dynamic electromagnetic induction rather than static material magnetization. In maritime and aviation contexts, such sources can lead to compass errors that vary with operational states, necessitating careful placement and shielding of equipment. Electrical currents flowing through wiring, motors, batteries, and generators create oscillating magnetic fields that induce compass deviations. For instance, alternating current (AC) systems in generators or large power circuits can produce fields strong enough to deflect compass needles, with deviations often exceeding 10 degrees during operation in aircraft due to electrically powered systems like alternators or heaters. In ships, similar currents from engine order telegraphs, voltage regulators, or minesweeping circuits contribute to transient errors when equipment is active. These fields are generated by the movement of charges, following principles of electromagnetism, and their intensity depends on current strength and proximity to the compass. Specific equipment such as radios, radar systems, and high-voltage lines further exacerbates these deviations by emitting localized electromagnetic fields. Radar transmitters and receivers, for example, can cause variable compass deflections in both ships and aircraft if positioned too closely, often requiring separate deviation cards for operation with and without such systems active. In older aircraft avionics, components involving electromagnetic deflection, like those in navigation displays, have been noted to contribute to similar localized fields. High-voltage lines in vessels or aircraft wiring loops amplify these effects, potentially leading to errors that demand equipment relocation or shielding to maintain compass accuracy within acceptable limits, typically ±10 degrees. Unlike the relatively steady deviations from ferromagnetic materials, those from non-ferromagnetic sources are highly variable, fluctuating with power usage, equipment activation, or movement. This intermittency—such as deviations appearing only when a radio transmitter is on or a motor starts—complicates navigation and underscores the need for dynamic monitoring in both ships and aircraft.Measurement and Compensation
Compass Swinging Procedures
Compass swinging, also known as compass adjustment or swinging the ship/aircraft, is an empirical process to determine and record the magnetic deviation of a compass by placing the vessel or aircraft on specific headings and comparing readings against known magnetic directions. This procedure accounts for onboard magnetic influences that cause the compass needle to deviate from true magnetic north.[6][10] In maritime contexts, the process begins with preparation in an open-water area free from external magnetic interference, such as steel structures or cranes, ensuring the vessel is on an even keel and all electrical equipment is secured to minimize transient effects. The ship is then maneuvered to align with the eight primary headings: cardinal points (north 000°, east 090°, south 180°, west 270°) and intercardinal points (northeast 045°, southeast 135°, southwest 225°, northwest 315°), using accurate references like a gyrocompass, GPS, or sun azimuth calculations from nautical almanacs. While steady on each heading for at least two minutes to avoid dynamic errors, the compass reading is recorded simultaneously with the known magnetic heading, which is derived by correcting the true heading for local magnetic variation. Deviation is calculated as the algebraic difference between the magnetic heading and the compass reading (Deviation = Magnetic Heading - Compass Heading), with results noted as east (E) or west (W).[6][11] Essential tools include the magnetic compass binnacle equipped with correctors such as fore-and-aft (B) and athwartship (C) magnets, Flinders bars, quadrantal spheres, and heeling magnets, all positioned per IMO Resolution A.382(X) to counter known deviation sources. Observations are logged on standardized forms like NAVSEA 3120/4 for tabulation, and environmental controls—such as calm seas, degaussing systems off during initial swings, and no nearby magnetic gear—are maintained to ensure accuracy. A deviation card or table is then produced for display near the compass, providing navigators with correction values for any heading.[6][12][13] Swinging is mandatory under SOLAS Chapter V, Regulation 19.2.1 for all ships, specifically after initial installation, when the compass becomes unreliable, following structural repairs or electrical alterations, and at intervals not exceeding two years per ISO 25862:2019 to keep residual deviation within limits (typically ≤5° for ships under 500 gross tonnage and ≤3° for larger vessels). For certain vessels, such as those transiting the Panama Canal, annual swinging is required with deviations not exceeding 7°. Lightning strikes or major refits also necessitate immediate re-swinging due to potential magnetic changes.[14][13][11] The following is a representative sample deviation table derived from a post-swinging record, showing residual deviations on the eight headings (values are illustrative based on typical adjustments):| Ship's Head (°) | Deviation (°) |
|---|---|
| 000 (N) | +3° E |
| 045 (NE) | +1° W |
| 090 (E) | -7° W |
| 135 (SE) | -4° E |
| 180 (S) | +2° W |
| 225 (SW) | +5° E |
| 270 (W) | -2° E |
| 315 (NW) | -1° W |