SOM_PAK is the package created by T.Kohonen. This package has a considerable quantity of options. I have got used to use SOM for research of data and this is the thing which most of all does not suffice me in RapidMiner.
RapidMiner uses rectangular grid. Sometimes hexagonal grid is better. The example can be seen here: http://en.wikipedia.org/wiki/Self-organizing_map
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