(var’ ē ō gram or ver ē ∂’ gram) A measure of the spatial continuity of a quantity; a plot of the difference between quantities as a function of their separation (see Figure V-1). Points close to each other generally differ by only small amounts and differences increase as points are separated by larger distances, until beyond some characteristic distance (the range) there is no systematic similarity, i.e., the curve levels off (the sill). Variograms may differ in different directions (anisotropic variograms). Also called a semivariogram. The difference between the constant value of the sill and a variogram curve is effectively a correlogram. Variograms are generally modeled for kriging or simulation by assuming a simple curve up to the elbow and a flat sill thereafter. Kriging uses smooth models that approximate the data rather than the data themselves.