What is a compass rose?

Have you ever looked at a map or nautical chart and noticed a circular symbol with arrows and letters? This is called a compass rose, and it’s used to indicate direction. A compass rose is an important tool to help people navigate — especially while at sea, where there may be no visible landmarks.

A compass rose on a nautical chart showing circular degree markings, the direction of true and magnetic north, and surrounding water depth soundings. — A compass rose, often found on nautical charts, appears as two rings, one smaller and set inside the other. The outside ring denotes "true" directions, and the smaller inside ring represents magnetic directions. True north refers to the geographical location of the North Pole (on a map) while magnetic north is the direction that a compass needle (or a magnetic object) will point. The difference between true and magnetic north varies depending on location.

While there are no strict rules for what a compass rose should look like, on nautical charts, they are generally shown as a smaller circle set inside a larger circle. The larger outer circle shows the direction of the true north or the location of the North Pole in relation to the area shown on the chart. The location of true north always stays the same. The smaller inner circle indicates the direction of magnetic north, which is the direction a compass needle points. Magnetic north is different from true north because of Earth’s magnetic field. The direction of Earth's magnetic field changes over time. Therefore, unlike true north, the location of magnetic north changes.

Because of this difference, navigators have to consider something called magnetic declination, sometimes called the variation, which is the angle of difference between true north and magnetic north, when plotting a course. Depending on where you are in the world, the angle between true north and magnetic north will differ. Declination is considered to be positive if magnetic north is east of true north and negative if it’s to the west. To make sure they stay on course, navigators must adjust their compass reading based on the declination value for their location. In addition to showing the direction of true and magnetic north, a nautical chart will indicate the declination value and the year it was calculated. Since the location of magnetic north changes over time, knowing the year a chart was created is important to understand the location of magnetic north at that time. For mariners traveling very long distances across the ocean, even a small error in declination could result in being hundreds of miles off course.

Understanding the parts of a compass rose and what they mean is essential for safe navigation. On the ocean, there are no roads or street signs. Therefore, mariners must rely on paper nautical charts and compasses to guide their way if they don't have GPS or electronic navigation devices on board. Without knowing how to read a compass rose on a chart or account for magnetic declination, vessels could easily sail off course.