haversine/haversine.js

A simple haversine module

// haversine
// By Nick Justice (niix)
// https://github.com/niix/haversine

var haversine = (function() {

  // convert to radians
  var toRad = function(num) {
    return num * Math.PI / 180
  }

  return function haversine(start, end, options) {
    var miles = 3960
    var km    = 6371
    options   = options || {}

    var R = options.unit === 'km' ? km : miles

    var dLat = toRad(end.latitude - start.latitude)
    var dLon = toRad(end.longitude - start.longitude)
    var lat1 = toRad(start.latitude)
    var lat2 = toRad(end.latitude)

    var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
            Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2)
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))

    return R * c
  }

})()

module.exports = haversine