Haversine formula

dist.go

func dist(lat1, lon1, lat2, lon2 float64) float64 {
	const R = 1000.0 * 6378.132 // Radius of earth in m.

	lat := lat2*math.Pi/180.0 - lat1*math.Pi/180.0
	lon := lon2*math.Pi/180.0 - lon1*math.Pi/180.0

	a := math.Sin(lat/2.0)*math.Sin(lat/2.0) + math.Cos(lat1*math.Pi/180.0)*math.Cos(lat2*math.Pi/180.0)*math.Sin(lon/2.0)*math.Sin(lon/2.0)
	c := 2.0 * math.Atan2(math.Sqrt(a), math.Sqrt(1.0-a))
	d := R * c
	return d
}