From 980a2f38d32189ff3b0fee805752ef39e9a58bb0 Mon Sep 17 00:00:00 2001
From: Patrick Peng Sun <patrick@lawipac.com>
Date: Fri, 28 Apr 2017 00:16:03 +1000
Subject: [PATCH] added function to calculate distance between two point of
 earth, in meters,  from Github gist

---
 calDistance.go | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 calDistance.go

diff --git a/calDistance.go b/calDistance.go
new file mode 100644
index 0000000..494a15f
--- /dev/null
+++ b/calDistance.go
@@ -0,0 +1,34 @@
+package main
+
+import "math"
+
+// haversin(θ) function
+func hsin(theta float64) float64 {
+	return math.Pow(math.Sin(theta/2), 2)
+}
+
+// Distance function returns the distance (in meters) between two points of
+//     a given longitude and latitude relatively accurately (using a spherical
+//     approximation of the Earth) through the Haversin Distance Formula for
+//     great arc distance on a sphere with accuracy for small distances
+//
+// point coordinates are supplied in degrees and converted into rad. in the func
+//
+// distance returned is METERS!!!!!!
+// http://en.wikipedia.org/wiki/Haversine_formula
+func Distance(lat1, lon1, lat2, lon2 float64) float64 {
+	// convert to radians
+	// must cast radius as float to multiply later
+	var la1, lo1, la2, lo2, r float64
+	la1 = lat1 * math.Pi / 180
+	lo1 = lon1 * math.Pi / 180
+	la2 = lat2 * math.Pi / 180
+	lo2 = lon2 * math.Pi / 180
+
+	r = 6378100 // Earth radius in METERS
+
+	// calculate
+	h := hsin(la2-la1) + math.Cos(la1)*math.Cos(la2)*hsin(lo2-lo1)
+
+	return 2 * r * math.Asin(math.Sqrt(h))
+}