title: How do I make a U-matrix?
body: <p>How exactly is an U-matrix constructed in order to visualise a <a href="http://en.wikipedia.org/wiki/Self-organizing_map" rel="nofollow noreferrer">self-organizing-map</a>? More specifically, suppose that I have an output grid of 3x3 nodes (that have already been trained), how do I construct a U-matrix from this? You can e.g. assume that the neurons (and inputs) have dimension 4.</p>
  
  <p>I have found several resources on the web, but they are not clear or they are contradictory. For example, the <a href="http://www.uni-marburg.de/fb12/datenbionik/pdf/pubs/1990/UltschSiemon90" rel="nofollow noreferrer">original paper</a> is full of typos.</p>