计算经纬度坐标集合四至

        var minx=200;
        var miny=200;
        var maxx=-180;
        var maxy=-180;
        for (let i = 0; i < data.length; i++) {
           var item = data[i];
           if (minx > item.gc84_l) minx = item.gc84_l;
           if (miny > item.gc84_b) miny = item.gc84_b;
           if (maxx < item.gc84_l) maxx = item.gc84_l;
           if (maxy < item.gc84_b) maxy = item.gc84_b;
       }