地磁偏角是指地球上任一处的磁北方向和正北方向之间的夹角。当地磁北向实际偏东时，地磁偏角为正，反之为负。

地磁偏角在历史上最早由中国北宋科学家沈括发现并记录在著作《梦溪笔谈》中：「方家以磁石磨针锋，则能指南，然常微偏东，不全南也。」而西方最早的记录则在此之后约400年。

Dip circles (also dip needles) are used to measure the angle between the horizon and the Earth's magnetic field (the dip angle). They were used in surveying, mining and prospecting as well as for the demonstration and study of magnetism.

Georg Hartmann first discovered dip angle in 1544, when he noticed the needle on a compass dipped towards the north hemisphere. Rather than explore this phenomenon, Hartmann sought ways to eliminate it. However, Robert Norman around 1576 investigated dip angle further and in 1581 described in print a device to measure this phenomenon.