"In cartography and geology, a topological map is a type of diagram that has been simplified so that only vital information remains and unnecessary detail has been removed. These maps lack scale, and distance and direction are subject to change and variation, but the relationship between points is maintained. A good example is the tube map of the London Underground or the map for the New York City Subway."The discussion thread at the MapPorn source includes comments by the artist regarding the technique he used to generate the map. When you look at it in detail, it is really quite interesting.