What does the Grid Magnetic Angle measure?

• A. The size of the angle between grid north and magnetic north
• B. The size of the angle between true north and grid north
• C. The size of the angle between true north and magnetic north
• D. The size of the angle between east and north

Answer: (A)

The Grid Magnetic Angle measures the angular difference between grid north—the reference direction of the map grid—and magnetic north, the direction toward the Earth's magnetic pole. This tells you how far you must rotate from grid bearings to align with magnetic bearings, which is essential when you’re using a compass while working with a map grid. It’s not the angle between grid north and true north (that’s grid convergence), nor the angle between true north and magnetic north (that’s magnetic declination), and it isn’t simply the angle between east and north. For example, if magnetic north lies 10 degrees east of grid north, the grid magnetic angle is 10 degrees, so a grid bearing of 30° would correspond to a magnetic bearing of 40° (assuming you add the GM angle when converting).