Remark 2.

For every variogram, $\gamma(0)=0$. However sometimes the data may display a jump at the origin. This apparent discontinuity is called the ``nugget effect'' and can be due to measurement errors or to microregionalization effects that are not evidenced at the scale of the actual data points. If the nugget effect is present the the variogram needs to be changed into:

$$\gamma(h) = \delta + \gamma_0(h)$$

where $\delta$ is the jump at the origin and $\gamma_0(h)$ the variogram without the jump. In Fig. 2.3 we report a variogram with the nugget effect together with some other special cases that may be encountered.