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Story: Magnetic field

Magnetic declination arrow

Magnetic declination arrow

This diagram appears on most topographic maps to show magnetic declination (the difference in angle between grid north and magnetic north). This is the correction that needs to be applied to a magnetic compass reading, to find true north. Grid north runs very close to true north, but because flat maps do not accurately represent curved surfaces, grid north and true north are slightly apart.

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How to cite this page

Tony Hurst, Magnetic field – The magnetic field and its direction, Te Ara – the Encyclopedia of New Zealand, https://teara.govt.nz/en/diagram/9218/magnetic-declination-arrow (accessed 4 June 2026).

Story by Tony Hurst, published 2 March 2009.

Comments

Richard Cullingworth
11 January 2011
The Grid Magnetic Angle diagram shows the difference between Grid North and Magnetic North. This is not the angle that is applied to Magnetic North to find True North unless Grid North and True North are exactly the same - sometimes they are not. Where Grid North and True North are not the same, some maps will show the difference (called Grivation) in the corners of the map. For airborne use in high latitudes (when GPS is not available), a grid is often superimposed on the map as the lines of longitude converge too much and magnetic compasses are unreliable. The aircraft navigation system is set to work on Grid North and there is always a significant difference between Grid North and True North.