What NPM package should I use for finding the nearest city. - mongodb

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

How many "pre-defined" cities are you working with? If it's a small number, you could probably just store the list in memory and do a linear scan.
Also, you should probably just give the mongo geospatial query a try to get an idea of exactly how expensive it is, before assuming that it's unreasonable - if you index the city locations and the, it will be pretty fast..
If you are dealing with a lot of points, still don't want to rely on mongo geo-indexing, and need something really specialized, maybe an R-Tree would be worth experimenting with. Here's an r-tree implementation for javascript. https://github.com/imbcmdth/RTree

Answer 2

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 :)