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// adapted from http://www.movable-type.co.uk/scripts/latlong.html
function getDistanceInKm(lat1, long1, lat2, long2) {
    function toRadians(deg) {
        // note: replace Math.PI/180 with 0.0174532925199433 for better performance
        return deg * (Math.PI / 180);
    };

    var RADIUS = 6371, // km
        diffLat = toRadians(lat2 - lat1),
        diffLon = toRadians(long2 - long1),
        lat1 = toRadians(lat1),
        lat2 = toRadians(lat2),

        a = Math.sin(diffLat / 2) * Math.sin(diffLat / 2) +
            Math.sin(diffLon / 2) * Math.sin(diffLon / 2) * Math.cos(lat1) * Math.cos(lat2),
        c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a)),
        d = RADIUS * c; // distance between 2 geolocations (in units of RADIUS e.g. km)

    return d;
}