js 经纬度计算直线距离

function getRad(d) {
    var PI = Math.PI; return d * PI / 180.0;
}
function getDistance(lat1, lng1, lat2, lng2) {
    var f = getRad((lat1 + lat2) / 2);
    var g = getRad((lat1 - lat2) / 2);
    var l = getRad((lng1 - lng2) / 2);
    var sg = Math.sin(g);
    var sl = Math.sin(l);
    var sf = Math.sin(f);
    var s, c, w, r, d, h1, h2;
    var a = 6378137.0;
    var fl = 1 / 298.257;
    sg = sg * sg;
    sl = sl * sl;
    sf = sf * sf;
    s = sg * (1 - sl) + (1 - sf) * sl;
    c = (1 - sg) * (1 - sl) + sf * sl;
    w = Math.atan(Math.sqrt(s / c));
    r = Math.sqrt(s * c) / w;
    d = 2 * w * a;
    h1 = (3 * r - 1) / 2 / c; h2 = (3 * r + 1) / 2 / s; s = d * (1 + fl * (h1 * sf * (1 - sg) - h2 * (1 - sf) * sg));
    s = s.toFixed(2);
    return s ;
}

getDistance(39.982419, 116.305958, 39.982464, 116.307760)