Let \( C_s = \{c _1 , c_2 , …d_n \}\) be the

set of cells at scale \(s\) where the occurrences of a node X where found. The \(C _{s−1} = \{d_ 1 , d_ 2 , …d _k \}\) is

the corresponding set of cells at an upper scale (ancestor of \(s \)) where the occurrences of a node X where found.

Note that the ratio:

\(r_s = \frac{\#C_{s-1}}{\#C_s}\)

gives us an indicator of how the occurrences are dispersed in the space.

If \(r_s\) is low means that the

spatial distribution is constrained in a region as small as the unit area of the upper scale while if \(r_s\) is close to 1 it tells us that the occurrences are as spatially distributed as the cells in the upper scale.

The method can be applied recursively to the sucessive scales to obtain a list of ratios \(r_1 , r_2 , ..r_s ,.. \) that can be fitted in model to estimate geographic extensions.