The compass is a fundamental instrument that operates by aligning its magnetic needle with Earth's magnetic field. This alignment facilitates navigation and orientation, offering a means to determine direction relative to magnetic north. However, the magnetic needle points to magnetic north, which differs slightly from true geographic north due to magnetic declination, which is the angular deviation between these two points. Declination varies based on geographic location and shifts over time due to the dynamic nature of Earth's magnetic field, which arises from convection currents in the liquid outer core.

The compass was indispensable for navigation and surveying before advanced instruments such as sextants and transits were invented. Surveyors often relied on a specialized compass, the surveyor's compass, to measure bearings. This device operated by aligning the magnetic needle with the magnetic north, after which the bearing could be read on a graduated circular scale. Bearings obtained in this manner provided critical data for mapping and land division.

The magnetic needle's sensitivity is essential for precise readings. To achieve this, the needle's pivot is carefully designed to minimize wear, while its ends are counterbalanced to prevent dipping due to Earth's magnetic inclination. Accurate readings also depend on avoiding interference from nearby metal objects, which can distort the magnetic field. Surveyors addressed this issue by measuring bearings from multiple points and averaging the results to account for anomalies.

Although largely supplanted by modern electronic tools such as GPS and theodolites, the compass retains historical significance. Its reliance on Earth's magnetic field highlights the intricate relationship between natural geophysical phenomena and technological innovation.