Distance & GPS
Trigonometric equation computing the great-circle distance between two GPS points on Earth.
The Haversine formula (19th century) computes the straight-line distance between two surface points accounting for curvature. It takes latitude and longitude of origin and destination and returns meters. The basis of nearly every "as-the-crow-flies" calculation in apps. It ignores roads: real driving distance requires a routing API (Google Maps, OSRM, Mapbox).