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How Marine Compasses Work

Types of Compasses

Deviation, Declination and Variation

Swinging the Compass

Using a Hand Bearing Compass

Compass Deviation and Variation

The three Norths

Here are three terms frequently encountered when using compasses to find bearings for navigation. Are you clear what each one means? And what about the three norths, too?

True north (Geographic north) is the location of the earth's axis of rotation and is the basis for lines of latitude and longitude (90°N, where all meridians of longitude intersect).

Magnetic north is the north direction shown on a compass determined by the earth's internal magnetic field. The true north pole and the magnetic north pole are currently about 1500km apart and magnetic north shifts with time. The compass rose on older charts will therefore have a magnetic compass for that time period which is unlikely to be reflective of the current variation.

Grid north is the north direction taken from grid lines on a map or chart. These vary from exactly true north-south lines due to curvature of the earth's surface and the effect of representing this curvature as a flat surface.


Deviation is the error in reading a bearing from the compass caused by the magnetic influence of some nearby object, such as a metal post or an engine. In the case of boats it may even be due to the materials from which the boat is constructed. Electric currents close to the compass may create a magnetic effect. To avoid this type of error the compass should be kept away from all such influences. Most boats have some measure of deviation which should be minimised by careful mounting of the compass. Deviation changes each time the boat alters course. There are people skilled in compass adjustment who can issue a card for a particular compass with deviations listed for changes in direction. This is particularly important for headings where a significant deviation exists.

Variation (also known as Declination)

This is the error caused by the earth's magnetic field. For a magnetic compass, the needle will point towards magnetic north rather than true north. Depending on where you are on the earth's surface this difference may be as much as 30°. Variation may be to east or west of true north, again depending on where you are on the earth's surface. Variation also changes as time progresses (as the magnetic north pole changes position). This information is always shown on maps and charts (usually in the compass rose) with a base year and a statement to say 'increasing x each year'. 

A good exercise is to look at a chart, locate this information and calculate the current variation for the chart you have in hand. A well known rhyme by which to remember how to apply variation (declination) is: 

Variation east, compass least
Variation west, compass best


Did you Know?

  • A compass needle will experience 'dip' (also known as magnetic inclination) due to the spherical shape of the earth. At the equator the lines of magnetic force will be almost parallel to the earth's surface but as you move towards the poles they will be increasingly angled toward the surface giving the compass needle more 'dip'. In theory at the centre of the poles (north or south) the lines of magnetic force should be exactly perpendicular to the earth's surface. In reality what happens to the compass needle is that it becomes unstable, wanting to dip a great deal but unable to do so.
  • Compasses are specific and pre-set for whichever hemisphere they are to be used in. This is because the needle 'dips' in an opposite direction in the northern hemisphere (head down) to that in the southern hemisphere (tail down). 


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