def _get_dim(data):
    """
    Data dimensionality with error checking
    """
    if data.ndim == 2:
        ndim = 2
    elif data.ndim == 3:
        ndim = 3
    else:
        raise RuntimeError(
            'Unsupported number of dimensions {}. We only supports 2 or 3D arrays.'
            .format(data.ndim))
    return ndim
