Magnetic deviation is an error that occurs when a magnetic compass is pulled away from the Earth’s natural magnetic field by nearby magnetic influences. This phenomenon introduces an angular difference between the direction the compass points and true magnetic north. For accurate navigation, especially in vehicles like ships or aircraft, this local magnetic interference must be understood and corrected. Failing to account for this error can lead to significant navigational mistakes.
Understanding Deviation Versus Variation
Magnetic compass errors are categorized into two distinct types: deviation and variation. Deviation is defined specifically as the local error caused by magnetic fields originating from the vessel or structure where the compass is mounted. This interference is unique to the compass’s immediate surroundings.
Variation, in contrast, is a global phenomenon representing the angular difference between true north and magnetic north. True north is the fixed geographic pole, while magnetic north is the location where the Earth’s magnetic field lines converge. The location of magnetic north slowly shifts over time, meaning the variation value is constant for a specific geographic location at a particular time.
Deviation is a function of the vessel’s heading and its internal magnetism, while variation is a function of the vessel’s geographic position on Earth. Navigators must correct for both of these errors to determine their true heading. A compass reading must first be corrected for deviation to find the magnetic heading, and that magnetic heading is then corrected for variation to determine the true heading.
Physical Sources of Magnetic Deviation
The primary sources of deviation stem from materials within the vessel that create their own magnetic fields, distorting the Earth’s field lines. These materials can be broadly separated into those that exhibit permanent magnetism and those that exhibit induced magnetism. Permanent magnetism, often called hard iron magnetism, is acquired by steel components during construction through processes like hammering and welding. This magnetism remains relatively constant regardless of the vessel’s heading, establishing fixed magnetic poles within the structure.
Induced magnetism, or soft iron magnetism, is temporary and changes based on the vessel’s alignment within the Earth’s magnetic field. Soft iron components, such as beams and girders, temporarily magnetize as the vessel turns, and the intensity and polarity of this induced field fluctuate with the heading. These magnetic effects are exacerbated by modern equipment, including electrical wiring, which generates fields from current flow, and electronic devices like speakers, batteries, or GPS units. Even small metallic objects stored near the compass can introduce noticeable magnetic interference.
Quantifying and Applying Deviation Correction
Since the deviation error is unique to each compass and changes with the vessel’s heading, navigators must precisely measure it through a process called “swinging the ship.” This procedure involves slowly steering the vessel through a full 360-degree circle while comparing the compass’s reading to a known, accurate bearing, such as one provided by a gyrocompass or a terrestrial landmark. By recording the difference between the compass heading and the known magnetic heading, the total error can be mapped.
The results of this measurement process are compiled into a Deviation Card or Deviation Table. This card lists the specific correction, typically in degrees East or West, that must be applied to the compass reading for a set of cardinal and intercardinal headings, often every 30 or 45 degrees. Navigators use this table during a voyage to convert the raw Compass Heading into an accurate Magnetic Heading.
Before creating the card, small compensating magnets and soft iron spheres are placed in the binnacle, the stand housing the compass, to physically reduce the largest components of the deviation. This process, known as compass adjustment, aims to neutralize the internal magnetic fields as much as possible. The Deviation Card then accounts for any residual errors that cannot be physically eliminated.