计算两点间距离，点与线段距离，三个点的夹角

        /// <summary>
        /// 将度转化为弧度
        /// </summary>
        /// <param name="degree">度</param>
        /// <returns>弧度</returns>
        public static double degreeToRad(double degree)
        {
            return Math.PI * degree / 180;
        }

        /// <summary>
        /// 将弧度转化为度
        /// </summary>
        /// <param name="rad">弧度</param>
        /// <returns>度</returns>
        public static double radToDegre(double rad)
        {
            return (180 * rad) / Math.PI;
        }

        //地球半径
        private const double EARTH_RADIUS = 6378.137;

        /// <summary>
        /// 从两个gps坐标点（经纬度）获得两点的直线距离，单位是米
        /// </summary>
        /// <param name="p1"></param>
        /// <param name="p2"></param>
        /// <param name="p3"></param>
        /// <param name="p4"></param>