Reproduction of the risk semivariogram model by the set of risk estimates. The semivariogram of risk estimates is computed in four directions; azimuth angles are measured in degrees clockwise from the NS axis. The solid curve denotes the anisotropic (i.e. direction-dependent) model used in Poisson kriging. This graph illustrates the underestimation of the spatial variability of the risk by the smooth map of kriging estimates.