BEARING CONVERSION
DIRECTION
THE MAGNETIC DECLINATION IS NEGATIVE
WHY CONVERT BETWEEN MAGNETIC AND GRID BEARINGS
Bearings taken with a compass are taken with reference to Magnetic North. Bearings measured on a map are taken with reference to Grid North (or True North for non-UTM maps). The Magnetic Declination at the location and time must be taken into account.
This is where the declination diagram drawn on the map comes into play.
DECLINATION DIAGRAM
The declination diagram for Perth is shown below. The values are correct for 2000.
The magnetic North is west of True North (and not east as the diagram states): the Magnetic Declination and Grid-Magnetic Angle are negative.
The Grid Convergence is 0.4°. The angle between True North and Grid North does not change with time.
The Grid/Magnetic Angle is 2.1° in 2000. This angle change with time as the magnetic field varies.
The Magnetic Declination (the angle between True North and Magnetic North) in 2000 was 2.1° – 0.4° = 1.7°.
In 2023, the Magnetic Declination is 1.66° in Perth and is moving in a westerly direction.
REMEMBER
- The angle of interest is the Grid/Magnetic Angle: the angle between Magnetic North and Grid North.
- Angles are always measured clockwise from the north-south line.
CONVERTING GRID BEARINGS TO MAGNETIC BEARINGS
You have measured a Grid Bearing on your map and you want to set this bearing on your compass.
The Grid Bearing is measured with respect to Grid North and is shown as a black arc
The Magnetic Bearing is measured with respect to Magnetic North and is shown as a dashed arc.
Note: this diagram is an illustration and is not to scale.
From the diagram, it is easy to see that magnetic bearing is larger than the grid bearing: we need to add the Grid/Magnetic angle (2.1°) to the Grid Bearing to calculate the Magnetic Bearing.
GRID BEARING + GRID/MAGNETIC ANGLE = MAGNETIC BEARING
CONVERTING MAGNETIC BEARINGS TO GRID BEARINGS
You have taken a Magnetic Bearing with your compass and you want to plot the Grid Bearing on the map.
The Magnetic Bearing is measured with respect to Magnetic North and is shown as a black arc
The Grid Bearing is measured with respect to Grid North and is shown as a dashed arc.
Note: this diagram is an illustration and is not to scale.
From the diagram, it is easy to see that Grid Bearing is smaller than the Magnetic Bearing: we need to subtract the Grid/Magnetic Angle (2.1°) from the Magnetic Bearing to calculate the Grid Bearing.
MAGNETIC BEARING – GRID/MAGNETIC ANGLE = GRID BEARING
IN SUMMARY
When the Magnetic Declination is negative, that is Magnetic North is moving in an westerly direction:
GRID BEARING + GRID/MAGNETIC ANGLE = MAGNETIC BEARING
MAGNETIC BEARING – GRID/MAGNETIC ANGLE = GRID BEARING
