Rhumb line
Also called: Loxodrome · Topic: Map Projections · NCERT: Class 11, Ch 4 "Map Projections"
Meaning
A rhumb line, also called a loxodrome, is a line that crosses every meridian at the same angle. So a ship following it keeps one constant compass bearing the whole way. On Mercator's projection, every straight line is a rhumb line. This makes Mercator charts very useful for navigation. But a rhumb line is usually longer than the great-circle route between the same two points. So on long voyages, navigators combine the two.
Example
A ship leaving Mumbai and holding one fixed compass bearing across the Arabian Sea is following a rhumb line. On a Mercator chart its path is a straight line. Navigators first draw the shortest route on a gnomonic chart, then copy it onto a Mercator chart as a chain of rhumb lines.
Don't confuse with
- Great circle: this is the shortest route between two points on the globe. It crosses meridians at changing angles, so the bearing keeps changing. It is a straight line on a gnomonic chart, not on a Mercator chart.