This sensor is a digital magnetic compass that measures the Earth's magnetic field to find the position of the Earth's magnetic poles. This sensor outputs a value from 0 to 359 degrees, which indicates the sensor's current absolute heading (the angle that the sensor faces) based off of magnetic north. A heading value of 0 degrees means that the sensor is facing due north; 90 degrees indicates that the sensor is facing east, 180 degrees is south, and 270 degrees is west:

The sensor also has a relative heading feature, which allows the user to set a target heading. The target heading becomes the new zero position. Relative heading values range from -179 to +180 degrees; a positive relative heading value indicates that the robot must turn right (clockwise) to reach the target direction, and a negative relative heading value means that the robot must turn left (counterclockwise). A relative heading value of zero indicates that the robot is perfectly on track with the target and does not need to adjust its trajectory. The GPS Car will rely on the relative heading feature in its navigation program.