Enter any 2 values (true bearing, magnetic bearing, and/or magnetic declination) into the calculator to determine the missing value. This calculator helps find the angular difference between true north (geographic) and magnetic north. Sign convention used: declination is positive for East and negative for West.
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Magnetic Declination Formula
The following formula is used to calculate the magnetic declination (using the common sign convention where East declination is positive and West declination is negative):
D = TB - MB
Variables:
- D is the magnetic declination (degrees; positive = East, negative = West)
- MB is the magnetic bearing (degrees)
- TB is the true bearing (degrees)
To calculate the magnetic declination, subtract the magnetic bearing from the true bearing (then interpret the sign: + = East, − = West).
What is Magnetic Declination?
Magnetic declination, also known as magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a compass needle points) and true north (the direction along the Earth’s surface towards the geographic North Pole). This angle varies depending on position on the Earth’s surface and changes over time.
How to Calculate Magnetic Declination?
The following steps outline how to calculate the Magnetic Declination:
- First, determine the true bearing (TB) in degrees.
- Next, determine the magnetic bearing (MB) in degrees.
- Use the formula D = TB – MB to calculate the magnetic declination (D).
- Finally, enter the values into the calculator to check your result.
Example Problem:
Use the following variables as an example problem to test your knowledge.
True Bearing (TB) = 120 degrees
Magnetic Bearing (MB) = 115 degrees
Magnetic Declination (D) = TB – MB = 120 – 115 = +5° (5°E).