Absence of Correlation

Very often, in practice, the correlation between two variables $Z(/boma{x})$ and $Z(/boma{x}+/boma{h})$ disappears when the distance $/vert /boma{h}/vert$ becomes too large:

and, in practice, we can put $C({/boma h})=0$, once $/vert /boma{h}/vert /geq a$. The distance $a$ beyond which $C({/boma h})$ can be considered to be equal to zero is called the range and it represents the transition from the state in which a spatial correlation exists ( $/vert /boma{h}/vert < a$) to the state in which there is absence of correlation ( $/vert /boma{h}/vert > a$).