Boussole

The compass, a vital direction-finding instrument, relies on Earth's magnetic field for its operation. Its needle aligns with magnetic north but deviates slightly from astronomic north, with the deviation varying by location.

This deviation is called magnetic declination, influenced by Earth's magnetic field, which originates from currents in its liquid outer core. Declination changes over time and with location, necessitating adjustments for accurate readings.

Historically, the compass was essential for navigation and surveying before advanced instruments like sextants and transits emerged. 

A surveyor's compass was commonly used to determine bearings by aligning its needle with magnetic north. Bearings were then read on a circular scale.

The needle's sensitivity is critical, and it is ensured by minimizing wear on its pivot and counterbalancing its ends to prevent downward dips. 

Magnetic interference from nearby metal objects can distort readings, a challenge addressed by measuring bearings from multiple points.

Though largely replaced by modern instruments, the compass remains a cornerstone in surveying history, illustrating the enduring significance of Earth's magnetic field.