Two maps can be quite different even if their average quantization
and/or topological errors are equal. Therefore a measure of similarity
between maps would be useful. Similarity could be based on comparing
the location of BMUs of input vectors 
on two different
maps [49]. This is however sensitive to the size and
orientation of the map.
A much better measure was proposed by Kaski and Lagus [18].
The method is an extension of the topological quantization error
presented in previous section. The shortest paths between each sample vector pair
and 
going through their BMUs i and j,
respectively, are measured on both maps 
and 
:
where k' is treated in the same way as in 3.5. The dissimilarity between two maps is taken as the expectation value of the relative absolute difference of these distances:
