  We suggest a class of direct sample estimates for the two-point quadratic
statistical functions of multidimensional data, which includes: estimates of
the sample autocovariance function (AcF), sample mean square increment (also,
structure) function, and the estimate of the power spectrum. The central
estimate for the class is the sample AcF, which is constructed as to represent
the finite Fourier transform of the periodogram estimate of the spectrum and is
positive semidefinite. The estimate explicitly account for the anisotropy of
the fields in all spatial directions and is illustrated on two examples: the
morphology of the Grab nebula and the surface roughness generated as a solution
of the anisotropic Kardar-Parisi-Zhang equation. We also provide an expression
of the covariance of the sample AcF in the case of data assumed to be drawn
from a two-dimensional Gaussian field with a known mean.
