def get_resolution(bbox, size):
    """
    Calculate the highest resolution needed to draw the bbox
    into an image with given size.

    >>> get_resolution((-180,-90,180,90), (256, 256))
    0.703125

    :returns: the resolution
    :rtype: float
    """
    w = abs(bbox[0] - bbox[2])
    h = abs(bbox[1] - bbox[3])
    return min(w/size[0], h/size[1])