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Magnetic Declination

What is magnetic declination and why it matters

Magnetic declination is the difference between true north and magnetic north. It changes depending on where you are standing on the earth. Bushwalkers need to take this into account when using a map and compass, as even a difference of 10° in a navigation bearing can lead to substantial navigation errors.

To recap:

  • True North is north, according to the earth’s axis. True north points to the North Pole.
  • Grid North refers to the direction northwards along the map projection grid lines. This comes from a 3-D object (the earth) being depicted on a 2D object (a map).
  • Magnetic North is the direction that a compass needle points to. Magnetic north is the direction of the Earth’s magnetic field lines.

The difference between True North and Magnetic North is referred to as the Magnetic Declination.

Bushwalkers can compensate for this difference by using charts of declination or local calibration that show the correct conversion (and rate of annual change). The easiest way to determine this is to use the tool in the map key that looks something like this:

The difference between grid north and true north is typically within 2°. For bushwalkers, grid north is the more relevant measurement as it allows you to use the UTM grid lines as your north reference (assuming that 2° difference between grid north and true north is an acceptable margin of error – most of this time in a bushwalking context this is fine). If the map does not have the numbers, but only intersecting lines, place the compass on the lines and measure the difference (for Sydney region, it’s roughly 12°).