Simple Way to Calibrate a Magnetic Compass

No sailor goes out to sea without a compass. Compasses have been used since times immemorial as a navigational and directional aid. In simple terms, a compass is an indicator instrument that aligns itself to the magnetic field of the North Pole of the Earth, thereby pointing north. The geographical North Pole and magnetic North Pole of the Earth are not the same, and not located at the same point on the Earth. A compass aligns itself to magnetic north and not the geographical North Pole.

Compass Navigation

• Compass navigation is the system used by mariners to plot their course using a compass, along with charts, maps and other calculation methods. A normal compass is an indicator of the direction, but there can be variations and deviations involved with regard to the exact location. However, users can calculate the distances traveled along with the magnetic references and plot the course. Modern compasses include an additional indicator to show the direction of travel.

Components

• The basic design of a compass includes the following components:

  a) Magnetic needle, which is an arrow-shaped magnet aligned to the earth’s magnetic field.

  b) Needle pivot, on which the needle is secured in place.

  c) Dial and compass points, which show the four directional cardinal points.

Calibrating Magnetic Compass

• Due to magnetic field variations and effects, a compass may require calibration and correction of deviation, depending upon where it is used. Magnetic compasses point to the magnetic north pole which is different from the geographical North Pole location. The distances between the two poles is around 1000 miles and vary with reference to the location of the compass. Therefore, magnetic compasses are required to be adjusted or calibrated while using a map which shows the geographical poles.

Calibrating Magnetic Compass With Respect to a Map

• (a) Take a topographical map and place the compass on the map near the North Pole location.

  (b) Align the map’s geographical north arrow and the compass heading arrow in line.

  (c) Rotate the compass dial and align the magnetic north indicator with the above line.

  (d) Next, rotate the map and align the compass heading with the magnetic north line of the map.

  (e) Both the map and compass are now aligned and calibrated to geographical north.

  A ship’s steering compass is fitted with two iron balls to correct the effects of magnetic materials and help in calibrating the compass. The calibration process involves preparing a deviation card and using the ship’s north head to align with shore locations. Deviations are noted in the card, and the lubber line in the compass is then aligned to the direction of the ship's travel. Aviation aircraft also use similar methodology to prepare and use compass deviation cards.

Calibrating Electronic Compasses

• Electronic compasses are calibrated with the help of sensors and calibration tables, along with a computer program, which uses inputs from global positioning systems known as landmark positions, and the physical readings of the sensors to map the deviation along with a gyroscope to record 360-degree movement. The computer finally generates correctional values which are fed into the electronic compass for correction.