Getting distance between two points based on latitude/longitude

I tried implementing this formula: The aplet does good for the two points I am testing:

enter image description here

Yet my code is not working.

    from math import sin, cos, sqrt, atan2

    R = 6373.0

    lat1 = 52.2296756
    lon1 = 21.0122287
    lat2 = 52.406374
    lon2 = 16.9251681

    dlon = lon2 - lon1
    dlat = lat2 - lat1
    a = (sin(dlat/2))**2 + cos(lat1) * cos(lat2) * (sin(dlon/2))**2
    c = 2 * atan2(sqrt(a), sqrt(1-a))
    distance = R * c

    print "Result", distance
    print "Should be", 278.546

The distance it returns is 5447.05546147. Why?

It's because in Python, all the trig functions use radians, not degrees.

You can either convert the numbers manually to radians, or use the radians function from the math module:

    from math import sin, cos, sqrt, atan2, radians

    # approximate radius of earth in km
    R = 6373.0

    lat1 = radians(52.2296756)
    lon1 = radians(21.0122287)
    lat2 = radians(52.406374)
    lon2 = radians(16.9251681)

    dlon = lon2 - lon1
    dlat = lat2 - lat1

    a = sin(dlat / 2)**2 + cos(lat1) * cos(lat2) * sin(dlon / 2)**2
    c = 2 * atan2(sqrt(a), sqrt(1 - a))

    distance = R * c

    print("Result:", distance)
    print("Should be:", 278.546, "km")

The distance is now returning the correct value of 278.545589351 km.

Edit: Just as a note, if you've stumbled across this post because you just need a quick and easy way of finding the distance between two points, I recommend you use the approach recommended in Kurt's answer below -- see his post for rationale.


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