return distance between two points in meters,
    coordinates are in degrees
    thanks to http://www.movable-type.co.uk/scripts/latlong.html

def gps_distance(lat1, lon1, lat2, lon2):
    '''return distance between two points in meters,
    coordinates are in degrees
    thanks to http://www.movable-type.co.uk/scripts/latlong.html'''
    lat1 = math.radians(lat1)
    lat2 = math.radians(lat2)
    lon1 = math.radians(lon1)
    lon2 = math.radians(lon2)
    dLat = lat2 - lat1
    dLon = lon2 - lon1

    a = math.sin(0.5*dLat)**2 + math.sin(0.5*dLon)**2 * math.cos(lat1) * math.cos(lat2)
    c = 2.0 * math.atan2(math.sqrt(a), math.sqrt(1.0-a))
    return radius_of_earth * c