//calculate the distance between two data points
function distance(lat1, lon1, lat2, lon2, unit) {
    var radlat1 = Math.PI * lat1/180;
    var radlat2 = Math.PI * lat2/180;
    var radlon1 = Math.PI * lon1/180;
    var radlon2 = Math.PI * lon2/180;
    var theta = lon1-lon2;
    var radtheta = Math.PI * theta/180;
    var dist = Math.sin(radlat1) * Math.sin(radlat2) + Math.cos(radlat1) * Math.cos(radlat2) * Math.cos(radtheta);
    dist = Math.acos(dist);
    dist = dist * 180/Math.PI;
    dist = dist * 60 * 1.1515;
    if (unit=="K") { dist = dist * 1.609344; }
    if (unit=="N") { dist = dist * 0.8684; }
    return dist;
}