Great Circle Routes

Drag two airports on a flat map or a globe and see the shortest route between them bend, with its length and bearing checked.

Why the route looks bent

A flat map has to stretch the sphere. On this one (equirectangular, longitude and latitude used as flat coordinates) a straight ruler line is not the shortest way. The shortest path on a sphere is an arc of a great circle: the circle you get by slicing the sphere through its centre. Drawn on the map, such an arc usually bulges toward the nearer pole: east-west distances shrink as you go poleward, so a detour toward the pole saves more than it costs. On the globe the same route is simply the tightest string you could pull between the two points.

The page measures the gap for the pair you chose: the dashed map-straight line is cut into short pieces, each piece's length is taken along the sphere and they are added up, then compared with the great circle. Winds, airspace and safety rules also shape real flight tracks; none of that is modelled here.

Checks the page runs on itself

Computed in your browser when the page loads. Every count and figure below is computed from the page's own data and code, except the web-search figures, which are typed in from a search and labelled as weaker.

Airport pairs, checked

Distances in km unless marked. "Sphere" is haversine on a sphere of radius km; "ellipsoid" is Vincenty's inverse formula on WGS84; "Karney" is GeographicLib's geodesic code (a separate algorithm and implementation, vendored, run here). "OpenFlights coordinates" repeat the haversine with each airport's position taken from a second coordinate file.

How this differs from Tissot Circles

Tissot Circles asks how a projection distorts: it puts circles of equal true size on Mercator and Equal Earth and measures how their shape and area change. This page asks the opposite question about one shortest path: given two points, where does the shortest route go, and how long is it, drawn on a plate carree map and a globe? Tissot Circles is about area and shape; this is about distance, bearing and the sphere-versus-ellipsoid gap. They share only the Natural Earth outline source.

What is not verified

Made by Claude, an AI model by Anthropic. Code: this page's own code under the repository's licence, and the vendored libraries under their own (d3-geo and d3-array, ISC; GeographicLib, MIT; licence files are in the folder). Data: Natural Earth land (public domain), OurAirports (public domain), OpenFlights airport positions (Open Database Licence), used only as the second coordinate source. See README.md.