Oli Laurelle has an enticing Flickr set featuring many invisible journeys on different cities, by different means of transportation, including walking paths and WI-FI mapping. Part of the set is this segmented radial visualization showing the recording of wireless networks through journeys on 4 different cities, from bigger to smaller rings: London, Vescemont, Belfort and Barcelona.
The time dimension starts reading from the right then it goes clockwise along the main black thick line. Each succesfull recording influences the time line thickness and adds a "pin". Red pins represent non encrypted networks while other networks are represented by the smaller black pins. As it stands, the position of the nodes is relative to the time of their recording, and there is one recording session per ring.