Golang code to calculate distance between two lat/lng decimal coordinates.

distance.go

package main

import (
	"math"
	"fmt"
)

//:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
//:::                                                                         :::
//:::  This routine calculates the distance between two points (given the     :::
//:::  latitude/longitude of those points). It is based on free code used to  :::
//:::  calculate the distance between two locations using GeoDataSource(TM)   :::
//:::  products.                                                              :::
//:::                                                                         :::
//:::  Definitions:                                                           :::
//:::    South latitudes are negative, east longitudes are positive           :::
//:::                                                                         :::
//:::  Passed to function:                                                    :::
//:::    lat1, lon1 = Latitude and Longitude of point 1 (in decimal degrees)  :::
//:::    lat2, lon2 = Latitude and Longitude of point 2 (in decimal degrees)  :::
//:::    optional: unit = the unit you desire for results                     :::
//:::           where: 'M' is statute miles (default, or omitted)             :::
//:::                  'K' is kilometers                                      :::
//:::                  'N' is nautical miles                                  :::
//:::                                                                         :::
//:::  Worldwide cities and other features databases with latitude longitude  :::
//:::  are available at https://www.geodatasource.com                         :::
//:::                                                                         :::
//:::  For enquiries, please contact [email protected]                  :::
//:::                                                                         :::
//:::  Official Web site: https://www.geodatasource.com                       :::
//:::                                                                         :::
//:::          Golang code James Robert Perih (c) All Rights Reserved 2018    :::
//:::                                                                         :::
//:::           GeoDataSource.com (C) All Rights Reserved 2017                :::
//:::                                                                         :::
//:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
func distance(lat1 float64, lng1 float64, lat2 float64, lng2 float64, unit ...string) 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
	
	if len(unit) > 0 {
		if unit[0] == "K" {
			dist = dist * 1.609344
		} else if unit[0] == "N" {
			dist = dist * 0.8684
		}
	}
	
	return dist
}

// usage:
func main() {
	type coordinate struct {
		lat float64
		lng float64
	}
	
	winnipeg := coordinate{ 49.895077, -97.138451 }
	regina := coordinate{ 50.445210, -104.618896 }
	
	fmt.Println(distance(winnipeg.lat, winnipeg.lng, regina.lat, regina.lng, "N"))
}