What is a great circle map?

A great circle map plots the shortest path between two points on the Earth's surface. Because the Earth is a sphere, the shortest route between two airports isn't a straight line on a flat map — it's an arc of a "great circle": a circle whose center passes through the center of the Earth. The equator and every line of longitude are great circles; the shortest route between any two cities follows one.

Why do flight routes look curved?

On common flat map projections (like Mercator), great circle routes appear to bow toward the poles. A flight from New York to Tokyo looks like it detours over Alaska — but that arc is actually the most direct track, thousands of miles shorter than the "straight" line on the flat map. That's why airliners crossing the Atlantic or Pacific fly what looks like a curve: they're flying the great circle.

Great circle distance

The great circle distance is the length of that shortest arc, usually quoted in nautical miles (nm) in aviation, or in miles and kilometers for everyone else. It's the baseline airlines and route planners use for range, block time, and fuel estimates. You can look up popular route distances or plot any pair of airports on the great circle map.

How to use this great circle mapper

Enter airports or routes in the search box on the home page. The mapper accepts IATA codes (SFO-JFK), "to" phrasing (JFK to NRT), city names (London-Paris), multi-stop routes (SFO-NRT-HND-ICN), and comma-separated combinations. Each leg is drawn as a great circle arc with its distance in nautical miles, and the URL updates so you can share the exact map.

gcmap, GC map, great circle mapper — same idea

Aviation enthusiasts have long shortened "great circle map" to gcmap or GC map. Whatever you call it, gcmap.io is a free, modern great circle mapper: interactive, mobile-friendly, secure, and fast — no signup required.