Show message when user close to a location

Question

I have got a location of a venue with these long/lat parameters: 51.4625733,-0.1868911

I am trying to find a jquery solution when user is close to this venue - let's say 100 meter then a message/alert should be displayed.

I am using this example to get a distance but it doesn't seem working correctly.

 function distance(lon1, lat1, lon2, lat2) {
   var R = 6371; // Radius of the earth in km
   var dLat = (lat2-lat1).toRad();  // Javascript functions in radians
   var dLon = (lon2-lon1).toRad(); 
   var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
      Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) * 
      Math.sin(dLon/2) * Math.sin(dLon/2); 
   var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
   var d = R * c; // Distance in km
   return d;
 }

 /** Converts numeric degrees to radians */
 if (typeof(Number.prototype.toRad) === "undefined") {
   Number.prototype.toRad = function() {
     return this * Math.PI / 180;
   }
 }

 window.navigator.geolocation.getCurrentPosition(function(pos) {
   console.log(pos); 
   console.log(
     distance(pos.coords.longitude, pos.coords.latitude,  51.4625733, -0.1868911)
   ); 
 });

http://jsfiddle.net/ws5kvm1k/

Your help is really appreciated.

Answer 1

I found a solution for this task so put it down for anyone who can find it useful.

// convert numeric degress to radians
toRad = function (value) {
    return value * Math.PI / 180;
};

Equirectangular = function (point1, point2) {
    var R = 6371; // earth radius in km
    var x = (toRad(point2.lon) - toRad(point1.lon)) * Math.cos((toRad(point1.lat) + toRad(point2.lat)) / 2);
    var y = (toRad(point2.lat) - toRad(point1.lat));
    return Math.sqrt(x * x + y * y) * R;
};

// START HERE

window.navigator.geolocation.getCurrentPosition(function (pos) {
    pointOne = {
        lat: pos.coords.latitude,
        lon: pos.coords.longitude
    };
    pointTwo = {
        lat: 51.4625733,
        lon: -0.1868911
    };

    x = Equirectangular(pointOne, pointTwo);
    d = 10; // in kilometers

    if (d == x || d > x) {
        $('body').append('<p><b>you are very close</b><br>' + x);
    } else {
        $('body').append('<br>' + x);
    }

});