已知两点经纬度坐标，求其直线距离

def calcDistance(Lat_A, Lng_A, Lat_B, Lng_B):
ra = 6378.140 # 赤道半径 (km)
rb = 6356.755 # 极半径 (km)
flatten = (ra - rb) / ra # 地球扁率
rad_lat_A = radians(Lat_A)
rad_lng_A = radians(Lng_A)
rad_lat_B = radians(Lat_B)
rad_lng_B = radians(Lng_B)
pA = atan(rb / ra * tan(rad_lat_A))
pB = atan(rb / ra * tan(rad_lat_B))
xx = acos(sin(pA) * sin(pB) + cos(pA) * cos(pB) * cos(rad_lng_A - rad_lng_B))
c1 = (sin(xx) - xx) * (sin(pA) + sin(pB)) * 2 / cos(xx / 2) * 2
c2 = (sin(xx) + xx) * (sin(pA) - sin(pB)) * 2 / sin(xx / 2) * 2
dr = flatten / 8 * (c1 - c2)
distance = float (ra * (xx + dr))
return distance