What does GM Angle specifically measure?

The GM Angle, or Grid-Magnetic Angle, specifically measures the difference between grid azimuth and magnetic azimuth. This angle is crucial for navigation as it allows a user to convert between the two types of azimuth readings. Grid azimuth refers to angular measurements relative to a grid system, often found on maps, while magnetic azimuth relates to the Earth's magnetic field. Understanding this angle is essential for accurately determining directions when navigating in the field, especially when using a map and compass. By knowing the GM Angle, one can effectively orient their compass to align with the grid on their map, ensuring correct navigation and positioning.