The page allows you to draw accurate ellipsoidal geodesics on Google Maps. You can specify the geodesic in one of two forms:
The sample data has lat1, lon1 in Wellington, New Zealand, lat2, lon2 in Salamanca, Spain, and s12 about 1.5 times the earth's circumference. Try clicking on the "compute" button next to the "Direct:" input box when the page first loads. You can navigate around the map using the normal Google Map controls.
The Javascipt code for computing the geodesic lines, circles, and envelopes is part of GeographicLib. The algorithms are derived in
Charles F. F. Karney,In putting together this Google Maps demonstration, I started with the sample code geometry-headings.html.
Algorithms for geodesics,
J. Geodesy 87(1), 43–55 (Jan. 2013);
DOI: 10.1007/s00190-012-0578-z (pdf);