Internal function to derive the anisotropy exponent from two roughness exponents

If any IQR is higher than 0.225, this function returns NA. The threshold value of 0.225 is the IQR of a normal distribution with a standard deviation equals to 0.5/3, a three-sigma range between 0 and 1.

get_zeta_(alpha_1, alpha_2, IQR_1, IQR_2, kruskal_flag)

Arguments

alpha_1	`numeric`, a roughness coefficient
alpha_2	`numeric`, a roughness coefficient
IQR_1	`numeric`, an IQR
IQR_2	`numeric`, an IQR
kruskal_flag	`logical` flag returned by `get_kruskal_flag()`

Value

a numeric anisotropy exponent