When using a compass, what is a critical function to ensure accuracy?

Calibration for local magnetic declination is essential for accurate navigation with a compass because it addresses the discrepancy between magnetic north (the direction a magnetic compass points) and true north (geographic north). Since the Earth's magnetic field varies in different geographic locations, understanding this declination allows navigators to correct their compass readings to reflect true bearings.

When a navigator fails to account for magnetic declination, their compass readings can be skewed, leading to inaccurate course plotting and potential navigation errors. This is particularly important in offshore or remote navigation where accurate positioning is critical for safety. By adjusting for local magnetic declination, a navigator can ensure that the compass readings align more closely with true geographic positions, significantly enhancing navigational accuracy.

The other options, while related to using a compass effectively, do not directly address the fundamental need to align compass readings with true north, which is essential for precise navigation.