How to Calculate True South With an Azimuth

The compass is one of the 10 essentials for outdoor survival. Knowing how to properly use it requires an understanding of declination. True north, also known as geographic north, is the axis on which the Earth spins, but compasses actually point to magnetic north, a moving position on the Earth formed by magnetic elements in the Earth's core. The difference between these two is called magnetic declination and knowing how to calculate it can make the difference between finding your way to true south or getting lost.

Things You'll Need

  • Compass
  • Map
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Instructions

    • 1

      Read the compass rose on the map for your area and note the magnetic declination, the mean annual change and the date that the map was printed.

    • 2

      Perform the following calculation. This example assumes a declination of 19.8 degrees west in 1987 (the date the map was printed) and a mean annual declination of one minute. The reading was done in the year 2011:

      One minute x 24 (number of years since 1987) = 24 minutes (mean annual change)

      19 degrees 48 minutes (the same as 19.8 degrees because there are 60 minutes per degree) - 24 minutes = 19 degrees 24 minutes

      Round down to the nearest degree. Therefore, magnetic declination is 19 degrees west.

    • 3

      Hold your compass so the sight is pointed away from you. Turn your body until you line up the sight to 180 degrees plus or minus the magnetic declination. Magnetic south is always at an azimuth bearing angle of 180 degrees, but if you are in a place that has a magnetic declination, you have to make the calculation to arrive at true south. Assuming the above example of 19 degrees west declination, you will line up the sight to 199 degrees. You are now facing an azimuth of true south.