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somself-organizing-maps

SOM map full understanding

I have some enqueries to fully understand how SOM maps work, if you check this SOM map:

Som Map

1 - Why there are some points in the map that doesn’t have any label (A, B or C)?

2 - I understand that in each node there are more than 1 elements. How many elements of a type must be in node to see its label in in the map?

3 - What is the meaning of the gray colors? Does black mean that there are a lot of points that match that cell? Or does it mean that there is a big distance between that cell and the near one?

4 - Why there is not any label (A, B or C) in any black cell? I realized later than this map is of size 10 x 5, and the labels can only be located at the points. Is that correct?

Thank you in advance!!

Solution

  • It's not a SOM map. it's a U-Matrix, which shows the distances between nodes. This will explain why there is a gap between every node and every other node. In the U-Matrix, the shade of the hexagon in the middle is lighter when the two nodes are near one another and darker when they are further apart. See http://users.ics.aalto.fi/jhollmen/dippa/node24.html for more info.

    The number of labels in each cell depends on the 'mode'. I'm not sure what implementation you're using but the modes for SOM Toolbox are:

    % From https://github.com/ilarinieminen/SOM-Toolbox/blob/master/som/som_autolabel.m
% The modes:
%  'add':   all labels from sFrom are added to sTo (even multiple
%           copies of same)  
%  'add1':  only one instance of each label is kept
%  'freq':  only one instance of each label is kept and '(#)', where 
%           # is the frequency of the label, is added to the end of 
%           the label. Labels are ordered according to frequency. 
%  'vote':  only the label with most instances is kept