A metric is developed providing a quantitative measure of the two-dimensional spatial coherence of scattered fields. The metric is based on fitting a function similar to bivariate Gaussian to measured two-dimensional coherence surfaces. This function provides a robust fit to the measured data for a range of coherence lengths and surface asymmetries. Through an eigendecomposition of the bivariate Gaussian covariance matrix, it is possible to define surface orientation as well as coherence lengths along the major and minor axes. The metric is applied to normal-incidence scattering data collected in recent field trials at Seneca Lake, NY.