def _set_estimator(estimator):
    """Translate the verbose Python estimator identifier to single char."""
    if estimator.lower() == 'matheron':
        cython_estimator = 'm'
    elif estimator.lower() == 'cressie':
        cython_estimator = 'c'
    else:
        raise ValueError('Unknown variogram estimator function ' + str(
            estimator))
    return cython_estimator
