A method of generating a geometric heading during positioning is provided. The method includes the following steps: estimating a current declination information of a current position according to a declination database, wherein the declination database gathers a magnetic map and a plurality of declination information corresponding to a difference between a magnetic north and a geometric north at all grid positions on the magnetic map; generating a predicted heading according to an angular velocity reading of a gyroscope and a magnetic heading reading of a magnetometer; and generating the geometric heading according to the current declination information of the current position and the predicted heading.