# Based on the formula for great-circle distance
# (http://en.wikipedia.org/wiki/Great-circle_distance)
#
# Note that this is not 100% accurate as the earth isn't a "true" sphere, but
# for our purposes it's plenty (guaranteed within 0.5%).

import math

def GeoDistance(lat1, lon1, lat2, lon2):
    """
    Calculate the distance between two points on earth in km
    """
    phi1 = math.radians(90.0 - lat1)
    phi2 = math.radians(90.0 - lat2)
    theta1 = math.radians(lon1)
    theta2 = math.radians(lon2)

    arc = math.acos(math.sin(phi1) * math.sin(phi2) *
            math.cos(theta1 - theta2) + math.cos(phi1)*math.cos(phi2))

    # 6371.01 is the average radius of the earth in km.
    return arc * 6371.01