def check_raster_bases(sys):
    """Return a list of all raster that have no basis."""
    rasters_wo_bases = []
    for sname, system in sys.items():
        for rname, raster in system.items():
            try:
                raster.basis_column()
            except KeyError:
                if sname:
                    rasters_wo_bases.append("'{}'/'{}'".format(sname, rname))
                else:
                    rasters_wo_bases.append("'{}'".format(rname))
    return rasters_wo_bases
