// the Great Circle Distance
func (p *Point) DistanceOf(q *Point) distance.Distance {
// convert degree -> rad
plat := p.Lat * math.Pi / 180.0
plon := p.Lon * math.Pi / 180.0
qlat := q.Lat * math.Pi / 180.0
qlon := q.Lon * math.Pi / 180.0
a := math.Cos(qlat) * math.Sin(plon-qlon)
b := math.Cos(plat)*math.Sin(qlat) - math.Sin(plat)*math.Cos(qlat)*math.Cos(plon-qlon)
c := math.Sin(plat)*math.Sin(qlat) + math.Cos(plat)*math.Cos(qlat)*math.Cos(plon-qlon)
dist := math.Atan2(math.Sqrt(a*a+b*b), c)
if dist < 0.0 {
dist += 2 * math.Pi
}
return distance.Distance(dist) * distance.EarthRadius
}
// and another way to compute the great circle distance
func (p *Point) DistanceOf2(q *Point) distance.Distance {
return distance.Distance(p.Angle(q)) * distance.EarthRadius
}