Find the distance between two points, taking into account the curvature of the Earth.

AtoB

function findDistance() {
  'use strict';
  var hasGeoLocation = navigator.geolocation ? true : false;
  if (hasGeoLocation) {
    var toLat, toLon, fromLat, fromLon, dLat, dLon, R, a, c, d, distance;
    navigator.geolocation.getCurrentPosition(function(position) {
// Define the toRad function for later calculation
      if (typeof(Number.prototype.toRad) === 'undefined') {
        Number.prototype.toRad = function() {
          return this * Math.PI / 180;
        };
      }
// Destination co-ordinates
      toLat = 51.524284;
      toLon = -0.076237;
// User co-ordinates
      fromLat = position.coords.latitude;
      fromLon = position.coords.longitude;
// Average Radius of the Earth (km)
      R = 6371;
// Perform the distance calculations using the Haversine formula to account for the Earth's curvature.
// This code is from http://www.movable-type.co.uk/scripts/latlong.html
      dLat = (toLat-fromLat).toRad();
      dLon = (toLon-fromLon).toRad();
      a = Math.sin(dLat/2) * Math.sin(dLat/2) + Math.cos(toLat.toRad()) * Math.cos(fromLat.toRad()) * Math.sin(dLon/2) * Math.sin(dLon/2);
      c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
      d = R * c;
// Round to one decimal place
      distance = Math.round(d*10)/10;
// Return the result
      document.getElementById('hello').innerHTML = 'You are ' + distance + 'km from the destination';
    });
  } else {
    window.alert('Your browser does not support GeoLocation');
  }
}
document.addEventListener('DOMContentLoaded', findDistance, false);