This JavaScript applet calculates distances and directions from one location on the surface of Earth towards another, both the ‘as the bird flies’ surface distance, and the ‘as the neutrino flies’ straight-line distance.

Azimuth refers to the compass heading when leaving your location traveling towards another along a great circle path (shortest surface route), where North=0°, East=90°, South=180° and West=270°. Back azimuth refers to the initial heading when leaving the distant location to come to where you are.

Inclination refers to the angle below horizontal of the straight line path to a distant location, where 0° is horizontal & local, and 90° points straight down through the center of the Earth to the antipode.

The straight line direction to another location on Earth may be pointed out with the two angles of azimuth and inclination.

Calculations assume Earth to be spherical with a 6366.707 km radius. If you come across any errors or have some ideas of how to improve the code, feel free to pass along your observations. This is Version 0.11, so everything might not work as well as it could…