Rhumb line
A rhumb line is a path on a sphere that crosses every meridian at the same angle. It appears as a straight line on a Mercator map.
Loxodrome: A curve on a surface that intersects every meridian at a constant angle. Loxodrome is the mathematical name commonly used for a rhumb line.
Nautical chart: A map designed for marine navigation, showing coastlines, depths, hazards, and navigational aids. Mercator nautical charts let navigators plot constant-bearing courses as straight segments.
Great circle: A circle on a sphere whose plane passes through the sphere's center; its arcs are the sphere's shortest paths. A great-circle arc usually saves distance but does not maintain a constant compass bearing.
Sphere: A three-dimensional surface whose points are all the same distance from a center. The classical rhumb line is defined on a spherical model of Earth.
Great-circle navigation: Navigation along great-circle arcs, which are the shortest paths between points on a sphere. Unlike a rhumb line, a great-circle route generally changes bearing.
Dead reckoning: A method of estimating position by advancing a known position using course, speed, and elapsed time. A maintained rhumb-line bearing supplies the course used in dead reckoning.
Geodesic: A locally shortest path between points on a curved surface. Rhumb lines are one family of paths on a sphere, distinct from its shortest geodesics.
Longitude: An angular coordinate measuring east–west position relative to a reference meridian. Meridians are the coordinate lines crossed at the rhumb line's fixed angle.
Bearing: A direction expressed as an angle measured clockwise from a reference direction, usually north. A rhumb line is defined by maintaining one bearing throughout its course.
Compass: An instrument that indicates direction relative to magnetic or geographic north. A compass course can hold the fixed direction that defines a rhumb line.