Rhumb Lines: Setting it Straight with Loxodromes

rhumb lines loxodrome mercator

Last Updated: Feb 2, 2017 @ 5:19 am

Rhumb Lines (Loxodromes)

When pilots want to fly a constant track direction, they’ll follow a rhumb lines (or loxodrome).

Rhumb lines have constant bearings and cross all meridians at the same angle.

Despite how rhumb lines look as if they are the shortest distance in certain map projections, they aren’t when travelling long distances on a sphere like the Earth. This is because the shortest distance is that of a great circle.

Now that you have a definition of a rhumb line, let’s look into it with a bit more detail.

Rhumb Lines on Map Projections

A rhumb line on a Mercator projection is a straight line. In the example below, there are 2 rhumb lines extending from New York to Madrid and London.

new york london madrid rhumb line

This is the reason why the Mercator projection is a popular map for sailors and pilots. Because the Mercator projection preserves angles and shapes (which makes the projection conformal), navigators can follow a single compass bearing resulting in a straight line.

We illustrate a rhumb line cutting all meridians at the same angle in a Mercator projection below:

rhumb line meridian angle

Now how about those same rhumb lines in a different map projection?

Here is how they look in the North Pole Azimuthal Equidistant map projection with a rhumb line to the north pole. Quite different looking, aren’t they?

READ MORE: Map Projections: Flatten the Sphere

polar projection rhumb line

Rhumb lines appear on the surface of the Earth as a curved line concave to the nearer pole. They spiral on a polar projection but it’s because of the projection how it still crosses all meridians at the same angle.

During short routes, the difference in distance between rhumb lines and great circles are almost unnoticeable. One of the problems for navigating a great circle is the need for constant changes in the course direction. That makes rhumb lines a simple method from getting from A to B.

READ MORE: Great Circle: Why are Geodesic Lines the Shortest Flight Path?

Conclusion

As an imaginary line on the earth’s surface, rhumb lines are a standard method of plotting a ship’s course on a chart.

However, a rhumb line is not the shortest distance between two points on a sphere. The shortest distance is called a great circle.

In mathematical terms, rhumb lines are called loxodromes from the Greek term for running at an inclined angle.

In the advanced toolbar in ArcGIS, you can construct these types of lines as a geodesic line. Try it out for yourself.

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