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:

Tue May 27 12:40:37 EET DST 1997