great circle distance in python

distance.py

"""Distance helpers."""

import math


EARTH_CIRCUMFERENCE = 6378137     # earth circumference in meters


def great_circle_distance(latlong_a, latlong_b):
    """
    >>> coord_pairs = [
    ...     # between eighth and 31st and eighth and 30th
    ...     [(40.750307,-73.994819), (40.749641,-73.99527)],
    ...     # sanfran to NYC ~2568 miles
    ...     [(37.784750,-122.421180), (40.714585,-74.007202)],
    ...     # about 10 feet apart
    ...     [(40.714732,-74.008091), (40.714753,-74.008074)],
    ...     # inches apart
    ...     [(40.754850,-73.975560), (40.754851,-73.975561)],
    ... ]
    
    >>> for pair in coord_pairs:
    ...     great_circle_distance(pair[0], pair[1]) # doctest: +ELLIPSIS
    83.325362855055...
    4133342.6554530...
    2.7426970360283...
    0.1396525521278...
    """
    lat1, lon1 = latlong_a
    lat2, lon2 = latlong_b

    dLat = math.radians(lat2 - lat1)
    dLon = math.radians(lon2 - lon1)
    a = (math.sin(dLat / 2) * math.sin(dLat / 2) +
            math.cos(math.radians(lat1)) * math.cos(math.radians(lat2)) * 
            math.sin(dLon / 2) * math.sin(dLon / 2))
    c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a))
    d = EARTH_CIRCUMFERENCE * c
    
    return d

Your code returns the wrong answer for the test ((47.64828, -122.52963), (47.61168, -122.33326)) compare it to google and several others. Your code understates the distance. Your code returns 15.284 km while other libraries returned 21.944km and google returned 12.7 miles which converts to 20.43km

joeatbayes, that is to be expected. There are different formulas based on different earth models with different accuracies.