returns shortest distance between two geolocations

calculate-great-circle-distance.ts

interface Location {
  lat: number,
  long: number
}

// Converts from degrees to radians.
function toRadians (degrees: number): number {
  return degrees * Math.PI / 180
}

// Converts from radians to degrees.
function toDegrees (radians: number): number {
  return radians * 180 / Math.PI
}

/**
 *
 * https://www.movable-type.co.uk/scripts/latlong.html
 * Haversine formula to calculate great circle distance between two points
 * where φ is latitude, λ is longitude, R is earth’s radius (mean radius = 6,371km);
 * note that angles need to be in radians to pass to trig functions!
 */

export function calculateGreatCircleDistance (Location1: Location, Location2: Location): number {
  const { lat: lat1, long: lon1 } = Location1
  const { lat: lat2, long: lon2 } = Location2
  const R = 6371e3 // earth radius in metres
  const φ1 = toRadians(lat1)
  const φ2 = toRadians(lat2)
  const Δφ = toRadians(lat2 - lat1)
  const Δλ = toRadians(lon2 - lon1)

  const a = Math.sin(Δφ / 2) * Math.sin(Δφ / 2) +
        Math.cos(φ1) * Math.cos(φ2) *
        Math.sin(Δλ / 2) * Math.sin(Δλ / 2)
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
  const d = R * c

  return d
}