Online Google Maps Compass Application

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How to adjust a compass for magnetic declination or variation

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A compass does not point to the true north - except by coincidence in some areas. The compass needle is attracted by magnetic force produced in the outer liquid part of the Earth's core, which varies in different parts of the world and is constantly changing. The magnetic north pole is currently located in the Baffin Island region of Canada, and from the UK, is west of true north. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination or variation.

Positive and Negative Magnetic Declination or Variation

A diagram in the map margin will show the difference (declination) at the centre of the map between magnetic north and true north. Where magnetic north is west of true north, the compass needle is deflected to the left. Because the angle to magnetic north is less than true north (360°), this is a negative declination. Where magnetic north is east of true north, the compass needle is deflected to the right. Here, the angle to magnetic north is greater than true north (0°), and is therefore a positive declination. This concept is illustrated in the diagrams below.
Negative magnetic declination - magnetic north is west of true north Positive magnetic declination  - magnetic north is east of true north
Negative declination - magnetic north is west of true north Positive declination - magnetic north is east of true north
Magnetic declination must be compensated for by adding the declination to the compass bearing if it is negative or subtracting if it is positive.



Positive Magnetic Declination or Variation Adjustment

If the magnetic declination or variation is positive as shown for the Tucson Aircraft Graveyard below, clicking OK on the message alert will automatically compensate the compass bearing (180°) by deducting the declination value of 10° 28' 48" (10.48°). Tucson Aircraft Graveyard: positive magnetic variation or declination of 10° 28' 48" (10.48°)
Tucson Aircraft Graveyard: positive magnetic variation or declination of 10° 28' 48" (10.48°)




A compensation adjustment will be made to the compass direction of travel arrow and a reduced compass bearing value will appear at the bottom right of the map with 169.52° displayed in this example. Tucson Aircraft Graveyard: compass automatically compensated for the positive declination value of 10°28' 48" (10.48°)
Compass automatically compensated for the positive declination value of 10° 28' 48" (10.48°)

Note that only the compass bearing is adjusted, not the map or true bearing which remains unchanged at 180°.



Negative Magnetic Declination or Variation Adjustment

If the magnetic declination or variation is negative as shown for the Cape Town area below, clicking OK on the message alert will automatically compensate the compass bearing (29°) by adding the declination value of 24° 30' 36" (24.51°). Cape Town: negative magnetic variation or declination of 24° 30' 36" (24.51°)
Cape Town: negative magnetic variation or declination of 24° 30' 36" (24.51°)


A compensation adjustment will be made to the compass direction of travel arrow and an increased compass bearing value will appear at the bottom right of the map with 53.51° displayed in this example. Compass automatically compensated for the negative declination value of 24° 30' 36" (24.51°)
Compass automatically compensated for the negative declination value of 24° 30' 36" (24.51°).

Note that only the compass bearing is adjusted, not the map or true bearing which remains unchanged at 29°.