Question

I am using Node.js and MongoDB.
Let say I have predefined few cities (e.g. Seattle, Miami, New York) with Lat & Lon. and there is a user click to my website and I know his IP address, and find out the lat & lon. Then I want to know which city that I’ve defined is the closest to the user.

I know I can do it using Mongo’s geospatial feature. but it would be quite ‘expensive’ to use DB to calculate that for every web request.

Is there a Node.js NPM package that can do the geo feature as I described above?

Answer 1

If you already have the user’s location and the location of each city, it should be quite fast to compute the distance to the nearest. Check out this site: http://www.movable-type.co.uk/scripts/latlong.html
I have used the first algorithm a few times.

  function getDistance(lat1,lat2,lon1,lon2){
    var R = 6371; // km
    var c = Math.PI / 180;
    var dLat = (lat2-lat1) * c;
    var dLon = (lon2-lon1) * c;
    var lat1 = lat1 * c;
    var lat2 = lat2 * c;

    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)); 
    var d = R * c;
    return d;
   }
var closest, dist = Number.MAX_VALUE;
for(var i = 0, l=cities.length;i<l;++i){
  if(getDistance(cities[i].longitude, cities[i].latitude, user.longitude, user.latitude) < max){
  closest = cities[i];
 }
}

  alert(closest.name + ' is the winner :)');

You might want to add some exception handling here 🙂