def default_refl_mask(ref_data):
    """
    Example of masking using the reflectance.

    :param ref_data: reflectance dataset from which to derive mask
    :type ref_data: xarray
    :rtype 2D numpy array (BOOL)
    """
    blue = ref_data.blue.where(ref_data.blue != ref_data.blue.attrs['nodata'])
    nir = ref_data.nir.where(ref_data.nir != ref_data.nir.attrs['nodata'])
    ratio = (blue - nir) / (nir + blue)
    mask = ratio > 0.0
    mask = mask.values.squeeze()
    return mask
