Why do nautical charts have two compass roses on them?
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Nautical charts have two compass roses on them to provide mariners with different information related to navigation. The primary reason for having two compass roses is to account for magnetic declination.
Magnetic declination is the angle difference between magnetic north and true north at a specific location. Magnetic north is the direction indicated by a compass needle, which is influenced by the Earth's magnetic field. True north, on the other hand, corresponds to the Earth's geographic North Pole.
To account for magnetic declination, nautical charts have two compass roses. One of the compass roses depicts true north, providing mariners with the essential reference point for navigation. This allows them to align their charts with the geographic features on the Earth's surface accurately.
The other compass rose indicates the magnetic north, accounting for the local magnetic declination. Mariners can use this information to correct their compass headings by adjusting for the angle of magnetic declination. This ensures that navigational courses are accurate and align with magnetic bearings.
Having both compass roses on nautical charts is crucial, as it enables mariners to navigate effectively and safely, taking into account both true and magnetic north references. They can then make the necessary adjustments to their compass readings based on the specific magnetic declination for that location.