This will return the distance between two points in miles (based on the Haversine formula).
To get the distance in kilometers, multiply with 1.60934.
func distance(lat1 float64, lng1 float64, lat2 float64, lng2 float64) float64 {
radlat1 := float64(math.Pi * lat1 / 180)
radlat2 := float64(math.Pi * lat2 / 180)
theta := float64(lng1 - lng2)
radtheta := float64(math.Pi * theta / 180)
dist := math.Sin(radlat1)*math.Sin(radlat2) + math.Cos(radlat1)*math.Cos(radlat2)*math.Cos(radtheta)
if dist > 1 {
dist = 1
}
dist = math.Acos(dist)
dist = dist * 180 / math.Pi
dist = dist * 60 * 1.1515
return dist
}