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"))
}

how does this compare to using ST_Distance()?

ST_Distance likely uses code that accomplishes the same.
If you require the other GIS functions ST-2 provides, you can use that library (creating, modelling, manipulating spheres).

If all you need is the distance between two points, save on code size, and use just this.

This code doesn't provide any guarantees, and is strictly a golang port of existing code snippets that geodatasource provides.