Swift计算两个经纬度之间的球面面积

func distanceBetween(#lat1: Float, lng1: Float, lat2: Float, lng2: Float) -> Float{
    
    var jl_jd: Float = 102834.74258026089786013677476285
    var jl_wd: Float = 111712.69150641055729984301412873
    var b: Float = abs((lng1 - lng2) * jl_jd)
    var a: Float = abs((lat1 - lat1) * jl_wd)
    return sqrt(a * a + b * b)
}