Distance & Bearing Between Coordinates (WGS 84)

Enter two latitude and longitude pairs and get the distance between them along the ellipsoid GPS uses, the bearing to set off on and the bearing you arrive on, and the point halfway along the route — or give a start, a bearing and a distance and get where you end up. The distance is the geodesic on the WGS 84 ellipsoid, computed with Vincenty's formulae, and it was checked against GeographicLib, the reference implementation of Karney's algorithm: across three thousand random pairs it agreed to within a tenth of a millimetre and to a hundred-millionth of a degree in bearing. Beside it the tool shows the haversine distance on a sphere of the mean Earth radius, the formula most online calculators and most code snippets use, and the percentage by which it misses. The miss depends on where you are and which way you go: JFK to Heathrow comes out 0.27% short on the sphere, a route along a meridian 0.27% long, and short hops near the poles close to half a percent out. The initial and final bearings differ because a geodesic — the shortest route over the curved surface — is not a line of constant compass heading; flying from New York to London you leave heading northeast at 51° and arrive heading east of southeast at 108°. The midpoint is the point halfway along that route, not the average of the coordinates, which lies somewhere else entirely for long distances. One case is handled honestly rather than hidden: for points almost exactly opposite each other on the globe, many routes of nearly equal length join them and Vincenty's iteration does not settle, so the tool says so and gives the spherical estimate instead. Coordinates can be typed in decimal degrees or in degrees, minutes and seconds with a hemisphere letter.