a) Typical variogram γ(h) and its equivalent covariance function C(h): if the data has some sort of spatial autocorrelation, nearby (small h) Z(x) observed values will be more similar than more distant Z(x) values (larger h); b) as the separation distance h grows, the quantity Z(x + h) − Z(x) in expression (16) will tend to increase on average, but less and less as the influence of Z(h) on Z(x + h) weakens; at some threshold distance h, called the range, the increase in variance becomes negligible and the asymptotical variance value is known as the sill