Convert angle in to distance along a great circle.

    Args:
        angle (float): Angle in degrees to convert to distance
        units (str): Unit type to be used for distances

    Returns:
        float: Distance in ``units``

    Raises:
        ValueError: Unknown value for ``units``

def angle_to_distance(angle, units='metric'):
    """Convert angle in to distance along a great circle.

    Args:
        angle (float): Angle in degrees to convert to distance
        units (str): Unit type to be used for distances

    Returns:
        float: Distance in ``units``

    Raises:
        ValueError: Unknown value for ``units``
    """
    distance = math.radians(angle) * BODY_RADIUS

    if units in ('km', 'metric'):
        return distance
    elif units in ('sm', 'imperial', 'US customary'):
        return distance / STATUTE_MILE
    elif units in ('nm', 'nautical'):
        return distance / NAUTICAL_MILE
    else:
        raise ValueError('Unknown units type %r' % units)