Monopole and quadrupole covariance matrix. The monopole-monopole and quadrupole-quadrupole elements are normalized to the variance: . The monopole-quadrupole elements are normalized to the mean of the quadrupole and monopole variances. The first off-diagonal elements of the monopole-monopole and quadrupole-quadrupole elements are ~20% of the diagonal elements. The diagonal elements of the quadrupole-monopole covariance are ~− 0.2 times the geometric mean of the monopole and quadrupole variances.