'a small amount of padding is added because the current
clipping algorithms seems to fail when the gridline ends at
the bbox boundary.'

def _add_pad(self, lon_min, lon_max, lat_min, lat_max):

    dlon = ((lon_max - lon_min) / self.nx)
    dlat = ((lat_max - lat_min) / self.ny)
    (lon_min, lon_max) = ((lon_min - dlon), (lon_max + dlon))
    (lat_min, lat_max) = ((lat_min - dlat), (lat_max + dlat))
    return (lon_min, lon_max, lat_min, lat_max)
