The Autocorrelation image is obtained by performing a cross correlation of a surface with itself.  The general shape of the autocorrelation image is often used to determine some meaningful information regarding the surface.  For example, for a surface with a predominant lay (anisotropic surface), the autocorrelation image will have a central lobe that extends along one axis.

 

 

For a surface that has no dominant direction (isotropic surface), the autocorrelation image will have a central circular peak.