What is the purpose of the outer rings on a compass rose on a chart?

The outer rings on a compass rose on a chart are designed primarily for determining true degrees. This feature is essential for navigators as it allows them to accurately interpret direction relative to true north, which is considered a fixed point on the Earth's surface. By aligning a vessel or a navigation tool with the compass rose, mariners can ensure they are heading in the correct direction based on geographic coordinates, rather than magnetic headings which may be influenced by variations in the Earth's magnetic field.

In contrast, the inner elements of a compass rose typically relate to magnetic directions, while the outer circle usually represents true direction. This distinction is critical for effective navigation, particularly in areas where magnetic deviation may affect the compass readings. Understanding the true degrees helps in planning courses and safely navigating to desired locations.