Marine Glossary

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great circle route

The shortest route between two points on a spherical earth, which distinguishes it from the other common route called rhumb-line route.

The great circle concept comes from imagining slicing the earth in half with a plane through the earth that goes through the departure, the destination, and the center of the earth. The intersection of that plane with the surface of the earth is the great circle route between the two points, and indeed on a truly spherical earth that is the shortest distance between them.

On a mercator chart, the great circle route is a curved track, meaning the true heading changes along the route, compared to a rhumb line route, which maintains a constant true heading from start to finish.

The great circle route is always considered as a first step in planning any ocean crossing, but it is rarely one that can be followed, or is chosen to be followed, by power or sailing vessels. First, the savings in distance compared to the simpler rhumb-line route is only significant if both the departure and destination are at high latitudes, and the distance is more than 2,000 miles, as illustrated in this video Great Circle vs Rhumb line route. Most voyages are planned with simple rhumb-line routes through prevailing wind patterns. See rhumb line route.

Note that the term "shortest distance" is a bit outdated in these days of precision GPS units, which allow users to select the shape of the earth ellipsoid. The truly shortest distance between two points on earth is called "ellipsoidal distance," a computation that is more complex and dependent upon the actual geoid selected. See article in the Library that shows that WSG84 distance between two points about 3,000 miles apart can differ from the great circle distance by as much as 7 or 8 miles. See ellipsoidal distance.

In qtVlm when you draw a region select box, which can be over a few meters or a few thousand miles, you get a report of both the great circle distance and initial heading and the rhumb line distance and its constant heading.

Great circle routes (GC) are often compared to rhumb line routes (RL) in navigation planning, so recall the basics that a RL is the direct straight line between the two points and it will have a constant true heading at every point on the route, whereas the GC route will always be shorter but it will curve toward the pole. To navigate a GC route we must break it up into a series of RL routes and then steer the constant RL heading along each leg. These headings will be high (more poleward) on the direct RL at first, and then low on the direct RL toward the end.

Abbreviation: GC